From c2c6409dead2a5988c8940fc29b3dac5f8a92e1a Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Wed, 11 Nov 2026 03:44:06 +0000
Subject: [PATCH 1/3] Rename the `optimistic` statement to `guess`

`guess x := e` replaces `optimistic x := e` (07 decision 18, ruled
2026-11-11). Semantics and lowering are unchanged (createOptimistic /
createOptimisticStore); only the surface word moves.

- Parsers (ref/src/parse.ts, compiler/src/parse.rs): `guess` is the
  keyword; `optimistic` stays reserved and is a parse error coded
  OPTIMISTIC whose hint names `guess` (guide/diagnostics.md, S-parse-hints).
- Codes: OPTIMISTIC_OUTSIDE_HOLD -> GUESS_OUTSIDE_HOLD,
  OPTIMISTIC_NO_QUESTION -> GUESS_NO_QUESTION, OPTIMISTIC_REVERTED ->
  GUESS_REVERTED; HELD_WRITE / STALE_WRITE_BACK / AC-1b messages say `guess`.
- Spec, guide, scenarios, examples, eval reference apps and the editor
  grammar use `guess`. eval/results and preregistrations are left as
  historical records.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWP7LmQhib6M9zGnmBH
---
 compiler/src/ast.rs                           |  2 +-
 compiler/src/check.rs                         |  8 ++--
 compiler/src/infer.rs                         |  2 +-
 compiler/src/parse.rs                         |  9 +++-
 eval/README.md                                |  2 +-
 eval/reference/chain1/rx/comments.rx          |  4 +-
 eval/reference/chain1/rx/issues.rx            | 14 +++---
 eval/reference/chain1/rx/labels.rx            |  8 ++--
 eval/reference/chain1/rx/settings.rx          |  4 +-
 eval/reference/chain2/rx/comments.rx          |  4 +-
 eval/reference/chain2/rx/issues.rx            | 14 +++---
 eval/reference/chain2/rx/labels.rx            |  8 ++--
 eval/reference/chain2/rx/settings.rx          |  4 +-
 eval/reference/chain3/rx/comments.rx          |  4 +-
 eval/reference/chain3/rx/issues.rx            | 14 +++---
 eval/reference/chain3/rx/labels.rx            |  8 ++--
 eval/reference/chain3/rx/settings.rx          |  4 +-
 eval/reference/chain4/rx/comments.rx          |  4 +-
 eval/reference/chain4/rx/issues.rx            | 18 ++++----
 eval/reference/chain4/rx/labels.rx            |  8 ++--
 eval/reference/chain4/rx/settings.rx          |  4 +-
 eval/reference/close/rx/main.rx               |  8 ++--
 eval/reference/comments/rx/main.rx            |  8 ++--
 eval/reference/count/rx/comments.rx           |  4 +-
 eval/reference/count/rx/issues.rx             | 14 +++---
 eval/reference/count/rx/settings.rx           |  4 +-
 eval/reference/desk/rx/comments.rx            |  4 +-
 eval/reference/desk/rx/issues.rx              | 14 +++---
 eval/reference/desk/rx/settings.rx            |  4 +-
 eval/reference/labels/rx/comments.rx          |  4 +-
 eval/reference/labels/rx/issues.rx            | 14 +++---
 eval/reference/labels/rx/labels.rx            |  8 ++--
 eval/reference/labels/rx/settings.rx          |  4 +-
 eval/reference/related/rx/comments.rx         |  4 +-
 eval/reference/related/rx/issues.rx           | 14 +++---
 eval/reference/related/rx/settings.rx         |  4 +-
 eval/reference/rename/rx/main.rx              | 10 ++---
 eval/reference/settings/rx/settings.rx        |  4 +-
 eval/reference/tracker/rx/issues.rx           |  4 +-
 examples/ai/feed.rx                           |  8 ++--
 examples/ai2/settings/NOTES.md                |  4 +-
 examples/ai2/settings/settings.rx             |  4 +-
 examples/ai2/tracker/NOTES.md                 |  2 +-
 examples/ai2/tracker/issues.rx                |  2 +-
 examples/cart.rx                              |  2 +-
 examples/notes/data.rx                        |  2 +-
 examples/shop/cart.rx                         |  2 +-
 examples/todos.rx                             | 12 ++---
 examples/users.rx                             |  4 +-
 guide/async.md                                | 28 ++++++------
 guide/diagnostics.md                          | 45 +++++++++++++++----
 guide/reference.md                            |  4 +-
 ref/compile/rt-web.ts                         |  6 +--
 ref/compile/rt.ts                             |  6 +--
 ref/oracle/README.md                          |  6 +--
 ref/oracle/lower.ts                           |  2 +-
 ref/src/check.ts                              |  8 ++--
 ref/src/infer.ts                              |  2 +-
 ref/src/model.ts                              |  2 +-
 ref/src/parse.ts                              |  7 ++-
 ref/src/runtime.ts                            |  4 +-
 scenarios/S-action-1.md                       |  6 +--
 scenarios/S-action-2.md                       |  2 +-
 scenarios/S-action-3.md                       |  2 +-
 scenarios/S-action-4.md                       |  2 +-
 scenarios/S-ai-feed.md                        |  4 +-
 scenarios/S-ai2-tracker.md                    |  2 +-
 scenarios/S-check-1.md                        |  8 ++--
 scenarios/S-effect-store-1.md                 |  2 +-
 scenarios/S-guess-reask-refresh.md            |  4 +-
 scenarios/S-infer-1.md                        | 12 ++---
 scenarios/S-infer-2.md                        |  2 +-
 scenarios/S-infer-3.md                        |  6 +--
 scenarios/S-infer-6.md                        |  2 +-
 scenarios/S-latest-reask.md                   |  2 +-
 scenarios/S-parse-hints.md                    |  8 ++++
 scenarios/S-remount-refresh.md                |  2 +-
 scenarios/S-solid-26.md                       |  2 +-
 scenarios/S-solid-32.md                       |  8 ++--
 scenarios/S-solid-33.md                       |  2 +-
 scenarios/S-solid-34.md                       |  2 +-
 scenarios/S-solid-35.md                       |  6 +--
 scenarios/S-solid-44.md                       |  2 +-
 scenarios/S-store-6.md                        | 10 ++---
 scenarios/S-store-7.md                        |  2 +-
 scenarios/S-store-8.md                        |  8 ++--
 spec/05-holds.md                              |  4 +-
 spec/06-stores.md                             | 24 +++++-----
 spec/07-surface.md                            | 22 ++++++---
 spec/08-constraints.md                        |  4 +-
 spec/09-modules.md                            |  4 +-
 spec/11-compiler.md                           |  2 +-
 spec/99-diagnostics.md                        |  7 +--
 tooling/vscode-rx/markdown.js                 |  2 +-
 tooling/vscode-rx/syntaxes/rx.tmLanguage.json |  2 +-
 95 files changed, 337 insertions(+), 281 deletions(-)

diff --git a/compiler/src/ast.rs b/compiler/src/ast.rs
index 1c2828fe..46bcbe77 100644
--- a/compiler/src/ast.rs
+++ b/compiler/src/ast.rs
@@ -110,7 +110,7 @@ pub enum PathSeg {
 pub struct Target {
     pub name: String,
     pub path: Vec<PathSeg>,
-    /// `use name := …` (RG-7, O-4). `None`: not written by this form (an `optimistic` target).
+    /// `use name := …` (RG-7, O-4). `None`: not written by this form (a `guess` target).
     pub ctx: Option<bool>,
 }
 impl Target {
diff --git a/compiler/src/check.rs b/compiler/src/check.rs
index 1fe90f97..7a26a9a5 100644
--- a/compiler/src/check.rs
+++ b/compiler/src/check.rs
@@ -2262,9 +2262,9 @@ impl<'a> Checker<'a> {
         for f in run_findings(stmts, action, bound, inferred) {
             let n = &f.name;
             match f.code {
-                Code::HeldWrite => self.err("HELD_WRITE", "AC-1c", format!("{n} := … comes before an `await` in a body that is an action because it writes and awaits (AC-1b), so it is held until the body ends (AC-1): for state that shows while the work runs, `optimistic {n} := …` (a source with a question, ST-5c); for a reset that lands now, `onFlush(() => {{ {n} := … }})` (EF-8); to hold it on purpose, declare the action (`action name() {{ … }}`); or write it after the `await`")),
+                Code::HeldWrite => self.err("HELD_WRITE", "AC-1c", format!("{n} := … comes before an `await` in a body that is an action because it writes and awaits (AC-1b), so it is held until the body ends (AC-1): for state that shows while the work runs, `guess {n} := …` (a source with a question, ST-5c); for a reset that lands now, `onFlush(() => {{ {n} := … }})` (EF-8); to hold it on purpose, declare the action (`action name() {{ … }}`); or write it after the `await`")),
                 Code::StaleOwnRead => self.err("STALE_OWN_READ", "AC-1d", format!("{n} is read after this change wrote it: the read sees the committed value, not the write (W-4) — keep the value you wrote in a local (`let v = …; {n} := v`) and read that, or write functionally (`{n} := v => …`, W-6)")),
-                Code::StaleWriteBack => self.err("STALE_WRITE_BACK", "AC-1e", format!("{n} := … after an `await` is computed from {n} as it was read before that `await`: whatever changed {n} in between (another action, a reload) is overwritten — write functionally (`{n} := v => …`, W-6), which applies to {n} as it stands at the write, or guess it (`optimistic`, AC-3) and let the answer replace the guess")),
+                Code::StaleWriteBack => self.err("STALE_WRITE_BACK", "AC-1e", format!("{n} := … after an `await` is computed from {n} as it was read before that `await`: whatever changed {n} in between (another action, a reload) is overwritten — write functionally (`{n} := v => …`, W-6), which applies to {n} as it stands at the write, or guess it (`guess`, AC-3) and let the answer replace the guess")),
             }
         }
     }
@@ -2351,12 +2351,12 @@ impl<'a> Checker<'a> {
                     }
                     if op == "optimistic" {
                         if !ctx.in_action {
-                            self.err("OPTIMISTIC_OUTSIDE_HOLD", "AC-3", "optimistic outside an action: a guess needs a hold to live in — an apply block is never an action (its return is the cleanup, EF-5): declare one, `action save(v) { optimistic … }`, and call it from the block".into());
+                            self.err("GUESS_OUTSIDE_HOLD", "AC-3", "a guess outside an action: it needs a hold to live in — an apply block is never an action (its return is the cleanup, EF-5): declare one, `action save(v) { guess … }`, and call it from the block".into());
                         }
                         if let Some(t) = tsym {
                             if self.syms[t].kind == SymKind::Source && !self.syms[t].question {
                                 self.err(
-                                    "OPTIMISTIC_NO_QUESTION",
+                                    "GUESS_NO_QUESTION",
                                     "AC-3",
                                     format!("{} is declared with := — a guess needs a question to confirm it (source {} = …, ST-5c)", target.name, target.name),
                                 );
diff --git a/compiler/src/infer.rs b/compiler/src/infer.rs
index 6bbb076e..02413af7 100644
--- a/compiler/src/infer.rs
+++ b/compiler/src/infer.rs
@@ -18,7 +18,7 @@ pub enum Trigger {
 pub fn trigger_text(t: Trigger) -> &'static str {
     match t {
         Trigger::WriteAwait => "it writes and awaits",
-        Trigger::Optimistic => "it has `optimistic`",
+        Trigger::Optimistic => "it has `guess`",
         Trigger::Affects => "it has `affects`",
     }
 }
diff --git a/compiler/src/parse.rs b/compiler/src/parse.rs
index 3393da02..370feb38 100644
--- a/compiler/src/parse.rs
+++ b/compiler/src/parse.rs
@@ -42,7 +42,7 @@ re!(LINE_COMMENT, r"//.*$");
 
 static KEYWORDS: LazyLock<HashSet<&'static str>> = LazyLock::new(|| {
     [
-        "source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined",
+        "source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined",
         "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal",
     ]
     .into_iter()
@@ -262,6 +262,7 @@ re!(JS_APIS, r"\b(useState|useReducer|useMemo|useCallback|useEffect|useRef|creat
 re!(JS_FUNCTION, r"^(export\s+)?(default\s+)?(async\s+)?function\b");
 re!(JS_LOCAL, r"^(const|let|var)\b");
 re!(JS_RETURN, r"^return\b");
+re!(OLD_OPTIMISTIC, r"(^|[{;]\s*)optimistic\s+[A-Za-z_$]");
 re!(NAME_BEFORE, r"([A-Za-z_$][A-Za-z0-9_$.]*)\s*$");
 re!(ARROW_BEFORE, r"=>\s*[A-Za-z_$][A-Za-z0-9_$.]*\s*$");
 re!(OP_ASSIGN, r"^(\+\+|--|(\*\*|\|\||&&|\?\?|[-+*/%])=)");
@@ -278,6 +279,9 @@ pub fn hint(src: &str, i: usize, msg: &str) -> Option<(&'static str, String)> {
     let line = src[start..end].trim();
     let before = &src[src[..i].rfind('\n').map(|j| j + 1).unwrap_or(0)..i];
     let name = NAME_BEFORE.captures(before).map(|c| c[1].to_string()).unwrap_or_else(|| "x".into());
+    if OLD_OPTIMISTIC.is_match(line) {
+        return Some(("OPTIMISTIC", "`optimistic` is spelled `guess`: `guess x := e` shows a guess until the action's answer replaces it (AC-3); it lowers to `createOptimistic`".into()));
+    }
     if msg.starts_with("W-2") && rest.starts_with(":=") && ARROW_BEFORE.is_match(before) {
         return Some(("REACTIVE_WRITE_IN_OWNED_SCOPE", format!("an arrow that writes takes a braced body, `() => {{ {} := … }}`: an expression-bodied arrow is a pure function (SC-3)", name)));
     }
@@ -926,6 +930,9 @@ fn parse_stmt(c: &mut Cursor) -> R<Stmt> {
     c.ws(true);
     let start = c.i;
     if c.keyword("optimistic") {
+        return c.fail("AC-3: `optimistic` is not a statement");
+    }
+    if c.keyword("guess") {
         c.ws(true);
         let name = c.ident_or("target")?;
         let path = parse_path(c)?;
diff --git a/eval/README.md b/eval/README.md
index 645cb461..ef04e4a6 100644
--- a/eval/README.md
+++ b/eval/README.md
@@ -86,7 +86,7 @@ predicted every frame right; the briefs here are stricter.
 - `tracker`: `assignToMe` cleared its error with a plain write (`assignError := ""`) inside an action that awaits,
   and a plain write in an async action is held until the action ends (AC-1), so the old error stayed up until the
   server answered. The fix is the `settings` app's own idiom: `assignError` a question source,
-  `optimistic assignError := ""`.
+  `guess assignError := ""`.
 
 ## Layout
 
diff --git a/eval/reference/chain1/rx/comments.rx b/eval/reference/chain1/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/chain1/rx/comments.rx
+++ b/eval/reference/chain1/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/chain1/rx/issues.rx b/eval/reference/chain1/rx/issues.rx
index 73d5be5a..2579778c 100644
--- a/eval/reference/chain1/rx/issues.rx
+++ b/eval/reference/chain1/rx/issues.rx
@@ -21,10 +21,10 @@ export component IssuesPage() {
   source closeError: string = ""
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -33,14 +33,14 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -51,9 +51,9 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/chain1/rx/labels.rx b/eval/reference/chain1/rx/labels.rx
index f1c9c11f..e37c0edc 100644
--- a/eval/reference/chain1/rx/labels.rx
+++ b/eval/reference/chain1/rx/labels.rx
@@ -19,9 +19,9 @@ export component Labels(props: { id: number }) {
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic adding := [...adding, { id: id, label: label }]
+      guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
       refresh labels
     } catch (e) {
@@ -30,9 +30,9 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic removing := [...removing, { id: id, label: label }]
+      guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
       refresh labels
     } catch (e) {
diff --git a/eval/reference/chain1/rx/settings.rx b/eval/reference/chain1/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/chain1/rx/settings.rx
+++ b/eval/reference/chain1/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain2/rx/comments.rx b/eval/reference/chain2/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/chain2/rx/comments.rx
+++ b/eval/reference/chain2/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/chain2/rx/issues.rx b/eval/reference/chain2/rx/issues.rx
index c59ae010..300add69 100644
--- a/eval/reference/chain2/rx/issues.rx
+++ b/eval/reference/chain2/rx/issues.rx
@@ -30,10 +30,10 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
 
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -42,14 +42,14 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -60,9 +60,9 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/chain2/rx/labels.rx b/eval/reference/chain2/rx/labels.rx
index f1c9c11f..e37c0edc 100644
--- a/eval/reference/chain2/rx/labels.rx
+++ b/eval/reference/chain2/rx/labels.rx
@@ -19,9 +19,9 @@ export component Labels(props: { id: number }) {
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic adding := [...adding, { id: id, label: label }]
+      guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
       refresh labels
     } catch (e) {
@@ -30,9 +30,9 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic removing := [...removing, { id: id, label: label }]
+      guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
       refresh labels
     } catch (e) {
diff --git a/eval/reference/chain2/rx/settings.rx b/eval/reference/chain2/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/chain2/rx/settings.rx
+++ b/eval/reference/chain2/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain3/rx/comments.rx b/eval/reference/chain3/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/chain3/rx/comments.rx
+++ b/eval/reference/chain3/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/chain3/rx/issues.rx b/eval/reference/chain3/rx/issues.rx
index 728712ef..90d04f88 100644
--- a/eval/reference/chain3/rx/issues.rx
+++ b/eval/reference/chain3/rx/issues.rx
@@ -31,10 +31,10 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
 
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -43,14 +43,14 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -61,9 +61,9 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/chain3/rx/labels.rx b/eval/reference/chain3/rx/labels.rx
index f1c9c11f..e37c0edc 100644
--- a/eval/reference/chain3/rx/labels.rx
+++ b/eval/reference/chain3/rx/labels.rx
@@ -19,9 +19,9 @@ export component Labels(props: { id: number }) {
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic adding := [...adding, { id: id, label: label }]
+      guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
       refresh labels
     } catch (e) {
@@ -30,9 +30,9 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic removing := [...removing, { id: id, label: label }]
+      guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
       refresh labels
     } catch (e) {
diff --git a/eval/reference/chain3/rx/settings.rx b/eval/reference/chain3/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/chain3/rx/settings.rx
+++ b/eval/reference/chain3/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain4/rx/comments.rx b/eval/reference/chain4/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/chain4/rx/comments.rx
+++ b/eval/reference/chain4/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/chain4/rx/issues.rx b/eval/reference/chain4/rx/issues.rx
index 51d7dc72..488ee123 100644
--- a/eval/reference/chain4/rx/issues.rx
+++ b/eval/reference/chain4/rx/issues.rx
@@ -34,13 +34,13 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
 
   action assignTo(id: number, who: string, fromRow: boolean) {
     if (fromRow) {
-      optimistic rowError := ""
+      guess rowError := ""
     } else {
-      optimistic assignError := ""
+      guess assignError := ""
     }
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -53,14 +53,14 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -72,12 +72,12 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
 
   action closeOne(id: number, fromRow: boolean) {
     if (fromRow) {
-      optimistic rowError := ""
+      guess rowError := ""
     } else {
-      optimistic closeError := ""
+      guess closeError := ""
     }
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/chain4/rx/labels.rx b/eval/reference/chain4/rx/labels.rx
index f1c9c11f..e37c0edc 100644
--- a/eval/reference/chain4/rx/labels.rx
+++ b/eval/reference/chain4/rx/labels.rx
@@ -19,9 +19,9 @@ export component Labels(props: { id: number }) {
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic adding := [...adding, { id: id, label: label }]
+      guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
       refresh labels
     } catch (e) {
@@ -30,9 +30,9 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic removing := [...removing, { id: id, label: label }]
+      guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
       refresh labels
     } catch (e) {
diff --git a/eval/reference/chain4/rx/settings.rx b/eval/reference/chain4/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/chain4/rx/settings.rx
+++ b/eval/reference/chain4/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/close/rx/main.rx b/eval/reference/close/rx/main.rx
index 7768b2cc..1f04fbb0 100644
--- a/eval/reference/close/rx/main.rx
+++ b/eval/reference/close/rx/main.rx
@@ -14,9 +14,9 @@ export default component App() {
   source assignError: string = ""
   source closeError: string = ""
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := keep(issues, t => t.id != id)
+      guess issues := keep(issues, t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
@@ -25,10 +25,10 @@ export default component App() {
   }
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = indexOf(issues, t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/comments/rx/main.rx b/eval/reference/comments/rx/main.rx
index a48b9b88..85d7afb3 100644
--- a/eval/reference/comments/rx/main.rx
+++ b/eval/reference/comments/rx/main.rx
@@ -17,9 +17,9 @@ export default component App() {
   source comments = selected == null ? [] : await fetchComments(selected)
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := ""
+    guess postError := ""
     try {
-      optimistic comments := [...comments, { author: author, text: `${text} (sending…)` }]
+      guess comments := [...comments, { author: author, text: `${text} (sending…)` }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
@@ -29,10 +29,10 @@ export default component App() {
 
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = indexOf(issues, t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/count/rx/comments.rx b/eval/reference/count/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/count/rx/comments.rx
+++ b/eval/reference/count/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/count/rx/issues.rx b/eval/reference/count/rx/issues.rx
index 4e32d57a..5825e67e 100644
--- a/eval/reference/count/rx/issues.rx
+++ b/eval/reference/count/rx/issues.rx
@@ -29,10 +29,10 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
 
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -41,14 +41,14 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -59,9 +59,9 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/count/rx/settings.rx b/eval/reference/count/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/count/rx/settings.rx
+++ b/eval/reference/count/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/desk/rx/comments.rx b/eval/reference/desk/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/desk/rx/comments.rx
+++ b/eval/reference/desk/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/desk/rx/issues.rx b/eval/reference/desk/rx/issues.rx
index 35ab63fe..36fc5323 100644
--- a/eval/reference/desk/rx/issues.rx
+++ b/eval/reference/desk/rx/issues.rx
@@ -20,10 +20,10 @@ export component IssuesPage() {
   source closeError: string = ""
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -32,14 +32,14 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -50,9 +50,9 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/desk/rx/settings.rx b/eval/reference/desk/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/desk/rx/settings.rx
+++ b/eval/reference/desk/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/labels/rx/comments.rx b/eval/reference/labels/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/labels/rx/comments.rx
+++ b/eval/reference/labels/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/labels/rx/issues.rx b/eval/reference/labels/rx/issues.rx
index 73d5be5a..2579778c 100644
--- a/eval/reference/labels/rx/issues.rx
+++ b/eval/reference/labels/rx/issues.rx
@@ -21,10 +21,10 @@ export component IssuesPage() {
   source closeError: string = ""
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -33,14 +33,14 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -51,9 +51,9 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/labels/rx/labels.rx b/eval/reference/labels/rx/labels.rx
index f1c9c11f..e37c0edc 100644
--- a/eval/reference/labels/rx/labels.rx
+++ b/eval/reference/labels/rx/labels.rx
@@ -19,9 +19,9 @@ export component Labels(props: { id: number }) {
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic adding := [...adding, { id: id, label: label }]
+      guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
       refresh labels
     } catch (e) {
@@ -30,9 +30,9 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    optimistic labelError := ""
+    guess labelError := ""
     try {
-      optimistic removing := [...removing, { id: id, label: label }]
+      guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
       refresh labels
     } catch (e) {
diff --git a/eval/reference/labels/rx/settings.rx b/eval/reference/labels/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/labels/rx/settings.rx
+++ b/eval/reference/labels/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/related/rx/comments.rx b/eval/reference/related/rx/comments.rx
index 3e9b7b2b..5476dd55 100644
--- a/eval/reference/related/rx/comments.rx
+++ b/eval/reference/related/rx/comments.rx
@@ -15,9 +15,9 @@ export component Comments(props: { id: number; user: string }) {
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    optimistic postError := null
+    guess postError := null
     try {
-      optimistic sending := [...sending, { id: id, author: author, text: text }]
+      guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
       refresh comments
     } catch (e) {
diff --git a/eval/reference/related/rx/issues.rx b/eval/reference/related/rx/issues.rx
index 7cd65f6f..878833c3 100644
--- a/eval/reference/related/rx/issues.rx
+++ b/eval/reference/related/rx/issues.rx
@@ -21,10 +21,10 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   source closeError: string = ""
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
@@ -33,14 +33,14 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   }
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -51,9 +51,9 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   }
 
   action closeOne(id: number) {
-    optimistic closeError := ""
+    guess closeError := ""
     try {
-      optimistic issues := issues.filter(t => t.id != id)
+      guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/related/rx/settings.rx b/eval/reference/related/rx/settings.rx
index 0df82318..6c45198c 100644
--- a/eval/reference/related/rx/settings.rx
+++ b/eval/reference/related/rx/settings.rx
@@ -16,8 +16,8 @@ export component SettingsPage() {
   source saveError: string = ""
 
   action save(next: Settings) {
-    optimistic settings := next
-    optimistic saveError := ""
+    guess settings := next
+    guess saveError := ""
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/rename/rx/main.rx b/eval/reference/rename/rx/main.rx
index a3f53de2..8bf3450f 100644
--- a/eval/reference/rename/rx/main.rx
+++ b/eval/reference/rename/rx/main.rx
@@ -17,14 +17,14 @@ export default component App() {
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
 
   action renameTo(id: number, title: string) {
-    optimistic renameError := ""
+    guess renameError := ""
     try {
       let i = indexOf(issues, t => t.id == id)
       if (i >= 0) {
-        optimistic issues[i].title := title
+        guess issues[i].title := title
       }
       if (detail != null && detail.id == id) {
-        optimistic detail.title := title
+        guess detail.title := title
       }
       await renameIssue(id, title)
       refresh issues
@@ -35,10 +35,10 @@ export default component App() {
   }
 
   action assignTo(id: number, who: string) {
-    optimistic assignError := ""
+    guess assignError := ""
     try {
       let i = indexOf(issues, t => t.id == id)
-      optimistic issues[i].assignee := who
+      guess issues[i].assignee := who
       await assign(id, who)
       refresh issues
     } catch (e) {
diff --git a/eval/reference/settings/rx/settings.rx b/eval/reference/settings/rx/settings.rx
index 6a834e8c..f57f9cab 100644
--- a/eval/reference/settings/rx/settings.rx
+++ b/eval/reference/settings/rx/settings.rx
@@ -29,8 +29,8 @@ export action retryLoad() {
 }
 
 export action save(next: Settings) {
-  optimistic settings := next     // the page shows what is being saved, now (H-11)
-  optimistic saveError := ""      // and stops showing the previous failure
+  guess settings := next     // the page shows what is being saved, now (H-11)
+  guess saveError := ""      // and stops showing the previous failure
   affects settings                // `pending? settings` is true until the save ends (AC-5)
   try {
     let saved = await saveSettings(next)
diff --git a/eval/reference/tracker/rx/issues.rx b/eval/reference/tracker/rx/issues.rx
index 298bb2a2..61cb3b84 100644
--- a/eval/reference/tracker/rx/issues.rx
+++ b/eval/reference/tracker/rx/issues.rx
@@ -42,8 +42,8 @@ export action select(id: number) {
 }
 
 export action assignToMe(id: number, user: string) {
-  optimistic assignError := ""
-  optimistic issues := list => withAssignee(list, id, user)
+  guess assignError := ""
+  guess issues := list => withAssignee(list, id, user)
   try {
     await assign(id, user)
     issues := list => withAssignee(list, id, user)
diff --git a/examples/ai/feed.rx b/examples/ai/feed.rx
index be7e5eb3..5079941d 100644
--- a/examples/ai/feed.rx
+++ b/examples/ai/feed.rx
@@ -22,7 +22,7 @@ source messages: Message[] = await* streamMessages(selected) -> []   // ST-5 + S
 msgs key id = messages                             // ST-4: each message keyed by id
 
 source draftText = ""                              // ST-5a: a constant question — provisional UI state a guess can clear
-source sending = false                             // same shape: `optimistic sending := true` reverts when the hold ends
+source sending = false                             // same shape: `guess sending := true` reverts when the hold ends
 source error := ""
 
 effect (len(msgs)) { log("count", $0) }            // EF: runs when the committed count changes
@@ -33,9 +33,9 @@ action send() {
     let text = draftText                           // snapshots (SC-2)
     let room = selected
     let msg = { id: len(messages) + 1, from: "me", text }
-    optimistic draftText := ""                     // AC-3: the guesses show now, display-ahead
-    optimistic sending := true
-    optimistic messages := m => [...m, msg]        // W-6: a functional guess appends
+    guess draftText := ""                     // AC-3: the guesses show now, display-ahead
+    guess sending := true
+    guess messages := m => [...m, msg]        // W-6: a functional guess appends
     await sendMessage(room, text)                  // AC-1: a hold-safe suspension
     draftText := ""                                // the truth, after the await (07 §5)
   } catch (e) {
diff --git a/examples/ai2/settings/NOTES.md b/examples/ai2/settings/NOTES.md
index a18ce4ae..e13e6ac1 100644
--- a/examples/ai2/settings/NOTES.md
+++ b/examples/ai2/settings/NOTES.md
@@ -34,7 +34,7 @@ Modules: `theme.rx` (contexts only) ← `settings.rx` (server truth + actions),
   frames would read `compactProfile [compact]Display name…`. A real page would
   not have these.
 - **`saveError` needs both a guess and a plain write on success.** I wrote
-  `optimistic saveError := ""` so a new save clears the old message at once.
+  `guess saveError := ""` so a new save clears the old message at once.
   But the source's truth is the last *plain* write (`"quota exceeded"`), so
   without `saveError := ""` after a successful `await`, the guess would revert
   and the old error would come back. It is correct per H-11, and easy to get
@@ -85,7 +85,7 @@ Modules: `theme.rx` (contexts only) ← `settings.rx` (server truth + actions),
    a value replaces the error". I assumed that happens automatically when the
    refreshed flight lands, even though the content readers were behind the
    fallback (S-solid-42 is cited for this).
-5. **A guess closing an `if` branch** (`optimistic saveError := ""` hiding
+5. **A guess closing an `if` branch** (`guess saveError := ""` hiding
    "Save failed" at once). H-7 says "a branch a lane condition opens is a
    mainline mount and attaches now". It does not explicitly say the same for a
    branch a lane condition *closes*. I assumed it is symmetric.
diff --git a/examples/ai2/settings/settings.rx b/examples/ai2/settings/settings.rx
index 6a834e8c..f57f9cab 100644
--- a/examples/ai2/settings/settings.rx
+++ b/examples/ai2/settings/settings.rx
@@ -29,8 +29,8 @@ export action retryLoad() {
 }
 
 export action save(next: Settings) {
-  optimistic settings := next     // the page shows what is being saved, now (H-11)
-  optimistic saveError := ""      // and stops showing the previous failure
+  guess settings := next     // the page shows what is being saved, now (H-11)
+  guess saveError := ""      // and stops showing the previous failure
   affects settings                // `pending? settings` is true until the save ends (AC-5)
   try {
     let saved = await saveSettings(next)
diff --git a/examples/ai2/tracker/NOTES.md b/examples/ai2/tracker/NOTES.md
index ab99f09b..5b1747b6 100644
--- a/examples/ai2/tracker/NOTES.md
+++ b/examples/ai2/tracker/NOTES.md
@@ -40,7 +40,7 @@ the list, and actions as the only writers.
   read (see Ambiguities 1). I'd want either a sync action to be stated to be
   an ordinary instant, or a lighter exported-setter form.
 - **The truth after a successful assign repeats the guess.**
-  `optimistic issues := f` followed later by `issues := f` with the same `f`.
+  `guess issues := f` followed later by `issues := f` with the same `f`.
   H-11 makes this necessary: without the plain write, the guess is "corrected
   by the truth at hand" when the body ends, which reverts a successful
   assignment. The ideal is `refresh issues` (the server is the truth), but
diff --git a/examples/ai2/tracker/issues.rx b/examples/ai2/tracker/issues.rx
index f0cdbd7d..30e051b8 100644
--- a/examples/ai2/tracker/issues.rx
+++ b/examples/ai2/tracker/issues.rx
@@ -41,7 +41,7 @@ export action select(id: number) {
 
 export action assignToMe(id: number, user: string) {
   assignError := ""
-  optimistic issues := list => withAssignee(list, id, user)
+  guess issues := list => withAssignee(list, id, user)
   try {
     await assign(id, user)
     issues := list => withAssignee(list, id, user)
diff --git a/examples/cart.rx b/examples/cart.rx
index 2346c366..daf070bb 100644
--- a/examples/cart.rx
+++ b/examples/cart.rx
@@ -33,7 +33,7 @@ source orderError: string := ""
 action placeOrder() {
   if (count === 0) return
   let items = cart                                               // SC-2: a snapshot of the lines being ordered
-  optimistic cart := []                                          // AC-3: the cart empties now, display-ahead
+  guess cart := []                                          // AC-3: the cart empties now, display-ahead
   affects cart                                                   // AC-5: `pending? cart` while the order is in flight
   orderError := ""
   try {
diff --git a/examples/notes/data.rx b/examples/notes/data.rx
index cade54c0..bc8ebc31 100644
--- a/examples/notes/data.rx
+++ b/examples/notes/data.rx
@@ -34,7 +34,7 @@ export action open(id: number) { route := { page: "note", id: id }; saveError :=
 export action back() { route := { page: "list" } }
 export action save(id: number, title: string, body: string) {
   saveError := ""
-  optimistic notes := ns => ns.map(n => n.id == id ? { ...n, title: title, body: body } : n)
+  guess notes := ns => ns.map(n => n.id == id ? { ...n, title: title, body: body } : n)
   try {
     try {
       let saved = await saveNote(id, title, body)
diff --git a/examples/shop/cart.rx b/examples/shop/cart.rx
index 57d5a988..d2b0d3d0 100644
--- a/examples/shop/cart.rx
+++ b/examples/shop/cart.rx
@@ -25,7 +25,7 @@ export action setQty(id: number, n: number) {
 export action placeOrder() {
   if (count === 0) return
   let items = cart                                 // SC-2: a snapshot of the lines being ordered
-  optimistic cart := []                            // AC-3: the cart empties now, display-ahead
+  guess cart := []                            // AC-3: the cart empties now, display-ahead
   affects cart                                     // AC-5: `pending? cart` while the order is in flight
   orderError := ""
   try {
diff --git a/examples/todos.rx b/examples/todos.rx
index e9d52c8b..856af618 100644
--- a/examples/todos.rx
+++ b/examples/todos.rx
@@ -1,7 +1,7 @@
 // TodoMVC against a server, built in layers (solid next's examples/todos). Each layer has its own lifetime and is
 // derived from the one below it; the screen reads only the top one.
 //
-//   3. guesses   `optimistic` writes on `todos` rows and on `adding`: shown now, gone when their action ends (AC-3)
+//   3. guesses   `guess` writes on `todos` rows and on `adding`: shown now, gone when their action ends (AC-3)
 //   2. client    `todos`: what the client owns over (1), here the failures awaiting a retry; the client writes it and
 //                a new answer from (1) re-derives it (ST-5a). The failures themselves outlive that, so they are a
 //                plain source `todos` is derived from
@@ -43,7 +43,7 @@ anyDone = live.some(t => t.done)
 // guess. A new row has no index yet, so an add guesses `adding`, a list of its own.
 action add(id: string, title: string) {
   try {
-    optimistic adding := as => [...as, { id: id, title: title, done: false, gone: false, saving: true, failure: null }]
+    guess adding := as => [...as, { id: id, title: title, done: false, gone: false, saving: true, failure: null }]
     await addTodo(id, title)
     failures := fs => without(fs, id)
   } catch (e) {
@@ -54,8 +54,8 @@ action add(id: string, title: string) {
 action toggle(id: string, done: boolean) {
   try {
     let i = todos.findIndex(t => t.id === id)
-    optimistic todos[i].done := done
-    optimistic todos[i].saving := true
+    guess todos[i].done := done
+    guess todos[i].saving := true
     await toggleTodo(id, done)
     failures := fs => without(fs, id)
   } catch (e) {
@@ -66,7 +66,7 @@ action toggle(id: string, done: boolean) {
 action remove(id: string) {
   try {
     let i = todos.findIndex(t => t.id === id)
-    optimistic todos[i].gone := true
+    guess todos[i].gone := true
     await removeTodo(id)
     failures := fs => without(fs, id)
   } catch (e) {
@@ -77,7 +77,7 @@ action remove(id: string) {
 // A bulk write that fails fans out to one failure per row, so each row gets its own retry.
 action clearDone(ids: string[]) {
   try {
-    optimistic todos := ts => ts.map(t => t.done ? { ...t, gone: true } : t)
+    guess todos := ts => ts.map(t => t.done ? { ...t, gone: true } : t)
     await removeTodos(ids)
     failures := fs => fs.filter(f => !ids.includes(f.id))
   } catch (e) {
diff --git a/examples/users.rx b/examples/users.rx
index 35a594b0..2ac1cbd9 100644
--- a/examples/users.rx
+++ b/examples/users.rx
@@ -16,8 +16,8 @@ total = len(users)
 status = pending? users ? "loading…" : `${total} users`
 
 action rename(id: number, name: string) {
-  optimistic users := us => us.map(u => u.id === id ? { ...u, name } : u)   // AC-3: shown now, verdict-inert
-  optimistic editing := null                                                 // the editor closes now; the plain write below lands at the commit
+  guess users := us => us.map(u => u.id === id ? { ...u, name } : u)   // AC-3: shown now, verdict-inert
+  guess editing := null                                                 // the editor closes now; the plain write below lands at the commit
   editing := null
   saveError := ""
   try {
diff --git a/guide/async.md b/guide/async.md
index 03fe5f35..08f981e0 100644
--- a/guide/async.md
+++ b/guide/async.md
@@ -64,8 +64,8 @@ export source profile: Profile = await loadProfile()   // a source with a questi
 export source saveError: string = ""
 
 export action save(next: Profile) {
-  optimistic profile := next        // shows now; reverts by itself if the save fails
-  optimistic saveError := ""        // the old error goes now
+  guess profile := next        // shows now; reverts by itself if the save fails
+  guess saveError := ""        // the old error goes now
   affects profile                   // `pending? profile` is true until the save ends
   try {
     let answer = await saveProfile(next)
@@ -76,7 +76,7 @@ export action save(next: Profile) {
 }
 ```
 
-A handler can be the action itself, with no keyword: an arrow that writes `optimistic`, declares `affects`, or both
+A handler can be the action itself, with no keyword: an arrow that writes `guess`, declares `affects`, or both
 writes and awaits is an action (AC-1b). Declare a named `action` when you call it from more than one place.
 
 An action's body is **one change**: every plain write in it, before or after an `await`, is held until the body
@@ -85,7 +85,7 @@ by what the screen should do:
 
 | You want | Write |
 | --- | --- |
-| Show a value now, and take it back if the action fails | `optimistic x := v` (x is a source with a question: `source x = …`) |
+| Show a value now, and take it back if the action fails | `guess x := v` (x is a source with a question: `source x = …`) |
 | `pending? x` true while the action runs | `affects x` |
 | Land a UI write now and keep it (close a menu, clear a message) | `onFlush(() => { x := v })` (EF-8) |
 | Record the server's answer | `x := answer` after the `await` |
@@ -101,7 +101,7 @@ export source todos: Todo[] key id = await loadTodos()
 export action markDone(id: number) {
   try {
     let i = todos.findIndex(t => t.id == id)  // -1 when absent
-    optimistic todos[i].done := true          // this row only
+    guess todos[i].done := true          // this row only
     await saveDone(id)
     refresh todos                             // the server's list confirms the guess
   } catch (e) { … }
@@ -121,12 +121,12 @@ flight, everything shown from `todos` waits for that action to end, including an
 The screen is never wrong in between: it shows the guesses, then the truth, in one frame. This is the design (H-11),
 not a bug, and moving the guess to a source of its own doesn't change it: the rows that show it read `todos`.
 
-A guess on a whole list (`optimistic adding := as => [...as, x]`) is one change with every other guess on that list:
+A guess on a whole list (`guess adding := as => [...as, x]`) is one change with every other guess on that list:
 two adds in flight end together, so a declined one stays until the other ends. That's the same rule, not something
 to work around.
 
 - **Show that something is in flight**, so the longer wait isn't silent: a row flag set by the guess
-  (`optimistic todos[i].saving := true`), or `pending? todos` with `affects todos`.
+  (`guess todos[i].saving := true`), or `pending? todos` with `affects todos`.
 - **Don't rebuild guesses by hand** (a plain list of pending ids with your own revert) to end them one by one: it
   loses the revert and the hold that keep the screen consistent.
 
@@ -144,7 +144,7 @@ shownAdding = adding.filter(a => a.photo == photo)   // shown after `tags`
 
 export action addTag(p: number, tag: string) {
   try {
-    optimistic adding := [...adding, { photo: p, tag: tag }]
+    guess adding := [...adding, { photo: p, tag: tag }]
     await saveTag(p, tag)
     refresh tags                              // the server's list now has the tag; the guess goes with the action
   } catch (e) { … }
@@ -165,7 +165,7 @@ and the screen reads only the top:
    (ST-5a). The client writes it (an edit, a draft, a flag, a failure to show), and a new answer from below resets
    it. Anything that must outlive that reset (in the todos app, the failures awaiting a retry) is a plain source
    this layer is derived from.
-3. **Guesses.** `optimistic` writes on layer 2, made by an action and gone when the action ends.
+3. **Guesses.** `guess` writes on layer 2, made by an action and gone when the action ends.
 
 `examples/todos.rx` is the whole app:
 
@@ -177,7 +177,7 @@ source adding: Row[] key id = []                            // 3. adds in flight
 
 export action add(id: string, title: string) {
   try {
-    optimistic adding := as => [...as, { id: id, title: title, saving: true }]
+    guess adding := as => [...as, { id: id, title: title, saving: true }]
     await addTodo(id, title)
     failures := fs => without(fs, id)
   } catch (e) {
@@ -198,7 +198,7 @@ export action add(id: string, title: string) {
 
 ## 4. Things that look like bugs and aren't
 
-- **A plain write in an action doesn't show until the server answers.** That's the hold. Use `optimistic` or
+- **A plain write in an action doesn't show until the server answers.** That's the hold. Use `guess` or
   `onFlush` (the table above). Don't move the code into a plain handler to escape it.
 - **A declined row guess stays until another action's re-ask lands.** While an action's `refresh` of the list is
   in flight, the list's rows wait for that action (H-11), declined guesses included. That's by design; see "When
@@ -210,7 +210,7 @@ export action add(id: string, title: string) {
   `loading on(k)` above the `errored`.
 - **The server accepted, and the screen went back to the old value.** The action guessed on one question and
   refreshed another, or none. A guess ends with its action; only a new answer for the question it guessed on shows
-  what the server took. Refresh every question you guessed on (`optimistic detail.title := t` needs `refresh detail`,
+  what the server took. Refresh every question you guessed on (`guess detail.title := t` needs `refresh detail`,
   not only `refresh issues`).
 - **A write landed and nothing showed it until the reload answered.** A guess needs a value to guess on. A rename sent
   while the detail is still loading, or for a row the list does not show yet (`findIndex` gave -1), guesses nothing,
@@ -220,7 +220,7 @@ export action add(id: string, title: string) {
 ## 5. Plain handlers
 
 A handler that writes and awaits is an action (AC-1b), so it never lands a write early by accident. A write before
-its `await` is a compile error (`HELD_WRITE`), because it would wait for the server: make it `optimistic` if it shows
+its `await` is a compile error (`HELD_WRITE`), because it would wait for the server: make it `guess` if it shows
 while the work runs, `onFlush` if it is a reset, or declare the action if it should wait. Plain handlers are for
 local state: typing, toggles, selection.
 
@@ -239,7 +239,7 @@ a value; an effect is for sending a value somewhere else (a parent's `on…` han
   the block does not run until the answer lands (R-2). The effect doesn't hold the screen while it waits.
 - **Failing inputs**: give an input that can fail a `catch` (`effect (x catch (e) => fallback) { … }`), so the block
   gets a plain value it can test for.
-- **Guesses**: an input computed from an `optimistic` write fires as soon as the guess shows, and again with the old
+- **Guesses**: an input computed from a `guess` write fires as soon as the guess shows, and again with the old
   value if the guess reverts (EF-2). It doesn't wait for the action to end.
 - **Lifetime**: an effect lives as long as its component. When the component goes away the effect stops, and its
   cleanup (a function the block returns) runs; nothing it would have seen later reaches it. State that must outlive
diff --git a/guide/diagnostics.md b/guide/diagnostics.md
index 1e0b26d3..3b8177c5 100644
--- a/guide/diagnostics.md
+++ b/guide/diagnostics.md
@@ -97,6 +97,33 @@ export default component App() {
 }
 ```
 
+### OPTIMISTIC
+
+A guess is written `guess x := e` (AC-3). The keyword was `optimistic` before 0.2; it lowers to the same
+`createOptimistic`, and shows until the action's answer replaces it.
+
+```rx before
+export default component App() {
+  source liked = false
+  action like() {
+    optimistic liked := true
+    refresh liked
+  }
+  <button onClick={() => { like() }}>♥</button>
+}
+```
+
+```rx after
+export default component App() {
+  source liked = false
+  action like() {
+    guess liked := true
+    refresh liked
+  }
+  <button onClick={() => { like() }}>♥</button>
+}
+```
+
 ### ASSIGNMENT
 
 A source is written with `:=` as a statement, in a handler or an action (W-1). There is no `=`, `+=` or `++` on state.
@@ -214,7 +241,7 @@ export default component App() {
 
 A write after an `await` that is computed from a read before it overwrites whatever changed the source in between
 (another action, a reload). Write functionally, so the write applies to the value as it stands (W-6), or guess it with
-`optimistic` and let the answer replace the guess (AC-3).
+`guess` and let the answer replace the guess (AC-3).
 
 ```rx before
 export default component App() {
@@ -379,7 +406,7 @@ export default component App() {
 ### HELD_WRITE
 
 A handler that writes and awaits is an action (AC-1b), and an action's writes land when its body ends (AC-1). So a
-write before the `await` does not show while the work runs. Say which you meant: `optimistic x := …` for state that
+write before the `await` does not show while the work runs. Say which you meant: `guess x := …` for state that
 shows while the work runs (on a source with a question, ST-5c); `onFlush(() => { x := … })` for a reset that lands now
 (EF-8); a declared `action` to hold it on purpose; or write it after the `await`. `async.md` ("What lands together, and
 what waits") is the long form.
@@ -433,7 +460,7 @@ export default component App() {
 }
 ```
 
-### OPTIMISTIC_OUTSIDE_HOLD
+### GUESS_OUTSIDE_HOLD
 
 A guess lives in an action's hold until the server answers it (AC-3). An effect's apply block is never an action, so
 declare one and call it from the block.
@@ -442,7 +469,7 @@ declare one and call it from the block.
 export default component App() {
   source draft := "a"
   source saved = "a"
-  effect (draft) { optimistic saved := $0 }
+  effect (draft) { guess saved := $0 }
   <p>{saved}</p>
 }
 ```
@@ -452,7 +479,7 @@ export default component App() {
   source draft := "a"
   source saved = "a"
   action save(v: string) {
-    optimistic saved := v
+    guess saved := v
     await fetch("save:" + v) catch (e) => null
   }
   effect (draft) { save($0) }
@@ -460,15 +487,15 @@ export default component App() {
 }
 ```
 
-### OPTIMISTIC_NO_QUESTION
+### GUESS_NO_QUESTION
 
-A guess is confirmed or replaced by the answer to a question, so `optimistic` writes a source declared with `=` (a
+A guess is confirmed or replaced by the answer to a question, so `guess` writes a source declared with `=` (a
 source with a question, ST-5c). A source seeded with `:=` has no question; write it plainly.
 
 ```rx before
 export default component App() {
   source shown := "Ann"
-  action fix() { optimistic shown := "Zed"; await fetch("x") catch (e) => null }
+  action fix() { guess shown := "Zed"; await fetch("x") catch (e) => null }
   <button onClick={() => { fix() }}>{shown}</button>
 }
 ```
@@ -476,7 +503,7 @@ export default component App() {
 ```rx after
 export default component App() {
   source shown = await fetch("name") catch (e) => "Ann"
-  action fix() { optimistic shown := "Zed"; await fetch("x") catch (e) => null }
+  action fix() { guess shown := "Zed"; await fetch("x") catch (e) => null }
   <button onClick={() => { fix() }}>{shown}</button>
 }
 ```
diff --git a/guide/reference.md b/guide/reference.md
index a36c9355..5a0c36f9 100644
--- a/guide/reference.md
+++ b/guide/reference.md
@@ -25,7 +25,7 @@ Control   ::= "if" Expr "{" … "}" ("else" "{" … "}")?               -- no `e
             | "for" Name "in" Expr "{" … "}"
             | "loading" ("on" "(" Expr ")")? "fallback" "{" … "}" "{" … "}"
             | "errored" "fallback" ("(" Name (":" Classes)? ")")? "{" … "}" "{" … "}"
-Stmt      ::= Target ":=" Expr | "optimistic" Target ":=" Expr | "refresh" Name | "affects" Name
+Stmt      ::= Target ":=" Expr | "guess" Target ":=" Expr | "refresh" Name | "affects" Name
             | ("let" | "const") Name "=" Expr | "await" Expr | ("let" | "const") Name "=" "await" Expr
             | "if" "(" Expr ")" … ("else" …)? | "try" … "catch" … "finally" … | "throw" Expr | "return" Expr? | Expr
                                                              -- imperative `else if` works; markup `{if}` has no `else if`
@@ -58,7 +58,7 @@ Expr      ::= TypeScript's, without assignment, `this`, `function`, `new` on rea
 - A failure inside the body must be caught there: `try { … } catch (e) { … }` (R-f-6, AC-7). The catch runs inside the
   same change: its writes land with the body's end. A rejected body commits its plain writes and reverts its guesses
   (AC-1, DV-2).
-- `optimistic x := e`: a guess on a source with a question (`source x = …`), shown now, kept until the action's change
+- `guess x := e`: a guess on a source with a question (`source x = …`), shown now, kept until the action's change
   commits or `x`'s own answer differs (AC-3, H-11). Guesses on one source hold together.
 - `refresh x`: asks `x` again, quietly (no `pending?`; the old value stays up until the new one lands). Inside an
   action, the answer lands with the action's commit (AC-4).
diff --git a/ref/compile/rt-web.ts b/ref/compile/rt-web.ts
index 849d001d..3b26a40b 100644
--- a/ref/compile/rt-web.ts
+++ b/ref/compile/rt-web.ts
@@ -162,12 +162,12 @@ export function createApp(opts: { root: any; api?: Record<string, (...a: any[])
         const w: Writer = {
           set: v => { if (typeof v === "function") { setStore(v as any); set(store); } else if (typeof v === "object" && v !== null) { setStore(() => v as any); set(store); } else set(v); },
           path: (keys, v) => setStore((d: any) => { let o = d; for (const k of keys.slice(0, -1)) o = o[k]; o[keys[keys.length - 1]] = v; }),
-          guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); },
+          guess: () => { throw new Error("GUESS_NO_QUESTION"); },
         };
         return [get, w];
       }
       const [get, set] = createSignal(init);
-      return [get, { set: set as (v: unknown) => void, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); } }];
+      return [get, { set: set as (v: unknown) => void, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("GUESS_NO_QUESTION"); } }];
     },
     /** `source x = e` (ST-5): createSignal(fn); with a guess written to it, a createOptimistic layer over that truth (AC-3);
      *  written or probed by path, createStore(fn, {}) whose derive writes its draft. */
@@ -191,7 +191,7 @@ export function createApp(opts: { root: any; api?: Record<string, (...a: any[])
         return [get, { set: () => { throw new Error("a root write to a derived store is not lowered yet"); }, path: (keys, v) => setStore((d: any) => { let o = d; for (const k of keys.slice(0, -1)) o = o[k]; o[keys[keys.length - 1]] = v; }), guess: () => { throw new Error("optimistic over a structured question source is not lowered yet"); } }];
       }
       const [get, set] = createSignal(fn as any, opts as any);
-      return [get, { set: set as any, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); } }];
+      return [get, { set: set as any, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("GUESS_NO_QUESTION"); } }];
     },
     memo: (name: string | null, fn: (prev: unknown) => unknown, opts?: { loadingValue?: unknown }): Acc => createMemo(named(name, fn) as any, opts as any) as Acc,
     deferred: (name: string, fn: (prev: unknown) => unknown, opts?: { loadingValue?: unknown }): Acc => createDeferred(named(name, fn) as any, opts as any) as Acc,
diff --git a/ref/compile/rt.ts b/ref/compile/rt.ts
index ad40b5f5..342753ac 100644
--- a/ref/compile/rt.ts
+++ b/ref/compile/rt.ts
@@ -172,12 +172,12 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
         const w: Writer = {
           set: v => { if (typeof v === "function") { setStore(v as any); set(store); } else if (typeof v === "object" && v !== null) { setStore(() => v as any); set(store); } else set(v); },
           path: (keys, v) => setStore((d: any) => { let o = d; for (const k of keys.slice(0, -1)) o = o[k]; o[keys[keys.length - 1]] = v; }),
-          guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); },
+          guess: () => { throw new Error("GUESS_NO_QUESTION"); },
         };
         return [get, w];
       }
       const [get, set] = createSignal(init);
-      return [get, { set: set as (v: unknown) => void, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); } }];
+      return [get, { set: set as (v: unknown) => void, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("GUESS_NO_QUESTION"); } }];
     },
     /** `source x = e` (ST-5): createSignal(fn); with a guess written to it, a createOptimistic layer over that truth (AC-3);
      *  written or probed by path, createStore(fn, {}) whose derive writes its draft. */
@@ -201,7 +201,7 @@ export function createApp(opts: { api?: Record<string, (...a: any[]) => unknown>
         return [get, { set: () => { throw new Error("a root write to a derived store is not lowered yet"); }, path: (keys, v) => setStore((d: any) => { let o = d; for (const k of keys.slice(0, -1)) o = o[k]; o[keys[keys.length - 1]] = v; }), guess: () => { throw new Error("optimistic over a structured question source is not lowered yet"); } }];
       }
       const [get, set] = createSignal(fn as any, opts as any);
-      return [get, { set: set as any, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("OPTIMISTIC_NO_QUESTION"); } }];
+      return [get, { set: set as any, path: () => { throw new Error("a path write to an atomic source"); }, guess: () => { throw new Error("GUESS_NO_QUESTION"); } }];
     },
     memo: (name: string | null, fn: (prev: unknown) => unknown, opts?: { loadingValue?: unknown }): Acc => createMemo(named(name, fn) as any, opts as any) as Acc,
     deferred: (name: string, fn: (prev: unknown) => unknown, opts?: { loadingValue?: unknown }): Acc => createDeferred(named(name, fn) as any, opts as any) as Acc,
diff --git a/ref/oracle/README.md b/ref/oracle/README.md
index cd271f1b..7a8f9ed8 100644
--- a/ref/oracle/README.md
+++ b/ref/oracle/README.md
@@ -8,7 +8,7 @@ an interpreter (`lower.ts`):
 | form | lowering |
 | --- | --- |
 | `source x := e` | `createSignal`; structured (object/array) → `createStore` behind a signal slot (ST-1, ST-2a) |
-| `source x = e` | `createSignal(fn)`; `createOptimistic(fn)` when an `optimistic x :=` exists; `createStore(fn, {})` when written or probed by path (ST-5) |
+| `source x = e` | `createSignal(fn)`; `createOptimistic(fn)` when an `guess x :=` exists; `createStore(fn, {})` when written or probed by path (ST-5) |
 | `x = e`, `await e [-> v]` | `createMemo(fn, { loadingValue })`; the memo returns the flight (a promise/async iterable) |
 | `deferred x = e` | `createDeferred` |
 | `x = draft(pre x -> seed) { … }` | `createProjection((draft) => …, seed)` (ST-3) |
@@ -18,8 +18,8 @@ an interpreter (`lower.ts`):
 | `peek`, `latest`, `pending?` | `untrack`, `latest(fn)`, `isPending(fn)` |
 | `{if}` `{for}` `{loading}` `{errored}` | `Show`, `For`, `Loading`, `Errored` from `solid-js` |
 | `action`, inline `action { }` | `action(function* …)` — a statement-level `await` is a `yield` of the flight |
-| `optimistic`, `refresh`, `affects` | the setter of a `createOptimistic`, `refresh(x)`, `affects(x)` |
-| `source x = e` written both plainly and optimistically | `createSignal(fn)` as the truth + `createOptimistic(() => truth())` as the guess layer; plain writes go to the truth, `optimistic` to the layer |
+| `guess`, `refresh`, `affects` | the setter of a `createOptimistic`, `refresh(x)`, `affects(x)` |
+| `source x = e` written both plainly and optimistically | `createSignal(fn)` as the truth + `createOptimistic(() => truth())` as the guess layer; plain writes go to the truth, `guess` to the layer |
 | nested `await` | a tail-position `await` is the memo's result (the memo returns the flight); any other position is unsupported |
 | `provide` / `use` | `setContext` / `getContext` against the declaring owner; a provided source is the source binding (RG-7c) |
 | `x := f` | the setter's updater form (W-6) |
diff --git a/ref/oracle/lower.ts b/ref/oracle/lower.ts
index b26eae1c..331f66ab 100644
--- a/ref/oracle/lower.ts
+++ b/ref/oracle/lower.ts
@@ -471,7 +471,7 @@ export class Oracle {
         if (b.k === "store") {
           const path = s.target.path.map(p => typeof p === "string" ? p : String(untrack(() => this.ev(p, scope, env))));
           if (s.k === "optimistic") {
-            if (!b.setGuess) throw new Error("OPTIMISTIC_NO_QUESTION");
+            if (!b.setGuess) throw new Error("GUESS_NO_QUESTION");
             b.setGuess((d: any) => { if (path.length === 0) { assignDraft(d, typeof v === "function" ? (v as (x: unknown) => unknown)(d) : v); return; } let o = d; for (const k of path.slice(0, -1)) o = o[k]; const last = path[path.length - 1]; o[last] = typeof v === "function" ? (v as (x: unknown) => unknown)(o[last]) : v; });   // W-6 per key
             return;
           }
diff --git a/ref/src/check.ts b/ref/src/check.ts
index 089b00b3..7b0113be 100644
--- a/ref/src/check.ts
+++ b/ref/src/check.ts
@@ -804,8 +804,8 @@ class Checker {
   /** AC-1c, AC-1d, AC-1e over one imperative run (ref/src/infer.ts). */
   private runChecks(stmts: Stmt[], action: boolean, bound: string[], inferred = false) {
     for (const f of runFindings(stmts, action, bound, inferred)) {
-      if (f.code === "HELD_WRITE") this.err("HELD_WRITE", "AC-1c", `${f.name} := … comes before an \`await\` in a body that is an action because it writes and awaits (AC-1b), so it is held until the body ends (AC-1): for state that shows while the work runs, \`optimistic ${f.name} := …\` (a source with a question, ST-5c); for a reset that lands now, \`onFlush(() => { ${f.name} := … })\` (EF-8); to hold it on purpose, declare the action (\`action name() { … }\`); or write it after the \`await\``);
-      else if (f.code === "STALE_WRITE_BACK") this.err("STALE_WRITE_BACK", "AC-1e", `${f.name} := … after an \`await\` is computed from ${f.name} as it was read before that \`await\`: whatever changed ${f.name} in between (another action, a reload) is overwritten — write functionally (\`${f.name} := v => …\`, W-6), which applies to ${f.name} as it stands at the write, or guess it (\`optimistic\`, AC-3) and let the answer replace the guess`);
+      if (f.code === "HELD_WRITE") this.err("HELD_WRITE", "AC-1c", `${f.name} := … comes before an \`await\` in a body that is an action because it writes and awaits (AC-1b), so it is held until the body ends (AC-1): for state that shows while the work runs, \`guess ${f.name} := …\` (a source with a question, ST-5c); for a reset that lands now, \`onFlush(() => { ${f.name} := … })\` (EF-8); to hold it on purpose, declare the action (\`action name() { … }\`); or write it after the \`await\``);
+      else if (f.code === "STALE_WRITE_BACK") this.err("STALE_WRITE_BACK", "AC-1e", `${f.name} := … after an \`await\` is computed from ${f.name} as it was read before that \`await\`: whatever changed ${f.name} in between (another action, a reload) is overwritten — write functionally (\`${f.name} := v => …\`, W-6), which applies to ${f.name} as it stands at the write, or guess it (\`guess\`, AC-3) and let the answer replace the guess`);
       else this.err("STALE_OWN_READ", "AC-1d", `${f.name} is read after this change wrote it: the read sees the committed value, not the write (W-4) — keep the value you wrote in a local (\`let v = …; ${f.name} := v\`) and read that, or write functionally (\`${f.name} := v => …\`, W-6)`);
     }
   }
@@ -841,8 +841,8 @@ class Checker {
             else if (target?.imported) this.err("WRITE_TO_NON_SOURCE", "MO-2", `${s.target.name} is a source of ${target.imported}: a module boundary is a value boundary — write it through an action ${target.imported} exports`);
           }
           if (s.k === "optimistic") {
-            if (!ctx.inAction) this.err("OPTIMISTIC_OUTSIDE_HOLD", "AC-3", "optimistic outside an action: a guess needs a hold to live in — an apply block is never an action (its return is the cleanup, EF-5): declare one, `action save(v) { optimistic … }`, and call it from the block");
-            if (target && target.kind === "source" && !target.question) this.err("OPTIMISTIC_NO_QUESTION", "AC-3", `${s.target.name} is declared with := — a guess needs a question to confirm it (source ${s.target.name} = …, ST-5c)`);
+            if (!ctx.inAction) this.err("GUESS_OUTSIDE_HOLD", "AC-3", "a guess outside an action: it needs a hold to live in — an apply block is never an action (its return is the cleanup, EF-5): declare one, `action save(v) { guess … }`, and call it from the block");
+            if (target && target.kind === "source" && !target.question) this.err("GUESS_NO_QUESTION", "AC-3", `${s.target.name} is declared with := — a guess needs a question to confirm it (source ${s.target.name} = …, ST-5c)`);
           }
           s.target.path.forEach(p => typeof p !== "string" && this.expr(p, env, ctx));
           this.expr(s.e, env, ctx);
diff --git a/ref/src/infer.ts b/ref/src/infer.ts
index ebd6a947..1cf2957c 100644
--- a/ref/src/infer.ts
+++ b/ref/src/infer.ts
@@ -41,7 +41,7 @@ export function inferredAction(stmts: Stmt[]): Trigger | null {
   go(stmts, new Map());
   return guess ?? (write && wait ? "write+await" : null);
 }
-export const triggerText = (t: Trigger): string => t === "write+await" ? "it writes and awaits" : `it has \`${t}\``;
+export const triggerText = (t: Trigger): string => t === "write+await" ? "it writes and awaits" : `it has \`${t === "optimistic" ? "guess" : t}\``;
 
 export type Finding = { code: "HELD_WRITE" | "STALE_OWN_READ" | "STALE_WRITE_BACK"; name: string };
 type Path = string[];                                                   // keys from the source; "*" a computed key
diff --git a/ref/src/model.ts b/ref/src/model.ts
index e4924907..7ea4ee91 100644
--- a/ref/src/model.ts
+++ b/ref/src/model.ts
@@ -12,7 +12,7 @@
 //   tracked   reactive scope (SC-1): the computation depends on it
 //   snapshot  imperative scope (SC-2), `peek` (E-5), a source's seed (E-4), a value parameter's argument (E-10)
 //   pre       `pre x` (C-2): the previous instant's value
-// and `probe: "latest" | "pending"` under `latest` / `pending?` (R-9, R-12). A write is `x := e` (W-1), `optimistic`
+// and `probe: "latest" | "pending"` under `latest` / `pending?` (R-9, R-12). A write is `x := e` (W-1), `guess`
 // (AC-3), `refresh` (AC-4) or `affects` (AC-5); `use p := e` writes the context's provided source (RG-7c). A call names a
 // fn, an action, a server fn or a JS import called by name.
 
diff --git a/ref/src/parse.ts b/ref/src/parse.ts
index bdc5143d..319bea21 100644
--- a/ref/src/parse.ts
+++ b/ref/src/parse.ts
@@ -15,6 +15,8 @@ export function hint(src: string, i: number, msg: string): [string, string] | nu
   const line = src.slice(lineStart(p), (src.indexOf("\n", p) + 1 || src.length + 1) - 1).trim();
   const before = src.slice(lineStart(i), i);
   const name = /([A-Za-z_$][A-Za-z0-9_$.]*)\s*$/.exec(before)?.[1] ?? "x";
+  if (/(^|[{;]\s*)optimistic\s+[A-Za-z_$]/.test(line))
+    return ["OPTIMISTIC", "`optimistic` is spelled `guess`: `guess x := e` shows a guess until the action's answer replaces it (AC-3); it lowers to `createOptimistic`"];
   if (msg.startsWith("W-2") && rest.startsWith(":=") && /=>\s*[A-Za-z_$][A-Za-z0-9_$.]*\s*$/.test(before))
     return ["REACTIVE_WRITE_IN_OWNED_SCOPE", `an arrow that writes takes a braced body, \`() => { ${name} := … }\`: an expression-bodied arrow is a pure function (SC-3)`];
   if (/^(\+\+|--|(\*\*|\|\||&&|\?\?|[-+*\/%])=|=(?![=>]))/.test(rest))
@@ -110,7 +112,7 @@ function parseClasses(c: Cursor): string[] | undefined {
   return out.some(n => n === "Error" || n === "unknown" || n === "any") ? undefined : out;
 }
 
-const KEYWORDS = new Set(["source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined", "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal"]);
+const KEYWORDS = new Set(["source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined", "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal"]);
 
 export function parseProgram(src: string): Program {
   const c = new Cursor(src);
@@ -288,7 +290,8 @@ export function parseStmts(src: string): Stmt[] {
 
 function parseStmt(c: Cursor): Stmt {
   c.ws();
-  if (c.keyword("optimistic")) {
+  if (c.keyword("optimistic")) c.fail("AC-3: `optimistic` is not a statement");
+  if (c.keyword("guess")) {
     c.ws(); const name = c.ident() ?? c.fail("target"); const path: Target["path"] = [];
     for (;;) { if (c.eat(".")) path.push(c.ident() ?? c.fail("member")); else if (c.eat("[")) { c.ws(); path.push(parseExpr(c)); c.ws(); c.expect("]"); } else break; }
     c.ws(); c.expect(":="); c.ws(); return { k: "optimistic", target: { name, path }, e: parseExpr(c) };
diff --git a/ref/src/runtime.ts b/ref/src/runtime.ts
index c0eee1bc..d60894d6 100644
--- a/ref/src/runtime.ts
+++ b/ref/src/runtime.ts
@@ -1100,9 +1100,9 @@ export class Runtime {
     else if (s.k === "refresh") { const n = this.nodeNamed(s.name, region); if (!n || (n.kind !== "eq") || !(n.question || n.flight || n.compute)) throw new RuleError("INVALID_REFRESH_TARGET", `${s.name} has no question to re-ask`); n.refreshInAction = !!run.action; this.pendingRefresh.push(n); }
     else if (s.k === "affects") { const n = this.nodeNamed(s.name, region); if (!n) throw new RuleError("INVALID_AFFECTS_TARGET", `${s.name} is not reactive`); this.pendingAffects.push(n); }
     else if (s.k === "optimistic") {
-      if (!run.action) throw new RuleError("OPTIMISTIC_OUTSIDE_HOLD", "optimistic outside an action body");
+      if (!run.action) throw new RuleError("GUESS_OUTSIDE_HOLD", "a guess outside an action body");
       const n = this.nodeNamed(s.target.name, region);
-      if (!n || !n.question) throw new RuleError("OPTIMISTIC_NO_QUESTION", `${s.target.name} is not a source with a question (source x = e)`);
+      if (!n || !n.question) throw new RuleError("GUESS_NO_QUESTION", `${s.target.name} is not a source with a question (source x = e)`);
       const path = s.target.path.map(p => typeof p === "string" ? p : String(this.ev(p, region, env)));
       this.pendingGuesses.push({ node: n, path, value: this.ev(s.e, region, env) });
     } else if (s.k === "write") {
diff --git a/scenarios/S-action-1.md b/scenarios/S-action-1.md
index c303067d..35a130b1 100644
--- a/scenarios/S-action-1.md
+++ b/scenarios/S-action-1.md
@@ -10,7 +10,7 @@ export default component App() {
   source status := ""
   source orders := 0
   action placeOrder() {
-    optimistic cart := []
+    guess cart := []
     status := "placing"
     try {
       let receipt = await fetch("checkout")
@@ -50,7 +50,7 @@ export default component App() {
   source cart = [1, 2]
   source status := ""
   action placeOrder() {
-    optimistic cart := []
+    guess cart := []
     status := "placing"
     try { let receipt = await fetch("checkout") } catch (e) { status := "declined" }
     fail("lost the receipt")
@@ -96,7 +96,7 @@ export default component App() {
   source cart = [1, 2]
   source status := ""
   action placeOrder() {
-    optimistic cart := []
+    guess cart := []
     status := "placing"
     let receipt = await fetch("checkout")
     cart := []
diff --git a/scenarios/S-action-2.md b/scenarios/S-action-2.md
index beb3cb3f..ab4995b1 100644
--- a/scenarios/S-action-2.md
+++ b/scenarios/S-action-2.md
@@ -12,7 +12,7 @@ export default component App() {
   source status := ""
   action rename(n) {
     try {
-      optimistic shown := n
+      guess shown := n
       await fetch("send")           // fire-and-forget: the answer is not the confirmation
       await until(() => shown == n) // the confirmation is the truth arriving
       status := "confirmed"
diff --git a/scenarios/S-action-3.md b/scenarios/S-action-3.md
index 7efa5f47..647d3fcc 100644
--- a/scenarios/S-action-3.md
+++ b/scenarios/S-action-3.md
@@ -10,7 +10,7 @@ export default component App() {
   source n := 0
   source guess = n
   action save() {
-    optimistic guess := value => value + 1
+    guess guess := value => value + 1
     try { await fetch("r1") } catch (error) {}
   }
   <p>{guess}</p>
diff --git a/scenarios/S-action-4.md b/scenarios/S-action-4.md
index 6072498b..ca12c0d9 100644
--- a/scenarios/S-action-4.md
+++ b/scenarios/S-action-4.md
@@ -10,7 +10,7 @@ export default component App() {
   source n := 0
   source guess = n
   action save(amount) {
-    optimistic guess := value => value + amount
+    guess guess := value => value + amount
     try {
       let got = await fetch("r" + amount)
       n := value => value + got
diff --git a/scenarios/S-ai-feed.md b/scenarios/S-ai-feed.md
index 1c92ae65..cfa8a0bb 100644
--- a/scenarios/S-ai-feed.md
+++ b/scenarios/S-ai-feed.md
@@ -62,11 +62,11 @@ change and lands with the body's end (AC-7).
 ## Rejected variants
 
 ```rx
--- reject: OPTIMISTIC_NO_QUESTION
+-- reject: GUESS_NO_QUESTION
 -- handler: pick()
 export default component App() {
   source selected := 1
-  action pick() { optimistic selected := 2; await fetch("x") }
+  action pick() { guess selected := 2; await fetch("x") }
 
   <p>{selected}</p>
 }
diff --git a/scenarios/S-ai2-tracker.md b/scenarios/S-ai2-tracker.md
index 481976ef..cb42669d 100644
--- a/scenarios/S-ai2-tracker.md
+++ b/scenarios/S-ai2-tracker.md
@@ -47,4 +47,4 @@ Predicted from the spec and not run (the test allows the checker only on `main.r
 | delete the `errored fallback(e) { … }` wrapper in `DetailPanel` | R-f-5 | `UNHANDLED_FAILURE` (`detail` fails `Error` or `NotFound`) |
 | in `assignToMe`, `await assign(id, user)` outside `try … catch` | R-f-6 | `UNCAUGHT_ACTION_FAILURE` |
 | in `ui.rx`, a handler `() => { selected := issue.id }` | MO-2a | `WRITE_TO_NON_SOURCE` |
-| `optimistic assignError := "…"` in an action (`assignError` is `:=`, no question) | AC-3 / ST-5c | `OPTIMISTIC_NO_QUESTION` |
+| `guess assignError := "…"` in an action (`assignError` is `:=`, no question) | AC-3 / ST-5c | `GUESS_NO_QUESTION` |
diff --git a/scenarios/S-check-1.md b/scenarios/S-check-1.md
index 9cab6844..32496d2c 100644
--- a/scenarios/S-check-1.md
+++ b/scenarios/S-check-1.md
@@ -199,21 +199,21 @@ export default component App() {
 ```
 
 ```rx
--- reject: OPTIMISTIC_OUTSIDE_HOLD
+-- reject: GUESS_OUTSIDE_HOLD
 export default component App() {
   source name := "Ann"
   source shown = name
-  effect (name) { optimistic shown := "Zed" }
+  effect (name) { guess shown := "Zed" }
 
   <p>{shown}</p>
 }
 ```
 
 ```rx
--- reject: OPTIMISTIC_NO_QUESTION
+-- reject: GUESS_NO_QUESTION
 export default component App() {
   source shown := "Ann"
-  action fix() { optimistic shown := "Zed"; await fetch("x") }
+  action fix() { guess shown := "Zed"; await fetch("x") }
 
   <p>{shown}</p>
 }
diff --git a/scenarios/S-effect-store-1.md b/scenarios/S-effect-store-1.md
index 4521d5eb..22a5419c 100644
--- a/scenarios/S-effect-store-1.md
+++ b/scenarios/S-effect-store-1.md
@@ -8,7 +8,7 @@ Rules exercised: EF-1, EF-2, H-11, ST-4, AC-3, AC-4.
 export default component App() {
   source rows key id = await fetch("l")
   effect (rows) { log(rows_title($0)) }
-  action ren() { try { optimistic rows[0].title := "x"; await fetch("save"); refresh rows } catch (e) {} }
+  action ren() { try { guess rows[0].title := "x"; await fetch("save"); refresh rows } catch (e) {} }
   errored fallback(e) { <p>error</p> } { loading fallback { <p>loading</p> } { <b>{for t in rows { <i>{t.title}</i> }}</b> } }
 }
 fn rows_title(rs) => rs.map(r => r.title).join(",")
diff --git a/scenarios/S-guess-reask-refresh.md b/scenarios/S-guess-reask-refresh.md
index aaffb6bf..7d208cb8 100644
--- a/scenarios/S-guess-reask-refresh.md
+++ b/scenarios/S-guess-reask-refresh.md
@@ -7,7 +7,7 @@ Rules exercised: AC-3, AC-4, H-11, D-34.
 ```rx
 export default component App() {
   source detail = await fetch("d")
-  action rename(t) { try { optimistic detail.title := t; await fetch("save"); refresh detail } catch (e) {} }
+  action rename(t) { try { guess detail.title := t; await fetch("save"); refresh detail } catch (e) {} }
   errored fallback(e) { <p>error</p> } { loading fallback { <p>loading</p> } { <b>{detail.title}</b> } }
 }
 ```
@@ -39,7 +39,7 @@ effect's view too.
 ```rx
 export default component App() {
   source detail = await fetch("d")
-  action rename(t) { try { optimistic detail.title := t; await fetch("save"); refresh detail } catch (e) {} }
+  action rename(t) { try { guess detail.title := t; await fetch("save"); refresh detail } catch (e) {} }
   effect (detail.title catch (e) => "err") { log($0) }
   <p>x</p>
 }
diff --git a/scenarios/S-infer-1.md b/scenarios/S-infer-1.md
index e4747eda..96606399 100644
--- a/scenarios/S-infer-1.md
+++ b/scenarios/S-infer-1.md
@@ -10,7 +10,7 @@ export default component App() {
   source error := ""
 
   <button onClick={() => {
-    optimistic liked := "yes"
+    guess liked := "yes"
     try {
       let r = await fetch("like")
       liked := r
@@ -30,7 +30,7 @@ that body.
 | instant | action                                                                                                   | frame  |
 | ------- | -------------------------------------------------------------------------------------------------------- | ------ |
 | 0       | mount                                                                                                    | `no `  |
-| 1       | handler `optimistic liked := "yes"; try { let r = await fetch("like"); liked := r } catch (e) { error := e.message }` | `yes ` |
+| 1       | handler `guess liked := "yes"; try { let r = await fetch("like"); liked := r } catch (e) { error := e.message }` | `yes ` |
 | 2       | resolve "like" "yes"                                                                                     | `yes ` |
 
 Instant 1: the guess shows at once (H-11). Instant 2: the truth confirms
@@ -41,7 +41,7 @@ it silently and the body ends.
 | instant | action                                                                                                   | frame        |
 | ------- | -------------------------------------------------------------------------------------------------------- | ------------ |
 | 0       | mount                                                                                                    | `no `        |
-| 1       | handler `optimistic liked := "yes"; try { let r = await fetch("like"); liked := r } catch (e) { error := e.message }` | `yes `       |
+| 1       | handler `guess liked := "yes"; try { let r = await fetch("like"); liked := r } catch (e) { error := e.message }` | `yes `       |
 | 2       | reject "like" "offline"                                                                                  | `no offline` |
 
 The guess reverts and the catch's write lands in the same frame (AC-7),
@@ -53,7 +53,7 @@ as in S-action-1.
 -- reject: UNCAUGHT_ACTION_FAILURE
 export default component App() {
   source liked = "no"
-  <button onClick={() => { optimistic liked := "yes"; let r = await fetch("like"); liked := r }}>{liked}</button>
+  <button onClick={() => { guess liked := "yes"; let r = await fetch("like"); liked := r }}>{liked}</button>
 }
 ```
 
@@ -61,11 +61,11 @@ An inferred action catches what its body can fail with, like a declared
 one (R-f-6): nothing awaits a handler.
 
 ```rx
--- reject: OPTIMISTIC_OUTSIDE_HOLD
+-- reject: GUESS_OUTSIDE_HOLD
 export default component App() {
   source draft := "a"
   source saved = "a"
-  effect (draft) { optimistic saved := $0 }
+  effect (draft) { guess saved := $0 }
   <p>{saved}</p>
 }
 ```
diff --git a/scenarios/S-infer-2.md b/scenarios/S-infer-2.md
index 8fdbd96f..e5bbafb1 100644
--- a/scenarios/S-infer-2.md
+++ b/scenarios/S-infer-2.md
@@ -74,7 +74,7 @@ export default component App() {
 
 Written as a plain handler, this showed `saving` at once. As the action
 the body now is, `saving` would wait for the server. The fixes are named
-by what the write is for: `optimistic` for state that shows while the work
+by what the write is for: `guess` for state that shows while the work
 runs, `onFlush` for a reset, a declared action to hold it on purpose.
 
 ```rx
diff --git a/scenarios/S-infer-3.md b/scenarios/S-infer-3.md
index c1f72260..ba39b636 100644
--- a/scenarios/S-infer-3.md
+++ b/scenarios/S-infer-3.md
@@ -9,7 +9,7 @@ export default component App() {
   source liked = "no"
   action like() { try { let r = await fetch("like"); liked := r } catch (e) {} }
 
-  <button onClick={() => { optimistic liked := "yes"; like() }}>{liked}</button>
+  <button onClick={() => { guess liked := "yes"; like() }}>{liked}</button>
 }
 ```
 
@@ -22,7 +22,7 @@ the same instant's change (D-7), so the guess lives until `like` ends.
 | instant | action                                         | frame |
 | ------- | ---------------------------------------------- | ----- |
 | 0       | mount                                          | `no`  |
-| 1       | handler `optimistic liked := "yes"; like()`    | `yes` |
+| 1       | handler `guess liked := "yes"; like()`    | `yes` |
 | 2       | resolve "like" "yes"                           | `yes` |
 
 ## Variant: the server declines
@@ -30,5 +30,5 @@ the same instant's change (D-7), so the guess lives until `like` ends.
 | instant | action                                         | frame |
 | ------- | ---------------------------------------------- | ----- |
 | 0       | mount                                          | `no`  |
-| 1       | handler `optimistic liked := "yes"; like()`    | `yes` |
+| 1       | handler `guess liked := "yes"; like()`    | `yes` |
 | 2       | reject "like" "offline"                        | `no`  |
diff --git a/scenarios/S-infer-6.md b/scenarios/S-infer-6.md
index 9e798c80..a19202b6 100644
--- a/scenarios/S-infer-6.md
+++ b/scenarios/S-infer-6.md
@@ -56,7 +56,7 @@ export default component App() {
   source items := []
   action add(x: string) {
     let before = items
-    optimistic items := [...before, x + "…"]
+    guess items := [...before, x + "…"]
     let r = await fetch(x) catch (e) => ""
     items := [...before, x]
   }
diff --git a/scenarios/S-latest-reask.md b/scenarios/S-latest-reask.md
index e0c8cb8f..e63603e2 100644
--- a/scenarios/S-latest-reask.md
+++ b/scenarios/S-latest-reask.md
@@ -7,7 +7,7 @@ Rules exercised: R-12, H-7, H-11, AC-3, AC-4, ST-4.
 ```rx
 export default component App() {
   source rows key id = await fetch("l")
-  action close(id) { try { optimistic rows := rows.filter(r => r.id != id); await fetch("close"); refresh rows } catch (e) {} }
+  action close(id) { try { guess rows := rows.filter(r => r.id != id); await fetch("close"); refresh rows } catch (e) {} }
   errored fallback(e) { <p>error</p> } { loading fallback { <p>loading</p> } { <p>{rows.length} / {(latest rows).length}</p> } }
 }
 ```
diff --git a/scenarios/S-parse-hints.md b/scenarios/S-parse-hints.md
index 5b49203f..6ef2f673 100644
--- a/scenarios/S-parse-hints.md
+++ b/scenarios/S-parse-hints.md
@@ -54,6 +54,14 @@ export default component App() {
 }
 ```
 
+```
+-- reject: OPTIMISTIC
+export default component App() {
+  source liked = false
+  <button onClick={() => { optimistic liked := true; liked := true }}>♥</button>
+}
+```
+
 ```
 -- reject: JS_API
 export default component App() {
diff --git a/scenarios/S-remount-refresh.md b/scenarios/S-remount-refresh.md
index ad6f200e..27fdc99b 100644
--- a/scenarios/S-remount-refresh.md
+++ b/scenarios/S-remount-refresh.md
@@ -7,7 +7,7 @@ Rules exercised: AC-4, H-11, ST-4, O-2.
 ```rx
 component List() {
   source rows key id = tag(await fetch("l"))
-  action ren() { try { optimistic rows[0].title := "x"; await fetch("save"); refresh rows } catch (e) {} }
+  action ren() { try { guess rows[0].title := "x"; await fetch("save"); refresh rows } catch (e) {} }
   errored fallback(e) { <p>error</p> } { loading fallback { <p>loading</p> } { <b>{for r in rows { <i>{r.title}</i> }}</b> } }
 }
 fn tag(rs) => rs.map(r => ({ id: r.id, title: r.title, seen: true }))
diff --git a/scenarios/S-solid-26.md b/scenarios/S-solid-26.md
index 22309cb6..da1d9826 100644
--- a/scenarios/S-solid-26.md
+++ b/scenarios/S-solid-26.md
@@ -43,7 +43,7 @@ export default component App() {
   source id := 1
   user = await fetch("u:" + id)
   source shown = user.name
-  action nick(n) { try { optimistic shown := n; await fetch("gate") } catch {} }
+  action nick(n) { try { guess shown := n; await fetch("gate") } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     {loading fallback { <p>…</p> } { <div><p>{shown}</p> <p>{pending? shown ? "updating" : ""}</p></div> }}
diff --git a/scenarios/S-solid-32.md b/scenarios/S-solid-32.md
index 5652150e..7c350675 100644
--- a/scenarios/S-solid-32.md
+++ b/scenarios/S-solid-32.md
@@ -9,7 +9,7 @@ export default component App() {
   source name := "Ann"
   source shown = name
   source saved := false
-  action rename(n) { try { optimistic shown := n; saved := true; await fetch("save"); name := n } catch {} }
+  action rename(n) { try { guess shown := n; saved := true; await fetch("save"); name := n } catch {} }
 
   <p>{shown} / {saved ? "saved" : "unsaved"} / {pending? shown ? "p" : "-"} {pending? saved ? "p" : "-"}</p>
 }
@@ -43,7 +43,7 @@ was a plain write and commits (AC-1, DV-2).
 
 | instant | action                              | frame                  |
 | ------- | ----------------------------------- | ---------------------- |
-| 1       | handler `optimistic shown := "Zed"` | `Ann / unsaved / - -`  |
+| 1       | handler `guess shown := "Zed"` | `Ann / unsaved / - -`  |
 
 A guess outside a hold does not exist (D-33). A handler with a guess is an
 action (AC-1b), so this compiles; the action's change opens and commits in
@@ -54,11 +54,11 @@ The `effect` form, which is never an action, is still rejected (S-check-1).
 
 
 ```rx
--- reject: OPTIMISTIC_NO_QUESTION
+-- reject: GUESS_NO_QUESTION
 -- handler: fix()
 export default component App() {
   source shown := "Ann"
-  action fix() { try { optimistic shown := "Zed"; await fetch("x") } catch {} }
+  action fix() { try { guess shown := "Zed"; await fetch("x") } catch {} }
 
   <p>{shown}</p>
 }
diff --git a/scenarios/S-solid-33.md b/scenarios/S-solid-33.md
index 0d5ddab6..850d1013 100644
--- a/scenarios/S-solid-33.md
+++ b/scenarios/S-solid-33.md
@@ -9,7 +9,7 @@ export default component App() {
   source value := 0
   source double = await fetch("d:" + str(value))
   related = await fetch("r:" + str(double))
-  action bump() { value := 1; optimistic double := 3 }
+  action bump() { value := 1; guess double := 3 }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     {loading fallback { <p>loading</p> } { <p>double={double} async={related}</p> }}
diff --git a/scenarios/S-solid-34.md b/scenarios/S-solid-34.md
index 02d25d98..7f3f6b32 100644
--- a/scenarios/S-solid-34.md
+++ b/scenarios/S-solid-34.md
@@ -8,7 +8,7 @@ Rules: AC-3, R-4, H-11.
 export default component App() {
   source id := 0
   source cat = await fetch("cat:" + str(id))
-  action pick(n, name) { try { id := n; optimistic cat := name; await fetch("w" + str(n)) } catch {} }
+  action pick(n, name) { try { id := n; guess cat := name; await fetch("w" + str(n)) } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     <p>{cat} / {latest cat} {pending? cat ? "p" : "-"}</p>
diff --git a/scenarios/S-solid-35.md b/scenarios/S-solid-35.md
index 9cadb111..58c5f6f8 100644
--- a/scenarios/S-solid-35.md
+++ b/scenarios/S-solid-35.md
@@ -8,7 +8,7 @@ Rules: AC-1, AC-3, H-11.
 export default component App() {
   source truth := 1
   source guess = truth
-  action attempt() { try { optimistic guess := 5; await fetch("unrelated") } catch {} }
+  action attempt() { try { guess guess := 5; await fetch("unrelated") } catch {} }
 
   <p>{guess} / {latest guess} {pending? guess ? "p" : "-"}</p>
 }
@@ -35,7 +35,7 @@ export default component App() {
   source truth := 1
   source guess = truth
   derived = await fetch("x:" + str(guess))
-  action attempt() { try { optimistic guess := 5; await fetch("unrelated") } catch {} }
+  action attempt() { try { guess guess := 5; await fetch("unrelated") } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     <p>{guess} {derived} / {latest guess} {pending? guess ? "p" : "-"}</p>
@@ -65,7 +65,7 @@ instant 3, and untracked `pending? guess` is `true` until the commit.
 ```rx
 export default component App() {
   source list = await fetch("list")
-  action add() { try { optimistic list := list + "+mine"; await fetch("save"); refresh list } catch {} }
+  action add() { try { guess list := list + "+mine"; await fetch("save"); refresh list } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     <p>{list}</p>
diff --git a/scenarios/S-solid-44.md b/scenarios/S-solid-44.md
index 9f4e1e1b..70a8bc6c 100644
--- a/scenarios/S-solid-44.md
+++ b/scenarios/S-solid-44.md
@@ -8,7 +8,7 @@ Rules: H-9, H-11, AC-3, AC-4, ST-5.
 export default component App() {
   source sel := 3
   source comments = await fetch("c:" + sel)
-  action post() { try { optimistic comments := 9; await fetch("post"); refresh comments } catch {} }
+  action post() { try { guess comments := 9; await fetch("post"); refresh comments } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     <div><span>{sel} {comments}</span> <span>{latest comments}</span> <span>{str(pending? comments)}</span></div>
diff --git a/scenarios/S-store-6.md b/scenarios/S-store-6.md
index 90889d98..eaf315d7 100644
--- a/scenarios/S-store-6.md
+++ b/scenarios/S-store-6.md
@@ -9,7 +9,7 @@ D-34 (`createOptimisticStore`).
 export default component App() {
   source todos key id = await fetch("todos")
   action toggle(i) {
-    try { optimistic todos[i].done := true; await fetch("save"); refresh todos } catch {}
+    try { guess todos[i].done := true; await fetch("save"); refresh todos } catch {}
   }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
@@ -21,7 +21,7 @@ export default component App() {
 }
 ```
 
-`optimistic todos[i].done := true` guesses one key of one row; the rest of
+`guess todos[i].done := true` guesses one key of one row; the rest of
 `todos` is the truth beneath it. The action saves, then re-asks the list
 (`refresh todos`) for the truth that confirms the guess.
 
@@ -65,7 +65,7 @@ corrected by the truth it was made over (D-34).
 export default component App() {
   source id := 1
   source state = await fetch("s:" + str(id))
-  action patch() { try { optimistic state.a := 5; await fetch("w") } catch {} }
+  action patch() { try { guess state.a := 5; await fetch("w") } catch {} }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
     {loading fallback { <p>loading</p> } { <p>{state.a} {state.b} {pending? state.a ? "a!" : "-"} {pending? state.b ? "b!" : "-"}</p> }}
@@ -87,10 +87,10 @@ body's end the guess reverts to the truth.
 ## Rejected variants
 
 ```rx
--- reject: OPTIMISTIC_NO_QUESTION
+-- reject: GUESS_NO_QUESTION
 export default component App() {
   source todos: { done: boolean }[] := [{ done: false }]
-  action toggle(i) { try { optimistic todos[i].done := true; await fetch("save") } catch {} }
+  action toggle(i) { try { guess todos[i].done := true; await fetch("save") } catch {} }
 
   <p>{len(todos)}</p>
 }
diff --git a/scenarios/S-store-7.md b/scenarios/S-store-7.md
index 2cc997c1..6ed60617 100644
--- a/scenarios/S-store-7.md
+++ b/scenarios/S-store-7.md
@@ -9,7 +9,7 @@ export default component App() {
   source filter := "open"
   source rows key id = await fetch("l:" + filter)
   shown = latest rows
-  action ren(i) { try { optimistic rows[i].title := "x"; await fetch("save") } catch (e) {} }
+  action ren(i) { try { guess rows[i].title := "x"; await fetch("save") } catch (e) {} }
   errored fallback(e) { <p>error: {e.message}</p> } {
     <b>{len(shown)}</b>{loading on(filter) fallback { <p>loading</p> } { <ul>{for t in rows { <li>{t.title}</li> }}</ul> }}
   }
diff --git a/scenarios/S-store-8.md b/scenarios/S-store-8.md
index 7c9fc7e1..b8aeac25 100644
--- a/scenarios/S-store-8.md
+++ b/scenarios/S-store-8.md
@@ -9,10 +9,10 @@ export default component App() {
   source todos key id = await fetch("todos")
   shown = latest todos
   action toggle(i) {
-    try { optimistic todos[i].done := true; await fetch("save") } catch {}
+    try { guess todos[i].done := true; await fetch("save") } catch {}
   }
   action rename(i) {
-    try { optimistic todos[i].title := "Z"; await fetch("ren") } catch {}
+    try { guess todos[i].title := "Z"; await fetch("ren") } catch {}
   }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
@@ -49,10 +49,10 @@ export default component App() {
   source todos key id = await fetch("todos")
   shown = latest todos
   action first(i) {
-    try { optimistic todos[i].title := "X"; await fetch("one") } catch {}
+    try { guess todos[i].title := "X"; await fetch("one") } catch {}
   }
   action second(i) {
-    try { optimistic todos[i].title := "Y"; await fetch("two") } catch {}
+    try { guess todos[i].title := "Y"; await fetch("two") } catch {}
   }
 
   errored fallback(e) { <p>error: {e.message}</p> } {
diff --git a/spec/05-holds.md b/spec/05-holds.md
index fe29966e..98424529 100644
--- a/spec/05-holds.md
+++ b/spec/05-holds.md
@@ -213,7 +213,7 @@ which showed `A 0` where Solid shows `A 1` — S-hold-3.)
   not `σ`, not `σ′`, not a probe. The instant begins when the run ends
   (W-3); from then `v` is in `σ′` (probes see it) and in `σ` at commit.
 
-- **H-11 (Optimistic).** [F-11, D-33, D-34] `optimistic x := e`, legal only
+- **H-11 (Optimistic).** [F-11, D-33, D-34] `guess x := e`, legal only
   inside an action body, declared or inferred (AC-3, AC-1b), and only on a source with a question (ST-5),
   writes a *guess* for `x`: from the instant's settle it is the graph's
   value (equations derive from it — S-solid-33: `related` re-asks from the
@@ -229,7 +229,7 @@ which showed `A 0` where Solid shows `A 1` — S-hold-3.)
   `true`, and the new question's answer is not the guess's truth — it neither
   confirms nor corrects the guess; it commits with the hold (S-solid-44). An
   input the guess's own action writes is the guess's question
-  (S-solid-34). A functional guess (`optimistic x := v => …`, W-6) applies
+  (S-solid-34). A functional guess (`guess x := v => …`, W-6) applies
   to `x` as the screen shows it: an input its action holds, the launching
   handler's own write included (AC-1), is not shown yet, so the updater
   sees the old answer (S-action-3; #33). `latest x` answers the guess
diff --git a/spec/06-stores.md b/spec/06-stores.md
index 6b0b750a..32a83a90 100644
--- a/spec/06-stores.md
+++ b/spec/06-stores.md
@@ -2,7 +2,7 @@
 
 **Status: OPEN (2026-10-21).** Structured values, sources with a standing
 question, drafts, keys, streams, and the imperative forms that tie writes to
-async work: `action`, `optimistic`, `refresh`, `affects`. Rules cite Solid's
+async work: `action`, `guess`, `refresh`, `affects`. Rules cite Solid's
 F-9, F-11 and the derived rules they restate. In `ref/`: ST-1…ST-6,
 AC-1…AC-5 (S-store-1…5, S-solid-8/9/15/20/26/32…35/37). Optimistic path writes: S-store-6. AC-6 `until`: S-action-2. AC-7 try/catch: S-action-1. A functional guess over a held input, and two actions guessing one slot (H-11): S-action-3, S-action-4. AC-1b…AC-1d (inferred actions, held writes, stale own reads): S-infer-1…5. AC-1e (stale write-back): S-infer-6.
 
@@ -84,7 +84,7 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
   a `createOptimistic(() => truth())` layer.
 - **ST-5b.** Its question (V-9) is `e`'s inputs. `refresh x` re-asks it
   quietly; `affects x` declares that in-flight work will change it (H-9).
-- **ST-5c.** It is the only legal target of `optimistic` (AC-3): a guess
+- **ST-5c.** It is the only legal target of `guess` (AC-3): a guess
   needs a truth that will confirm or correct it.
 
 ## ST-6 — Streams
@@ -142,7 +142,7 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
   adoption).
 - **AC-1b (Inferred action).** [ruled 2026-10-28] An arrow function in
   imperative scope, including an element's event handler (02 SC-4), is an
-  action when its own statements include `optimistic x := e` (AC-3),
+  action when its own statements include `guess x := e` (AC-3),
   `affects x` (AC-5), or both a write to a source (W-1) and a
   statement-level `await` (07 decision 16). It is then `(ps) => action { B }`
   with every consequence of AC-1: one change across its awaited work, its
@@ -156,7 +156,7 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
   An `effect`'s apply block is never inferred: its return is the effect's
   cleanup (EF-5), which an action's body cannot give, and the next run's
   cleanup could not cancel an action in flight. A server function's body is
-  never inferred. A guess in either stays `OPTIMISTIC_OUTSIDE_HOLD`.
+  never inferred. A guess in either stays `GUESS_OUTSIDE_HOLD`.
   Lowers to Solid's `action` exactly as `action { … }` does, nothing added
   (11). Solid: a TypeScript function's promises are invisible to the
   transition, so its author writes `action(function* () { … })`; here every
@@ -168,7 +168,7 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
   so a later write in the body hides it and otherwise it lands late, and
   nothing in the text says so. The fix names what the write is for:
   ephemeral state that shows while the work runs ("saving", a spinner, a
-  pending row) is a guess, `optimistic x := v` on a source with a question
+  pending row) is a guess, `guess x := v` on a source with a question
   (ST-5c; often a constant one, `source status = ""`); a reset that lands
   now and stays (clear an error, clear an input, close a menu) is
   `onFlush(() => { x := v })` (02 EF-8). A write meant to be held — a
@@ -205,7 +205,7 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
   index computed from `x` (`let i = indexOf(x, …); … x[i].f := v`) is not
   the written value and does not count. The fix is the functional write
   (`x := v => …`, W-6), which applies to `x` as it stands at the write, or
-  a guess (`optimistic`, AC-3) that the answer replaces. A guess or a write
+  a guess (`guess`, AC-3) that the answer replaces. A guess or a write
   before the `await` is the change's own and is not checked; nor is an
   awaited answer (`let r = await …`), which is the server's, not a
   snapshot of `x`. The check is the language's (Solid cannot see the read,
@@ -217,17 +217,17 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
 - **AC-2 (Not special).** An action is only a way to tie writes to async
   work; the hold is the thing (H-2). `refresh`, `affects` and probes behave
   the same inside and outside one.
-- **AC-3 (optimistic).** `optimistic x := e`, legal only inside an action
+- **AC-3 (guess).** `guess x := e`, legal only inside an action
   body, declared or inferred (AC-1b); in an `effect`'s apply block, which
-  is never an action, it is `OPTIMISTIC_OUTSIDE_HOLD`, and only on a source with a
-  question (ST-5; `OPTIMISTIC_NO_QUESTION`): writes a guess that is the
+  is never an action, it is `GUESS_OUTSIDE_HOLD`, and only on a source with a
+  question (ST-5; `GUESS_NO_QUESTION`): writes a guess that is the
   graph's and the screen's value for `x` from the instant's settle,
   display-ahead and verdict-inert, until the hold it was made over commits
   or `x`'s own answer differs — the full rule is H-11. `x`'s truth is
   whatever its question gives: a constant (`source draft = "Ann"`), another
   source (`source draft = name`, S-solid-32), or an `await`. Lowers to
   `createOptimistic`/`createOptimisticStore` by usage. Path targets
-  (`optimistic x.a := v`, `optimistic todos[i].done := true`) guess per key,
+  (`guess x.a := v`, `guess todos[i].done := true`) guess per key,
   as Solid's per-node override: the rest of `x` is the truth beneath the
   guess, a new answer corrects only where it differs at a guessed key, and
   equal there it confirms silently (S-store-6). A question re-asked by an
@@ -299,11 +299,11 @@ selected = draft(pre selected -> {}) { d[id] := true; d[prevId] := false }
 | `ACTION_CALLED_IN_OWNED_SCOPE`  | U   | AC-1: an action is `io` (IO-2)                   |
 | `INVALID_REFRESH_TARGET`        | S   | AC-4: not a source with a question / await eq    |
 | `INVALID_AFFECTS_TARGET`        | S   | AC-5                                             |
-| `OPTIMISTIC_OUTSIDE_HOLD` (new) | S   | AC-3: an apply block (never an action, AC-1b)    |
+| `GUESS_OUTSIDE_HOLD` (new) | S   | AC-3: an apply block (never an action, AC-1b)    |
 | `HELD_WRITE` (new)              | S   | AC-1c: an early write in an inferred action      |
 | `STALE_OWN_READ` (new)          | S   | AC-1d: a read of the change's own write          |
 | `STALE_WRITE_BACK` (new)        | S   | AC-1e: a pre-`await` read written back after it  |
-| `OPTIMISTIC_NO_QUESTION` (new)  | S   | AC-3 / ST-5c                                     |
+| `GUESS_NO_QUESTION` (new)  | S   | AC-3 / ST-5c                                     |
 
 ## Scenarios
 
diff --git a/spec/07-surface.md b/spec/07-surface.md
index 3784d3bc..f2df3daf 100644
--- a/spec/07-surface.md
+++ b/spec/07-surface.md
@@ -198,7 +198,7 @@ effect (count) {
 }
 
 action rename(n: string) {
-  optimistic user.name := n          // AC-3: only here
+  guess user.name := n          // AC-3: only here
   let saved = await api.rename(n)    // AC-1: a hold-safe suspension
   user := saved
 }
@@ -206,7 +206,7 @@ action rename(n: string) {
 action { refresh user; affects user }   // inline, one run (AC-1)
 
 <button onClick={() => {             // an action without the keyword: it guesses (AC-1b)
-  optimistic liked := true
+  guess liked := true
   try { let r = await api.like(id); liked := r } catch (e) { likeError := e.message }
 }}>♥</button>
 ```
@@ -230,7 +230,7 @@ action { refresh user; affects user }   // inline, one run (AC-1)
 - An arrow that guesses, declares `affects`, or both writes and awaits is
   an action (06 AC-1b); an `effect`'s apply block never is. In such an
   arrow a plain write before an `await` is `HELD_WRITE` (AC-1c): make it
-  `optimistic` if it is ephemeral state that shows while the work runs,
+  `guess` if it is ephemeral state that shows while the work runs,
   `onFlush(() => { … })` if it is a reset that lands now, or declare the
   action to hold it on purpose. In every run, reading a source the run's
   own change wrote is `STALE_OWN_READ` (AC-1d): keep the value in a local.
@@ -252,7 +252,7 @@ action { refresh user; affects user }   // inline, one run (AC-1)
   ```rx
   action placeOrder() {
     let items = cart              // a snapshot of what is being ordered
-    optimistic cart := []         // the guess shows now (AC-3)
+    guess cart := []         // the guess shows now (AC-3)
     affects cart                  // `pending? cart` is true until the body ends (AC-5) — the guess alone is verdict-inert
     try {
       let receipt = await checkout(items)
@@ -268,7 +268,7 @@ action { refresh user; affects user }   // inline, one run (AC-1)
   `await`, inside the `try`, and report failure from the `catch` (AC-7,
   R-f-6). An optimistic write over a *structured* question source is a
   root write (functional or whole); path targets are not in `ref/` yet.
-- `refresh x`, `affects x`, `optimistic x := e` are statements (AC-3..5).
+- `refresh x`, `affects x`, `guess x := e` are statements (AC-3..5).
 
 ## 6. Components (RG-4)
 
@@ -445,7 +445,7 @@ Control   ::= "if" Expr "{" RBlock "}" ("else" "{" RBlock "}")?
             | "loading" ("on" "(" Expr ")")? "fallback" "{" RBlock "}" "{" RBlock "}"
             | "errored" "fallback" ("(" Name (":" Classes)? ")")? "{" RBlock "}" "{" RBlock "}"
 Block     ::= "{" Stmt* "}"
-Stmt      ::= Target ":=" Expr | "optimistic" Target ":=" Expr      -- Target `use` Name Path: WRITE_TO_NON_SOURCE (O-4)
+Stmt      ::= Target ":=" Expr | "guess" Target ":=" Expr      -- Target `use` Name Path: WRITE_TO_NON_SOURCE (O-4)
             | "refresh" Name | "affects" Name | "action" Block
             | ("let" | "const") Name Type? "=" Expr
             | "if" "(" Expr ")" (Block | Stmt) ("else" (Block | Stmt))?
@@ -525,3 +525,13 @@ Expr      ::= TSExpr with §3's removals and additions
     apply block is not inferred (its return is the cleanup, EF-5). The
     declared `action` stays for names, reuse, and holding a write on
     purpose.
+18. **`guess x := e`, not `optimistic x := e`** (ruled 2026-11-11): the
+    statement is named for what it writes, a guess (AC-3, H-11), which the
+    answer replaces. The word was `optimistic` through 0.1, after Solid's
+    `createOptimistic`, which it still lowers to. A guess on state nothing
+    will answer reads wrong on its face (`guess saveError := …`: the eval's
+    pilot runs wrote that nine times as `optimistic`, `GUESS_NO_QUESTION`).
+    `optimistic` stays reserved, and a line that uses it is the parse error
+    `OPTIMISTIC`, which names `guess`. `source` was weighed against `state`
+    and `signal` the same day and kept (decision 2): `signal` reads as atomic
+    beside stores, and an equation is state too.
diff --git a/spec/08-constraints.md b/spec/08-constraints.md
index 56ba85df..f59d1e0a 100644
--- a/spec/08-constraints.md
+++ b/spec/08-constraints.md
@@ -24,7 +24,7 @@ runtime check.
 
 | constraint | rules | enforcer |
 | --- | --- | --- |
-| no write, statement or `io` call in reactive scope; `optimistic` only in an action; a draft writes only `d` | W-1, W-2, IO-2, AC-3, ST-3a | grammar: no position exists (bin U) |
+| no write, statement or `io` call in reactive scope; `guess` only in an action; a draft writes only `d` | W-1, W-2, IO-2, AC-3, ST-3a | grammar: no position exists (bin U) |
 | `io` rises and never falls; `fn` is pure, `action` is `io`; `pure` only at the JS boundary | R-io-1..4 | row inference in the checker |
 | `pending` enters only through `await`; `lane` only through probes; rows flow through equations and across modules | R-1, R-10, R-13, R-14 | row inference |
 | no instantaneous cycle | C-1 | the static input graph (CN-3) |
@@ -79,7 +79,7 @@ runtime check.
   first load. A probe waits like a read (R-9). Only the first load is
   checked: an answer that lands after the first paint is a transition
   (H-2), legal anywhere. Whether a later hold shows on screen is the
-  affordances' business (`pending?`, `latest`, an `optimistic` guess; a
+  affordances' business (`pending?`, `latest`, a `guess` guess; a
   `loading` region is an escape, not an affordance), and a hold that shows
   none is Solid's run-time `SILENT_HOLD` report; the static form stays
   advice (99, the advisory rows). With SSR the held mount stays correct
diff --git a/spec/09-modules.md b/spec/09-modules.md
index 134b2541..8333995d 100644
--- a/spec/09-modules.md
+++ b/spec/09-modules.md
@@ -41,7 +41,7 @@ see the list at the end.
 ## MO-2 — The value boundary
 
 - **MO-2a.** An imported source is read-only in the importing module:
-  `x := …` and `optimistic x := …` through an import are
+  `x := …` and `guess x := …` through an import are
   `WRITE_TO_NON_SOURCE` (rule MO-2). The exporting module exports actions as
   its write interface (`export action select(id) { selected := id }`).
 - **MO-2b.** `refresh x` and `affects x` of an imported question are
@@ -188,7 +188,7 @@ export component Save(props: { v: unrendered action go catches(nothing) })
   text (S-module-3 is S-module-1 with only the text). Beyond decision 5's
   words, a line carries:
   - a source's kind: `export source x … = …` asks a question (ST-5),
-    `:= …` does not (`refresh x` and `optimistic x := …` depend on it);
+    `:= …` does not (`refresh x` and `guess x := …` depend on it);
   - a fn's parameters (`ref`, with their written types), `rejects(…)` (absent:
     the fn returns nothing that rejects — unlike a JS declaration's, MO-8a,
     where absent is `Error`), and `instantiates` (RG-6);
diff --git a/spec/11-compiler.md b/spec/11-compiler.md
index 223e84e3..81a43f08 100644
--- a/spec/11-compiler.md
+++ b/spec/11-compiler.md
@@ -198,7 +198,7 @@ spellings again. Compile-time only; nothing of it reaches run time.
   `latest` / `pending?` it carries the probe (R-9, R-12); a read of one
   field of props or a store records the field.
 - **CP-6c (writes and calls).** A write is `x := e` (W-1, with whether it
-  writes a path), `optimistic` (AC-3), `refresh` (AC-4) or `affects`
+  writes a path), `guess` (AC-3), `refresh` (AC-4) or `affects`
   (AC-5); `use p := e` is recorded for the diagnostic it gets (O-4). A call
   names a fn, action, server fn or JS import called by name.
 - **CP-6d (one walk, two implementations).** The facts are recorded by the
diff --git a/spec/99-diagnostics.md b/spec/99-diagnostics.md
index eff454bd..46e2272a 100644
--- a/spec/99-diagnostics.md
+++ b/spec/99-diagnostics.md
@@ -62,7 +62,7 @@ Bins:
 | `UNSTABLE_MEMO_OUTPUT`            | R   | data-dependent (partly S once ST decides structured equality)      |
 | `UNSTABLE_LIST_IDENTITY`          | S   | ST: key is on the type; unkeyed `for` over records is a diagnostic |
 | `IMMUTABLE_UPDATE_IN_STORE`       | U   | ST: a structured equation is a delta over `pre`, not a replaced container |
-| `OPTIMISTIC_REVERTED`             | R   | data-dependent                                                     |
+| `GUESS_REVERTED`                  | R   | data-dependent                                                     |
 | `GRAPH_GROWTH`                    | R   | profiling                                                          |
 | `REACTIVITY_HALTED`               | R   | an error escaping every boundary                                   |
 | `INVARIANT_VIOLATION`             | R   | runtime internal                                                   |
@@ -99,11 +99,11 @@ named.
 | `WRITE_TO_NON_SOURCE`             | S   | W-1   | S-core-1, S-check-1 |
 | `DRAFT_WRITE_OUTSIDE_DRAFT`       | S   | ST-3a | S-store-2 |
 | `ASYNC_STORE_SETTER`              | S   | ST-3b | S-check-1 |
-| `OPTIMISTIC_OUTSIDE_HOLD`         | S   | AC-3 (an `effect`'s apply block: never an action, AC-1b) | S-check-1, S-infer-1 |
+| `GUESS_OUTSIDE_HOLD`         | S   | AC-3 (an `effect`'s apply block: never an action, AC-1b) | S-check-1, S-infer-1 |
 | `HELD_WRITE`                      | S   | AC-1c | S-infer-2 |
 | `STALE_OWN_READ`                  | S   | AC-1d | S-infer-5 |
 | `STALE_WRITE_BACK`                | S   | AC-1e | S-infer-6 |
-| `OPTIMISTIC_NO_QUESTION`          | S   | AC-3 / ST-5c | S-check-1, S-solid-32 |
+| `GUESS_NO_QUESTION`          | S   | AC-3 / ST-5c | S-check-1, S-solid-32 |
 | `INVALID_REFRESH_TARGET`, `INVALID_AFFECTS_TARGET` | S | AC-4, AC-5 | S-check-1 |
 | `REGION_ESCAPE`                   | S   | O-3, O-3a, O-3b | S-check-1 (seven shapes), S-module-1 |
 | `UNSTABLE_LIST_IDENTITY`          | S   | ST-4b (narrow form: `map` into fresh records, unkeyed) | S-store-3 |
@@ -163,6 +163,7 @@ section with a fix in `guide/diagnostics.md`, and the compiler's tests check its
 | `JS_FUNCTION`| `function f() { … }`                                     |
 | `JS_RETURN`  | `return <div>…</div>` in a component                     |
 | `JS_API`     | `createSignal(…)`, `useState(…)`, … on a failing line    |
+| `OPTIMISTIC` | `optimistic x := …`, the old spelling of `guess x := …` (AC-3) |
 | `ASSIGNMENT` | `x = …`, `x += …`, `x++` where a write `x := …` was meant (W-1) |
 | `REACTIVE_WRITE_IN_OWNED_SCOPE` | `() => x := …`: the arrow needs a braced body (W-2, SC-3) |
 
diff --git a/tooling/vscode-rx/markdown.js b/tooling/vscode-rx/markdown.js
index 06c3da23..fda0ac9c 100644
--- a/tooling/vscode-rx/markdown.js
+++ b/tooling/vscode-rx/markdown.js
@@ -7,7 +7,7 @@ const typescript = require("highlight.js/lib/languages/typescript");
 /** The words .rx adds to TypeScript's: the ones syntaxes/rx.tmLanguage.json highlights (spec/07). */
 const RX_KEYWORDS = [
   "source", "component", "action", "pure", "server", "client", "fn", "error", "context", "use", "provide", "pre",
-  "latest", "peek", "pending", "optimistic", "affects", "refresh", "errored", "loading", "fallback", "portal",
+  "latest", "peek", "pending", "guess", "affects", "refresh", "errored", "loading", "fallback", "portal",
   "deferred", "key", "rejects", "fails", "effect",
 ];
 
diff --git a/tooling/vscode-rx/syntaxes/rx.tmLanguage.json b/tooling/vscode-rx/syntaxes/rx.tmLanguage.json
index 2381a671..1711ec8b 100644
--- a/tooling/vscode-rx/syntaxes/rx.tmLanguage.json
+++ b/tooling/vscode-rx/syntaxes/rx.tmLanguage.json
@@ -247,7 +247,7 @@
           "name": "keyword.operator.reactive.rx"
         },
         {
-          "match": "\\b(optimistic|refresh|affects)\\b",
+          "match": "\\b(guess|refresh|affects)\\b",
           "name": "keyword.other.action.rx"
         },
         {

From a2989597f9b5966f09fe02dd0e667124c2384d15 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Wed, 11 Nov 2026 03:59:07 +0000
Subject: [PATCH 2/3] Don't reserve `optimistic`: it names Solid's primitive,
 not a keyword

Drops the OPTIMISTIC parse hint, its guide section, spec row and
S-parse-hints variant; 07 decision 18 says why.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWP7LmQhib6M9zGnmBH
---
 compiler/src/parse.rs      |  9 +--------
 guide/diagnostics.md       | 27 ---------------------------
 ref/src/parse.ts           |  5 +----
 scenarios/S-parse-hints.md |  8 --------
 spec/07-surface.md         |  4 ++--
 spec/99-diagnostics.md     |  1 -
 6 files changed, 4 insertions(+), 50 deletions(-)

diff --git a/compiler/src/parse.rs b/compiler/src/parse.rs
index 370feb38..b0a4e017 100644
--- a/compiler/src/parse.rs
+++ b/compiler/src/parse.rs
@@ -42,7 +42,7 @@ re!(LINE_COMMENT, r"//.*$");
 
 static KEYWORDS: LazyLock<HashSet<&'static str>> = LazyLock::new(|| {
     [
-        "source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined",
+        "source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined",
         "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal",
     ]
     .into_iter()
@@ -262,7 +262,6 @@ re!(JS_APIS, r"\b(useState|useReducer|useMemo|useCallback|useEffect|useRef|creat
 re!(JS_FUNCTION, r"^(export\s+)?(default\s+)?(async\s+)?function\b");
 re!(JS_LOCAL, r"^(const|let|var)\b");
 re!(JS_RETURN, r"^return\b");
-re!(OLD_OPTIMISTIC, r"(^|[{;]\s*)optimistic\s+[A-Za-z_$]");
 re!(NAME_BEFORE, r"([A-Za-z_$][A-Za-z0-9_$.]*)\s*$");
 re!(ARROW_BEFORE, r"=>\s*[A-Za-z_$][A-Za-z0-9_$.]*\s*$");
 re!(OP_ASSIGN, r"^(\+\+|--|(\*\*|\|\||&&|\?\?|[-+*/%])=)");
@@ -279,9 +278,6 @@ pub fn hint(src: &str, i: usize, msg: &str) -> Option<(&'static str, String)> {
     let line = src[start..end].trim();
     let before = &src[src[..i].rfind('\n').map(|j| j + 1).unwrap_or(0)..i];
     let name = NAME_BEFORE.captures(before).map(|c| c[1].to_string()).unwrap_or_else(|| "x".into());
-    if OLD_OPTIMISTIC.is_match(line) {
-        return Some(("OPTIMISTIC", "`optimistic` is spelled `guess`: `guess x := e` shows a guess until the action's answer replaces it (AC-3); it lowers to `createOptimistic`".into()));
-    }
     if msg.starts_with("W-2") && rest.starts_with(":=") && ARROW_BEFORE.is_match(before) {
         return Some(("REACTIVE_WRITE_IN_OWNED_SCOPE", format!("an arrow that writes takes a braced body, `() => {{ {} := … }}`: an expression-bodied arrow is a pure function (SC-3)", name)));
     }
@@ -929,9 +925,6 @@ fn parse_path(c: &mut Cursor) -> R<Vec<PathSeg>> {
 fn parse_stmt(c: &mut Cursor) -> R<Stmt> {
     c.ws(true);
     let start = c.i;
-    if c.keyword("optimistic") {
-        return c.fail("AC-3: `optimistic` is not a statement");
-    }
     if c.keyword("guess") {
         c.ws(true);
         let name = c.ident_or("target")?;
diff --git a/guide/diagnostics.md b/guide/diagnostics.md
index 3b8177c5..b634c891 100644
--- a/guide/diagnostics.md
+++ b/guide/diagnostics.md
@@ -97,33 +97,6 @@ export default component App() {
 }
 ```
 
-### OPTIMISTIC
-
-A guess is written `guess x := e` (AC-3). The keyword was `optimistic` before 0.2; it lowers to the same
-`createOptimistic`, and shows until the action's answer replaces it.
-
-```rx before
-export default component App() {
-  source liked = false
-  action like() {
-    optimistic liked := true
-    refresh liked
-  }
-  <button onClick={() => { like() }}>♥</button>
-}
-```
-
-```rx after
-export default component App() {
-  source liked = false
-  action like() {
-    guess liked := true
-    refresh liked
-  }
-  <button onClick={() => { like() }}>♥</button>
-}
-```
-
 ### ASSIGNMENT
 
 A source is written with `:=` as a statement, in a handler or an action (W-1). There is no `=`, `+=` or `++` on state.
diff --git a/ref/src/parse.ts b/ref/src/parse.ts
index 319bea21..946d9182 100644
--- a/ref/src/parse.ts
+++ b/ref/src/parse.ts
@@ -15,8 +15,6 @@ export function hint(src: string, i: number, msg: string): [string, string] | nu
   const line = src.slice(lineStart(p), (src.indexOf("\n", p) + 1 || src.length + 1) - 1).trim();
   const before = src.slice(lineStart(i), i);
   const name = /([A-Za-z_$][A-Za-z0-9_$.]*)\s*$/.exec(before)?.[1] ?? "x";
-  if (/(^|[{;]\s*)optimistic\s+[A-Za-z_$]/.test(line))
-    return ["OPTIMISTIC", "`optimistic` is spelled `guess`: `guess x := e` shows a guess until the action's answer replaces it (AC-3); it lowers to `createOptimistic`"];
   if (msg.startsWith("W-2") && rest.startsWith(":=") && /=>\s*[A-Za-z_$][A-Za-z0-9_$.]*\s*$/.test(before))
     return ["REACTIVE_WRITE_IN_OWNED_SCOPE", `an arrow that writes takes a braced body, \`() => { ${name} := … }\`: an expression-bodied arrow is a pure function (SC-3)`];
   if (/^(\+\+|--|(\*\*|\|\||&&|\?\?|[-+*\/%])=|=(?![=>]))/.test(rest))
@@ -112,7 +110,7 @@ function parseClasses(c: Cursor): string[] | undefined {
   return out.some(n => n === "Error" || n === "unknown" || n === "any") ? undefined : out;
 }
 
-const KEYWORDS = new Set(["source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "optimistic", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined", "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal"]);
+const KEYWORDS = new Set(["source", "effect", "component", "ref", "await", "latest", "loading", "errored", "fallback", "on", "deferred", "action", "guess", "refresh", "affects", "if", "else", "pre", "peek", "fn", "let", "for", "in", "true", "false", "undefined", "null", "provide", "context", "use", "import", "export", "from", "pure", "return", "type", "interface", "try", "catch", "finally", "throw", "portal"]);
 
 export function parseProgram(src: string): Program {
   const c = new Cursor(src);
@@ -290,7 +288,6 @@ export function parseStmts(src: string): Stmt[] {
 
 function parseStmt(c: Cursor): Stmt {
   c.ws();
-  if (c.keyword("optimistic")) c.fail("AC-3: `optimistic` is not a statement");
   if (c.keyword("guess")) {
     c.ws(); const name = c.ident() ?? c.fail("target"); const path: Target["path"] = [];
     for (;;) { if (c.eat(".")) path.push(c.ident() ?? c.fail("member")); else if (c.eat("[")) { c.ws(); path.push(parseExpr(c)); c.ws(); c.expect("]"); } else break; }
diff --git a/scenarios/S-parse-hints.md b/scenarios/S-parse-hints.md
index 6ef2f673..5b49203f 100644
--- a/scenarios/S-parse-hints.md
+++ b/scenarios/S-parse-hints.md
@@ -54,14 +54,6 @@ export default component App() {
 }
 ```
 
-```
--- reject: OPTIMISTIC
-export default component App() {
-  source liked = false
-  <button onClick={() => { optimistic liked := true; liked := true }}>♥</button>
-}
-```
-
 ```
 -- reject: JS_API
 export default component App() {
diff --git a/spec/07-surface.md b/spec/07-surface.md
index f2df3daf..28d150b7 100644
--- a/spec/07-surface.md
+++ b/spec/07-surface.md
@@ -531,7 +531,7 @@ Expr      ::= TSExpr with §3's removals and additions
     `createOptimistic`, which it still lowers to. A guess on state nothing
     will answer reads wrong on its face (`guess saveError := …`: the eval's
     pilot runs wrote that nine times as `optimistic`, `GUESS_NO_QUESTION`).
-    `optimistic` stays reserved, and a line that uses it is the parse error
-    `OPTIMISTIC`, which names `guess`. `source` was weighed against `state`
+    `optimistic` is not reserved: Solid's primitive keeps its name, and the
+    keyword need not. `source` was weighed against `state`
     and `signal` the same day and kept (decision 2): `signal` reads as atomic
     beside stores, and an equation is state too.
diff --git a/spec/99-diagnostics.md b/spec/99-diagnostics.md
index 46e2272a..54543cfd 100644
--- a/spec/99-diagnostics.md
+++ b/spec/99-diagnostics.md
@@ -163,7 +163,6 @@ section with a fix in `guide/diagnostics.md`, and the compiler's tests check its
 | `JS_FUNCTION`| `function f() { … }`                                     |
 | `JS_RETURN`  | `return <div>…</div>` in a component                     |
 | `JS_API`     | `createSignal(…)`, `useState(…)`, … on a failing line    |
-| `OPTIMISTIC` | `optimistic x := …`, the old spelling of `guess x := …` (AC-3) |
 | `ASSIGNMENT` | `x = …`, `x += …`, `x++` where a write `x := …` was meant (W-1) |
 | `REACTIVE_WRITE_IN_OWNED_SCOPE` | `() => x := …`: the arrow needs a braced body (W-2, SC-3) |
 

From bffcff84144dbfd14b852f7ac615f1cec216a444 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Wed, 11 Nov 2026 05:41:06 +0000
Subject: [PATCH 3/3] Clear errors with onFlush, not a guess

An error clear is a reset that lands now (EF-8), not a guess at the
server's answer: `guess saveError := ""` worked only because the source
was given a constant question to revert to. The guide's action example,
the examples and the eval reference apps now write
`onFlush(() => { saveError := "" })`, and those error sources are
declared with `:=`. S-ai2-settings gains the flush instant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWP7LmQhib6M9zGnmBH
---
 eval/README.md                         |  2 +-
 eval/reference/chain1/rx/comments.rx   |  4 ++--
 eval/reference/chain1/rx/issues.rx     | 12 ++++++------
 eval/reference/chain1/rx/labels.rx     |  6 +++---
 eval/reference/chain1/rx/settings.rx   |  4 ++--
 eval/reference/chain2/rx/comments.rx   |  4 ++--
 eval/reference/chain2/rx/issues.rx     | 12 ++++++------
 eval/reference/chain2/rx/labels.rx     |  6 +++---
 eval/reference/chain2/rx/settings.rx   |  4 ++--
 eval/reference/chain3/rx/comments.rx   |  4 ++--
 eval/reference/chain3/rx/issues.rx     | 12 ++++++------
 eval/reference/chain3/rx/labels.rx     |  6 +++---
 eval/reference/chain3/rx/settings.rx   |  4 ++--
 eval/reference/chain4/rx/comments.rx   |  4 ++--
 eval/reference/chain4/rx/issues.rx     | 18 +++++++++---------
 eval/reference/chain4/rx/labels.rx     |  6 +++---
 eval/reference/chain4/rx/settings.rx   |  4 ++--
 eval/reference/close/rx/main.rx        |  8 ++++----
 eval/reference/comments/rx/main.rx     |  8 ++++----
 eval/reference/count/rx/comments.rx    |  4 ++--
 eval/reference/count/rx/issues.rx      | 12 ++++++------
 eval/reference/count/rx/settings.rx    |  4 ++--
 eval/reference/desk/rx/comments.rx     |  4 ++--
 eval/reference/desk/rx/issues.rx       | 12 ++++++------
 eval/reference/desk/rx/settings.rx     |  4 ++--
 eval/reference/labels/rx/comments.rx   |  4 ++--
 eval/reference/labels/rx/issues.rx     | 12 ++++++------
 eval/reference/labels/rx/labels.rx     |  6 +++---
 eval/reference/labels/rx/settings.rx   |  4 ++--
 eval/reference/related/rx/comments.rx  |  4 ++--
 eval/reference/related/rx/issues.rx    | 12 ++++++------
 eval/reference/related/rx/settings.rx  |  4 ++--
 eval/reference/rename/rx/main.rx       |  8 ++++----
 eval/reference/settings/rx/settings.rx |  4 ++--
 eval/reference/tracker/rx/issues.rx    |  4 ++--
 examples/ai2/settings/settings.rx      |  8 ++++----
 guide/async.md                         |  4 ++--
 scenarios/S-ai2-settings.md            | 12 +++++++-----
 38 files changed, 128 insertions(+), 126 deletions(-)

diff --git a/eval/README.md b/eval/README.md
index ef04e4a6..6b576b49 100644
--- a/eval/README.md
+++ b/eval/README.md
@@ -86,7 +86,7 @@ predicted every frame right; the briefs here are stricter.
 - `tracker`: `assignToMe` cleared its error with a plain write (`assignError := ""`) inside an action that awaits,
   and a plain write in an async action is held until the action ends (AC-1), so the old error stayed up until the
   server answered. The fix is the `settings` app's own idiom: `assignError` a question source,
-  `guess assignError := ""`.
+  `guess assignError := ""` (since 2026-11-11 the references clear errors with `onFlush`, EF-8).
 
 ## Layout
 
diff --git a/eval/reference/chain1/rx/comments.rx b/eval/reference/chain1/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/chain1/rx/comments.rx
+++ b/eval/reference/chain1/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/chain1/rx/issues.rx b/eval/reference/chain1/rx/issues.rx
index 2579778c..13838f04 100644
--- a/eval/reference/chain1/rx/issues.rx
+++ b/eval/reference/chain1/rx/issues.rx
@@ -16,12 +16,12 @@ export component IssuesPage() {
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -33,7 +33,7 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -51,7 +51,7 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/chain1/rx/labels.rx b/eval/reference/chain1/rx/labels.rx
index e37c0edc..1fb4ff95 100644
--- a/eval/reference/chain1/rx/labels.rx
+++ b/eval/reference/chain1/rx/labels.rx
@@ -12,14 +12,14 @@ export component Labels(props: { id: number }) {
     ...labels.filter(l => !removing.some(r => r.id == props.id && r.label == l)),
     ...adding.filter(a => a.id == props.id).map(a => a.label),
   ]
-  source labelError: string = ""
+  source labelError: string := ""
   source labelInput: string := ""
 
   // a label error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
@@ -30,7 +30,7 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
diff --git a/eval/reference/chain1/rx/settings.rx b/eval/reference/chain1/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/chain1/rx/settings.rx
+++ b/eval/reference/chain1/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain2/rx/comments.rx b/eval/reference/chain2/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/chain2/rx/comments.rx
+++ b/eval/reference/chain2/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/chain2/rx/issues.rx b/eval/reference/chain2/rx/issues.rx
index 300add69..a400f55d 100644
--- a/eval/reference/chain2/rx/issues.rx
+++ b/eval/reference/chain2/rx/issues.rx
@@ -24,13 +24,13 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -42,7 +42,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -60,7 +60,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/chain2/rx/labels.rx b/eval/reference/chain2/rx/labels.rx
index e37c0edc..1fb4ff95 100644
--- a/eval/reference/chain2/rx/labels.rx
+++ b/eval/reference/chain2/rx/labels.rx
@@ -12,14 +12,14 @@ export component Labels(props: { id: number }) {
     ...labels.filter(l => !removing.some(r => r.id == props.id && r.label == l)),
     ...adding.filter(a => a.id == props.id).map(a => a.label),
   ]
-  source labelError: string = ""
+  source labelError: string := ""
   source labelInput: string := ""
 
   // a label error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
@@ -30,7 +30,7 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
diff --git a/eval/reference/chain2/rx/settings.rx b/eval/reference/chain2/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/chain2/rx/settings.rx
+++ b/eval/reference/chain2/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain3/rx/comments.rx b/eval/reference/chain3/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/chain3/rx/comments.rx
+++ b/eval/reference/chain3/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/chain3/rx/issues.rx b/eval/reference/chain3/rx/issues.rx
index 90d04f88..a19469ef 100644
--- a/eval/reference/chain3/rx/issues.rx
+++ b/eval/reference/chain3/rx/issues.rx
@@ -25,13 +25,13 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -43,7 +43,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -61,7 +61,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/chain3/rx/labels.rx b/eval/reference/chain3/rx/labels.rx
index e37c0edc..1fb4ff95 100644
--- a/eval/reference/chain3/rx/labels.rx
+++ b/eval/reference/chain3/rx/labels.rx
@@ -12,14 +12,14 @@ export component Labels(props: { id: number }) {
     ...labels.filter(l => !removing.some(r => r.id == props.id && r.label == l)),
     ...adding.filter(a => a.id == props.id).map(a => a.label),
   ]
-  source labelError: string = ""
+  source labelError: string := ""
   source labelInput: string := ""
 
   // a label error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
@@ -30,7 +30,7 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
diff --git a/eval/reference/chain3/rx/settings.rx b/eval/reference/chain3/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/chain3/rx/settings.rx
+++ b/eval/reference/chain3/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/chain4/rx/comments.rx b/eval/reference/chain4/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/chain4/rx/comments.rx
+++ b/eval/reference/chain4/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/chain4/rx/issues.rx b/eval/reference/chain4/rx/issues.rx
index 488ee123..97122ac4 100644
--- a/eval/reference/chain4/rx/issues.rx
+++ b/eval/reference/chain4/rx/issues.rx
@@ -25,18 +25,18 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
   // the newest row action's failure; row actions and the detail's leave each other's errors alone
-  source rowError: string = ""
+  source rowError: string := ""
 
 
   action assignTo(id: number, who: string, fromRow: boolean) {
     if (fromRow) {
-      guess rowError := ""
+      onFlush(() => { rowError := "" })
     } else {
-      guess assignError := ""
+      onFlush(() => { assignError := "" })
     }
     try {
       let i = issues.findIndex(t => t.id == id)
@@ -53,7 +53,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -72,9 +72,9 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void; onOpenHel
 
   action closeOne(id: number, fromRow: boolean) {
     if (fromRow) {
-      guess rowError := ""
+      onFlush(() => { rowError := "" })
     } else {
-      guess closeError := ""
+      onFlush(() => { closeError := "" })
     }
     try {
       guess issues := issues.filter(t => t.id != id)
diff --git a/eval/reference/chain4/rx/labels.rx b/eval/reference/chain4/rx/labels.rx
index e37c0edc..1fb4ff95 100644
--- a/eval/reference/chain4/rx/labels.rx
+++ b/eval/reference/chain4/rx/labels.rx
@@ -12,14 +12,14 @@ export component Labels(props: { id: number }) {
     ...labels.filter(l => !removing.some(r => r.id == props.id && r.label == l)),
     ...adding.filter(a => a.id == props.id).map(a => a.label),
   ]
-  source labelError: string = ""
+  source labelError: string := ""
   source labelInput: string := ""
 
   // a label error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
@@ -30,7 +30,7 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
diff --git a/eval/reference/chain4/rx/settings.rx b/eval/reference/chain4/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/chain4/rx/settings.rx
+++ b/eval/reference/chain4/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/close/rx/main.rx b/eval/reference/close/rx/main.rx
index 1f04fbb0..738dc2b3 100644
--- a/eval/reference/close/rx/main.rx
+++ b/eval/reference/close/rx/main.rx
@@ -11,10 +11,10 @@ export default component App() {
   source filter: string := "open"
   source selected: number | null := null
   source issues key id = await fetchIssues(filter)
-  source assignError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source closeError: string := ""
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := keep(issues, t => t.id != id)
       await closeIssue(id)
@@ -25,7 +25,7 @@ export default component App() {
   }
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = indexOf(issues, t => t.id == id)
       guess issues[i].assignee := who
diff --git a/eval/reference/comments/rx/main.rx b/eval/reference/comments/rx/main.rx
index 85d7afb3..d9f2f0d5 100644
--- a/eval/reference/comments/rx/main.rx
+++ b/eval/reference/comments/rx/main.rx
@@ -11,13 +11,13 @@ export default component App() {
   source filter: string := "open"
   source selected: number | null := null
   source issues key id = await fetchIssues(filter)
-  source assignError: string = ""
-  source postError: string = ""
+  source assignError: string := ""
+  source postError: string := ""
   source commentInput: string := ""
   source comments = selected == null ? [] : await fetchComments(selected)
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := ""
+    onFlush(() => { postError := "" })
     try {
       guess comments := [...comments, { author: author, text: `${text} (sending…)` }]
       await addComment(id, author, text)
@@ -29,7 +29,7 @@ export default component App() {
 
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = indexOf(issues, t => t.id == id)
       guess issues[i].assignee := who
diff --git a/eval/reference/count/rx/comments.rx b/eval/reference/count/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/count/rx/comments.rx
+++ b/eval/reference/count/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/count/rx/issues.rx b/eval/reference/count/rx/issues.rx
index 5825e67e..0bac8b0e 100644
--- a/eval/reference/count/rx/issues.rx
+++ b/eval/reference/count/rx/issues.rx
@@ -23,13 +23,13 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -41,7 +41,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -59,7 +59,7 @@ export component IssuesPage(props: { onOpenCount: (n: number) => void }) {
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/count/rx/settings.rx b/eval/reference/count/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/count/rx/settings.rx
+++ b/eval/reference/count/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/desk/rx/comments.rx b/eval/reference/desk/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/desk/rx/comments.rx
+++ b/eval/reference/desk/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/desk/rx/issues.rx b/eval/reference/desk/rx/issues.rx
index 36fc5323..7cd02652 100644
--- a/eval/reference/desk/rx/issues.rx
+++ b/eval/reference/desk/rx/issues.rx
@@ -15,12 +15,12 @@ export component IssuesPage() {
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -32,7 +32,7 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -50,7 +50,7 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/desk/rx/settings.rx b/eval/reference/desk/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/desk/rx/settings.rx
+++ b/eval/reference/desk/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/labels/rx/comments.rx b/eval/reference/labels/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/labels/rx/comments.rx
+++ b/eval/reference/labels/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/labels/rx/issues.rx b/eval/reference/labels/rx/issues.rx
index 2579778c..13838f04 100644
--- a/eval/reference/labels/rx/issues.rx
+++ b/eval/reference/labels/rx/issues.rx
@@ -16,12 +16,12 @@ export component IssuesPage() {
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -33,7 +33,7 @@ export component IssuesPage() {
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -51,7 +51,7 @@ export component IssuesPage() {
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/labels/rx/labels.rx b/eval/reference/labels/rx/labels.rx
index e37c0edc..1fb4ff95 100644
--- a/eval/reference/labels/rx/labels.rx
+++ b/eval/reference/labels/rx/labels.rx
@@ -12,14 +12,14 @@ export component Labels(props: { id: number }) {
     ...labels.filter(l => !removing.some(r => r.id == props.id && r.label == l)),
     ...adding.filter(a => a.id == props.id).map(a => a.label),
   ]
-  source labelError: string = ""
+  source labelError: string := ""
   source labelInput: string := ""
 
   // a label error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { labelError := "" }
 
   action add(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess adding := [...adding, { id: id, label: label }]
       await addLabel(id, label)
@@ -30,7 +30,7 @@ export component Labels(props: { id: number }) {
   }
 
   action remove(id: number, label: string) {
-    guess labelError := ""
+    onFlush(() => { labelError := "" })
     try {
       guess removing := [...removing, { id: id, label: label }]
       await removeLabel(id, label)
diff --git a/eval/reference/labels/rx/settings.rx b/eval/reference/labels/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/labels/rx/settings.rx
+++ b/eval/reference/labels/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/related/rx/comments.rx b/eval/reference/related/rx/comments.rx
index 5476dd55..8517b69a 100644
--- a/eval/reference/related/rx/comments.rx
+++ b/eval/reference/related/rx/comments.rx
@@ -8,14 +8,14 @@ export component Comments(props: { id: number; user: string }) {
   source comments = await fetchComments(props.id)
   source sending: { id: number; author: string; text: string }[] = []
   shownSending = sending.filter(s => s.id == props.id)
-  source postError: string | null = null
+  source postError: string | null := null
   source commentInput: string := ""
 
   // a post error belongs to the issue it was made on: selecting another clears it
   effect (props.id) { postError := null }
 
   action postComment(id: number, author: string, text: string) {
-    guess postError := null
+    onFlush(() => { postError := null })
     try {
       guess sending := [...sending, { id: id, author: author, text: text }]
       await addComment(id, author, text)
diff --git a/eval/reference/related/rx/issues.rx b/eval/reference/related/rx/issues.rx
index 878833c3..db6ad3f6 100644
--- a/eval/reference/related/rx/issues.rx
+++ b/eval/reference/related/rx/issues.rx
@@ -16,12 +16,12 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   source issues key id = await fetchIssues(filter)
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
   source titleInput: string := ""
-  source assignError: string = ""
-  source renameError: string = ""
-  source closeError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
+  source closeError: string := ""
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       guess issues[i].assignee := who
@@ -33,7 +33,7 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   }
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = issues.findIndex(t => t.id == id)
       if (i >= 0) {
@@ -51,7 +51,7 @@ export component IssuesPage(props: { onOpenHelp: (query: string, id: number) =>
   }
 
   action closeOne(id: number) {
-    guess closeError := ""
+    onFlush(() => { closeError := "" })
     try {
       guess issues := issues.filter(t => t.id != id)
       await closeIssue(id)
diff --git a/eval/reference/related/rx/settings.rx b/eval/reference/related/rx/settings.rx
index 6c45198c..5093aaca 100644
--- a/eval/reference/related/rx/settings.rx
+++ b/eval/reference/related/rx/settings.rx
@@ -13,11 +13,11 @@ fn validName(name: string): string =>
 
 export component SettingsPage() {
   source settings: Settings = await loadSettings()
-  source saveError: string = ""
+  source saveError: string := ""
 
   action save(next: Settings) {
     guess settings := next
-    guess saveError := ""
+    onFlush(() => { saveError := "" })
     affects settings
     try {
       let saved = await saveSettings(next)
diff --git a/eval/reference/rename/rx/main.rx b/eval/reference/rename/rx/main.rx
index 8bf3450f..55ce3ff3 100644
--- a/eval/reference/rename/rx/main.rx
+++ b/eval/reference/rename/rx/main.rx
@@ -11,13 +11,13 @@ export default component App() {
   source filter: string := "open"
   source selected: number | null := null
   source issues key id = await fetchIssues(filter)
-  source assignError: string = ""
-  source renameError: string = ""
+  source assignError: string := ""
+  source renameError: string := ""
   source titleInput: string := ""
   source detail = selected == null ? null : found(await fetchIssue(selected), selected)
 
   action renameTo(id: number, title: string) {
-    guess renameError := ""
+    onFlush(() => { renameError := "" })
     try {
       let i = indexOf(issues, t => t.id == id)
       if (i >= 0) {
@@ -35,7 +35,7 @@ export default component App() {
   }
 
   action assignTo(id: number, who: string) {
-    guess assignError := ""
+    onFlush(() => { assignError := "" })
     try {
       let i = indexOf(issues, t => t.id == id)
       guess issues[i].assignee := who
diff --git a/eval/reference/settings/rx/settings.rx b/eval/reference/settings/rx/settings.rx
index f57f9cab..7d93eab6 100644
--- a/eval/reference/settings/rx/settings.rx
+++ b/eval/reference/settings/rx/settings.rx
@@ -22,7 +22,7 @@ export source settings: Settings = await loadSettings()
 
 // What the last save failed with, or "". A question with a constant truth, so
 // a new save can clear it as a guess the moment it starts (ST-5a).
-export source saveError: string = ""
+export source saveError: string := ""
 
 export action retryLoad() {
   refresh settings
@@ -30,7 +30,7 @@ export action retryLoad() {
 
 export action save(next: Settings) {
   guess settings := next     // the page shows what is being saved, now (H-11)
-  guess saveError := ""      // and stops showing the previous failure
+  onFlush(() => { saveError := "" })      // and stops showing the previous failure
   affects settings                // `pending? settings` is true until the save ends (AC-5)
   try {
     let saved = await saveSettings(next)
diff --git a/eval/reference/tracker/rx/issues.rx b/eval/reference/tracker/rx/issues.rx
index 61cb3b84..43e64616 100644
--- a/eval/reference/tracker/rx/issues.rx
+++ b/eval/reference/tracker/rx/issues.rx
@@ -24,7 +24,7 @@ export source selected: number | null := null
 // The server's message from the last declined assignment ("" when none). A
 // question with a constant truth, so a new assignment can clear it as a guess
 // the moment it starts (ST-5a); a plain write would be held with the action.
-export source assignError: string = ""
+export source assignError: string := ""
 
 // Only asked while something is selected (E-11: the null arm opens no flight).
 export detail = selected == null ? null : found(await fetchIssue(selected), selected)
@@ -42,7 +42,7 @@ export action select(id: number) {
 }
 
 export action assignToMe(id: number, user: string) {
-  guess assignError := ""
+  onFlush(() => { assignError := "" })
   guess issues := list => withAssignee(list, id, user)
   try {
     await assign(id, user)
diff --git a/examples/ai2/settings/settings.rx b/examples/ai2/settings/settings.rx
index f57f9cab..2a60047a 100644
--- a/examples/ai2/settings/settings.rx
+++ b/examples/ai2/settings/settings.rx
@@ -20,9 +20,9 @@ export fn validate(s: Settings): Settings =>
 // The truth. A source with a question (ST-5): `refresh` re-asks it, a save guesses it.
 export source settings: Settings = await loadSettings()
 
-// What the last save failed with, or "". A question with a constant truth, so
-// a new save can clear it as a guess the moment it starts (ST-5a).
-export source saveError: string = ""
+// What the last save failed with, or "". A new save clears it in an onFlush run
+// of its own, so it goes the moment the save starts (EF-8).
+export source saveError: string := ""
 
 export action retryLoad() {
   refresh settings
@@ -30,7 +30,7 @@ export action retryLoad() {
 
 export action save(next: Settings) {
   guess settings := next     // the page shows what is being saved, now (H-11)
-  guess saveError := ""      // and stops showing the previous failure
+  onFlush(() => { saveError := "" })      // and stops showing the previous failure
   affects settings                // `pending? settings` is true until the save ends (AC-5)
   try {
     let saved = await saveSettings(next)
diff --git a/guide/async.md b/guide/async.md
index 08f981e0..635abcd3 100644
--- a/guide/async.md
+++ b/guide/async.md
@@ -61,11 +61,11 @@ forecast = city == "" ? null : city.length < 3 ? throw TooShort(city) : await fe
 
 ```rx
 export source profile: Profile = await loadProfile()   // a source with a question: it can take a guess
-export source saveError: string = ""
+export source saveError: string := ""
 
 export action save(next: Profile) {
   guess profile := next        // shows now; reverts by itself if the save fails
-  guess saveError := ""        // the old error goes now
+  onFlush(() => { saveError := "" })   // the old error goes now, in its own instant (EF-8)
   affects profile                   // `pending? profile` is true until the save ends
   try {
     let answer = await saveProfile(next)
diff --git a/scenarios/S-ai2-settings.md b/scenarios/S-ai2-settings.md
index 3cf087e3..764d27d6 100644
--- a/scenarios/S-ai2-settings.md
+++ b/scenarios/S-ai2-settings.md
@@ -22,8 +22,9 @@ file: ../examples/ai2/settings/main.rx
 | 7 | handler `save(draft)` | `Saved: Bea · dark · compact (saving…) Profile [compact] Display name 3/20 Appearance [compact] Theme: dark light dark Toggle density Preview: dark / compact Save` | `pending? settings` true (affects) |
 | 8 | reject "saveSettings:{"name":"Bea","theme":"dark","density":"compact"}" "quota exceeded" | `Saved: Ann · light · compact Save failed: quota exceeded Profile [compact] Display name 3/20 Appearance [compact] Theme: dark light dark Toggle density Preview: dark / compact Save` | `pending? settings` false |
 | 9 | handler `draftDensity := "comfortable"` | `Saved: Ann · light · compact Save failed: quota exceeded Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | — |
-| 10 | handler `save(draft)` | `Saved: Bea · dark · comfortable (saving…) Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | `pending? settings` true |
-| 11 | resolve "saveSettings:{"name":"Bea","theme":"dark","density":"comfortable"}" {"name":"Bea","theme":"dark","density":"comfortable"} | `Saved: Bea · dark · comfortable Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | `pending? settings` false |
+| 10 | handler `save(draft)` | `Saved: Bea · dark · comfortable (saving…) Save failed: quota exceeded Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | `pending? settings` true |
+| 11 | (flush) | `Saved: Bea · dark · comfortable (saving…) Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | `pending? settings` true |
+| 12 | resolve "saveSettings:{"name":"Bea","theme":"dark","density":"comfortable"}" {"name":"Bea","theme":"dark","density":"comfortable"} | `Saved: Bea · dark · comfortable Profile [comfortable] Display name 3/20 Appearance [comfortable] Theme: dark light dark Toggle density Preview: dark / comfortable Save` | `pending? settings` false |
 
 Why, one line per instant:
 
@@ -34,8 +35,9 @@ Why, one line per instant:
 - **4.** An empty name makes `validate(draft)` fail with `NameRequired`. The `catch` turns that into its message, so `if nameError` opens the message branch (R-f-4, RG-1). The counter reads `0/20`.
 - **5.** 27 characters fails with `NameTooLong(27)`. `nameError` goes from one truthy string to another, so the branch is kept and the narrowed binding updates (RG-1b, RG-1c).
 - **6.** Two writes in one handler are one instant (W-3). The name is valid again, so `nameError` is "" and the branch closes. The ThemePicker label and the Preview both show the theme through the context. The saved line does not move.
-- **7.** `save` guesses `settings := draft` and `saveError := ""`, then `affects settings` and awaits. The guess's readers form a lane and show at once (H-11, H-7). `pending? settings` is true because of `affects` (AC-5, H-9). The form is not recreated (the `if` goes truthy→truthy) and its draft does not change.
+- **7.** `save` guesses `settings := draft`, clears `saveError` in an `onFlush` run of its own (EF-8; nothing to clear yet), then `affects settings` and awaits. The guess's readers form a lane and show at once (H-11, H-7). `pending? settings` is true because of `affects` (AC-5, H-9). The form is not recreated (the `if` goes truthy→truthy) and its draft does not change.
 - **8.** The rejection throws at the `await`. The `catch` writes `saveError` inside the still-open change (AC-7). At the end of the body the guess on `settings` is corrected to the unchanged truth (Ann), and that commits in one frame with the error message (H-11, AC-1). The draft is local to the form, so it keeps Bea/dark.
 - **9.** A plain local write changes the `density` provision, so both Panels and the Preview update. Nothing else moves.
-- **10.** A new key (the density differs) means a new flight. The `settings` guess shows now. The `saveError` guess `""` closes the "Save failed" branch now (H-11; a lane condition's branch change attaches immediately, H-7). `pending? settings` is true.
-- **11.** The answer is written as the truth (`settings := saved`), equal to the guess, so it confirms silently. The plain `saveError := ""` is the truth for that source too, so it does not revert to "quota exceeded". The change commits, and `pending? settings` is false.
+- **10.** A new key (the density differs) means a new flight. The `settings` guess shows now (H-11). `pending? settings` is true. The old error is still up: its clear is registered with `onFlush`, not written by this run.
+- **11.** The `onFlush` run is a change of its own and commits at once, while the save is still out (EF-8, S-flush-1): the "Save failed" branch closes.
+- **12.** The answer is written as the truth (`settings := saved`), equal to the guess, so it confirms silently. The plain `saveError := ""` is the truth for that source too, so it does not revert to "quota exceeded". The change commits, and `pending? settings` is false.