diff --git a/package.json b/package.json
index 098ca497..55f8e3e5 100644
--- a/package.json
+++ b/package.json
@@ -26,7 +26,7 @@
   "scripts": {
     "start": "firebase emulators:exec --project=rxfire-525a3 \"vitest --ui\"",
     "build": "tsc && vite build && npm run build:types-cjs && npm run docs",
-    "build:types-cjs": "node -e \"require('fs').copyFileSync('dist/index.d.ts','dist/index.d.cts')\"",
+    "build:types-cjs": "node scripts/emit-cjs-types.mjs",
     "test": "firebase emulators:exec --project=rxfire-525a3 \"vitest\"",
     "test:firestore": "firebase emulators:exec --only firestore --ui --project=rxfire-525a3 \"vitest firestore\"",
     "test:database": "firebase emulators:exec --only database --project=rxfire-525a3 \"vitest database\"",
diff --git a/scripts/emit-cjs-types.mjs b/scripts/emit-cjs-types.mjs
new file mode 100644
index 00000000..cbfe1cc4
--- /dev/null
+++ b/scripts/emit-cjs-types.mjs
@@ -0,0 +1,20 @@
+// Writes a CommonJS declaration file (`.d.cts`) next to every `dist/*.d.ts`, for the `require` export condition.
+// A copy of `index.d.ts` is not enough: its relative imports resolve to `.d.ts` files, which TypeScript reads
+// as ESM in this `"type": "module"` package, so CommonJS consumers get TS1479 on each import (#850).
+// Rewriting `./x.js` to `./x.cjs` points each import at the matching `.d.cts` instead.
+import { readdirSync, readFileSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const dir = 'dist';
+const relativeSpecifier = /(\bfrom\s*|\bimport\s*\(\s*)(['"])(\.{1,2}\/[^'"]*)\2/g;
+
+for (const file of readdirSync(dir).filter((f) => f.endsWith('.d.ts'))) {
+  const source = readFileSync(join(dir, file), 'utf8');
+  const cjs = source.replace(relativeSpecifier, (_, keyword, quote, specifier) => {
+    if (!specifier.endsWith('.js')) {
+      throw new Error(`${file}: relative import without a .js extension: ${specifier}`);
+    }
+    return `${keyword}${quote}${specifier.slice(0, -'.js'.length)}.cjs${quote}`;
+  });
+  writeFileSync(join(dir, file.replace(/\.d\.ts$/, '.d.cts')), cjs);
+}