openapi-to-mcp

A minimal MCP server that auto-generates tools from your OpenAPI spec. Optimized for Vercel's serverless architecture.

Quick Start

1. Fork this repo
2. Set OPENAPI_SPEC_URL to your OpenAPI spec URL
3. Deploy to Vercel

Configuration

Variable	Required	Description
OPENAPI_SPEC_URL	Yes	URL or file path to your OpenAPI spec (YAML or JSON)
API_BASE_URL	No	Override the API base URL (defaults to servers[0].url from the spec)
MCP_SERVER_NAME	No	Override the MCP server name (defaults to info.title from the spec)
MCP_TOOL_PREFIX	No	Prefix prepended to every tool name, for namespacing (e.g. acme_)
PORT	No	Local dev server port (default: 3000)

How It Works

1. On cold start, the server fetches and parses your OpenAPI spec
2. Each API endpoint becomes an MCP tool; its JSON Schema validates arguments before your API is called (tools whose schema fails to compile are skipped with a logged error)
3. When a tool is called, the request is forwarded to your API with the caller's headers (API key, Bearer token, etc.) passed through unchanged; MCP transport headers (Mcp-*) are stripped
4. The spec and tools are cached in memory, so warm invocations skip the fetch

The server is stateless and serves both MCP 2026-07-28 and 2025-era clients from the root path. GET returns 405.

Tool Annotations

Hints are derived from the HTTP method:

Method	readOnly	destructive	idempotent
GET	yes		yes
POST
PUT		yes	yes
PATCH		yes
DELETE		yes	yes

openWorldHint is always false. Set x-mcp-destructive: true|false on an operation to override.

Running Locally

1. Install dependencies

pnpm install

2. Configure environment variables

Create .env and set OPENAPI_SPEC_URL to your OpenAPI spec. For local dev, this can be a remote URL or an absolute file path:

# Remote URL
OPENAPI_SPEC_URL=https://api.example.com/openapi.yaml

# Or an absolute file path (local dev only)
OPENAPI_SPEC_URL=/path/to/your/openapi.yml

3. Start the dev server

pnpm dev

The server starts at http://localhost:3000.

4. Connect your MCP client to the local server

How you connect to the MCP server depends on your client (Claude Code, Cursor, VS Code, etc.). Check your client's docs for where to add MCP servers. Most clients use a similar JSON format. Here's an example for Claude Code (.mcp.json):

{
  "mcpServers": {
    "my-api": {
      "type": "http",
      "url": "http://localhost:3000",
      "headers": {
        "Authorization": "Bearer <your-token>"
      }
    }
  }
}

Use whatever headers your API requires. See Auth.

Deploying to Vercel

1. Push this repo to GitHub
2. Import it at vercel.com/new
3. Add OPENAPI_SPEC_URL under Settings > Environment Variables. The other variables in Configuration are optional.

That's it. Your MCP server is live at https://your-project.vercel.app (or a custom domain like mcp.example.com). Connect clients the same way as local, just swap the URL.

Auth

All headers from your MCP client config are forwarded to the target API on every tool call. The MCP server does not validate or store credentials. Your API handles authentication.

Configure whatever headers your API expects in the client config:

// API key auth
"headers": { "X-Api-Key": "<your-key>" }

// Bearer token
"headers": { "Authorization": "Bearer <your-token>" }

// Multiple headers
"headers": { "Authorization": "Bearer <token>", "X-Org-Id": "org_123" }