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docs(csp-cors): document CORS headers for the MCP endpoint in browser-based hosts - #801
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…-based hosts Browser-based hosts (basic-host, web inspectors, web assistants) call the MCP endpoint cross-origin. Servers that hand-roll CORS around createMcpHandler must allow the SEP-2243 headers Mcp-Method and Mcp-Name, or the 2026-07-28 preflight fails and clients fall back to the 2025 handshake.
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Summary
docs/csp-cors.mdcovers CORS for requests an app makes from its iframe, but not the other boundary: a browser-based host (examples/basic-host, web inspectors, web assistants) calling the MCP endpoint cross-origin. This adds a short section for that case.Why
The Express examples use
app.use(cors()), which reflects requested headers, so they work. Servers that hand-roll CORS aroundcreateMcpHandler(Cloudflare Workers, Deno, Bun) usually allow-list headers explicitly. On 2026-07-28 exchanges the TypeScript client also sendsMcp-MethodandMcp-Name(SEP-2243). If those are missing, the preflight fails with "Request header field mcp-method is not allowed by Access-Control-Allow-Headers", and a client usingversionNegotiation: { mode: "auto" }silently falls back to the 2025 handshake or fails to connect. I hit this when a browser-based host connected to a Worker-hosted MCP Apps server.Changes
docs/csp-cors.md: when it applies, a table of the request headers the client sends and when, and a minimalfetch-handler example.Header names were checked against
@modelcontextprotocol/client2.1 (RESERVED_REQUEST_HEADER_NAMESand_applyBodyDerivedHeadersin the Streamable HTTP transport).