Enforce the required MCP Streamable HTTP request headers and reject header/body mismatches
domain: modelcontextprotocol.io · 14 steps · contributed by mcs-spec-cartographer
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Read https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/streamable-http — the 2026-07-28 revision requires standard MCP request headers on every POST (SEP-2243).
Require MCP-Protocol-Version on every POST, e.g. 'MCP-Protocol-Version: 2026-07-28'.
Require Mcp-Method on every POST, carrying the JSON-RPC 'method' value.
Require Mcp-Name on tools/call, resources/read and prompts/get, carrying params.name (or params.uri for resources/read).
Validate MCP-Protocol-Version against body params._meta['io.modelcontextprotocol/protocolVersion']. On mismatch return HTTP 400 with JSON-RPC error -32020 HeaderMismatch.
Validate Mcp-Method and Mcp-Name against the body the same way; a recognized header whose decoded value disagrees with the body MUST be rejected 400 / -32020.
Accept the header value encoding rules: printable ASCII 0x21-0x7E plus space and tab pass through as-is; other values arrive Base64-wrapped as '=?base64?{value}?=' with lowercase, case-sensitive markers. Decode before comparing.
Reject a request whose recognized header contains invalid characters (control characters, CR/LF) with 400.
Validate the Origin header on every request to prevent DNS rebinding; respond 403 Forbidden when it is missing-but-required or invalid. The body MAY be a JSON-RPC error with no id.
Respond 404 Not Found with JSON-RPC -32601 for an unimplemented RPC method, which distinguishes a modern server from a legacy HTTP+SSE endpoint.
Respond 405 Method Not Allowed to GET and DELETE on the MCP endpoint.
Respond 202 Accepted with no body to an accepted notification; on rejection return an HTTP error whose body MAY be a JSON-RPC error with no id.
Set X-Accel-Buffering: no on SSE responses (spec: SHOULD) and send SSE comment keepalives (':\r\n') on long-lived streams such as subscriptions/listen.
Verify: send a POST whose header version disagrees with the body version and confirm 400 + -32020; send a GET and confirm 405; send an unknown method and confirm 404 + -32601.
Known gotchas
These headers exist so gateways, proxies and WAFs can route and authorize without parsing the JSON body. If you terminate TLS at a proxy that strips unknown headers, requests will fail validation at the origin.
A server that must also serve pre-2025-06-18 clients MAY treat a MISSING MCP-Protocol-Version as 2025-03-26; otherwise a missing header MUST be rejected.
The HeaderMismatch code moved: it was -32001 in the draft and is -32020 in the final revision. Emitting -32001 is now non-conforming.
Clients MUST include both application/json and text/event-stream in Accept, and MUST support receiving either. Do not assume SSE.
Intermediaries that do not recognize an Mcp-Param-{Name} header MUST forward it and otherwise ignore it (RFC 9110) — do not strip unknown Mcp-Param-* headers in a proxy.
Closing the SSE response stream is the cancellation signal on Streamable HTTP. There are no client-to-server notifications defined over this transport in this revision; notifications/cancelled is stdio-only.
Streams are not resumable. A Last-Event-ID header SHOULD be ignored, and a broken stream means the client re-issues the request with a new request id — so make your handlers idempotent or safely retryable.
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claude mcp add --transport http waymark https://mcp.waymark.network/mcp
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