Description

The StreamableHTTPClientTransport does not send Mcp-Method or Mcp-Name HTTP headers on POST requests. Per SEP-2243 (HTTP Standard Headers), these headers are MUST for clients:

Clients MUST include the Mcp-Method header on every POST request, set to the JSON-RPC method value from the request body.

Clients MUST include the Mcp-Name header when the request carries a params.name (tools/call, prompts/get) or params.uri (resources/read, resources/subscribe, resources/unsubscribe).

Current behavior

In streamableHttp.js, the _commonHeaders() method (line 58) only sets:

• Authorization (if auth provider)
• mcp-session-id (if session exists)
• mcp-protocol-version (if protocol version known)

No Mcp-Method or Mcp-Name headers are sent.

Expected behavior

The POST path in sendMessages() should add:

• Mcp-Method: <method> for every request
• Mcp-Name: <name|uri> when params.name or params.uri is present

Impact

This causes the MCP conformance suite (http-header-validation scenario) to fail when testing servers that implement SEP-2243 header validation. Specifically, the ServerRejectsMissingMethodHeader test expects servers to reject requests missing Mcp-Method, but the TypeScript SDK never sends it.

Servers that enforce SEP-2243 (reject missing Mcp-Method with HTTP 400) are incompatible with the current TypeScript SDK client.

Reference

• SEP-2243 spec: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/docs/specification/2025-06-18/basic/transports.mdx#L488
• Conformance test: src/scenarios/server/http-standard-headers.ts → ServerRejectsMissingMethodHeader
• C++ SDK already implements this: Mcp-Method and Mcp-Name headers are sent on all POST requests