SEP-2243: HTTP Header Standardization for Streamable HTTP Transport

[caomengxuan666]

Description

Per SEP-2243 (HTTP Standard Headers), MCP clients MUST send the following HTTP headers on POST requests:

• Mcp-Method: Set to the JSON-RPC method value from the request body (REQUIRED on every POST)
• Mcp-Name: Set from params.name (for tools/call, prompts/get) or params.uri (for resources/read, resources/subscribe, resources/unsubscribe) when present

Context

The cxxmcp (C++ SDK) conformance testing revealed that the official conformance test suite's reference client (TypeScript SDK) does not send these headers, making the conformance suite self-contradictory. See:

• typescript-sdk#2176 — TypeScript SDK bug report
• conformance#323 — Conformance suite self-contradiction
• typescript-sdk#2178 — Fix PR for TypeScript SDK

Impact

Servers that enforce SEP-2243 (reject missing Mcp-Method with HTTP 400) are incompatible with clients that don't send these headers. This affects cross-SDK interoperability.

Request

Please implement Mcp-Method and Mcp-Name header support per SEP-2243 to ensure cross-SDK compatibility.

[nikita-kibitkin]

I can take the Mcp-Method client-side part of this.

#994 already covers Mcp-Name for Streamable HTTP, but Mcp-Method is still not emitted by HttpClientStreamableHttpTransport. I would add the HttpHeaders constant and set Mcp-Method on every Streamable HTTP POST from the JSON-RPC request/notification method, next to the existing MCP-Protocol-Version handling, with tests around initialize and a regular request/notification.

I would keep server-side validation out of this PR and leave it as a follow-up, since #994 already starts touching that area for Mcp-Name and I do not want to mix client emission with server enforcement in the same small change. If #994 merges first, I will rebase on top of it.

Does that split work, or would you prefer Mcp-Method to be folded into #994 instead?