## Motivation and Context
When an `initialize` POST carries an `MCP-Protocol-Version` HTTP header that disagrees with
`params.protocolVersion` in the JSON-RPC body, the Streamable HTTP server accepts the request
and negotiates using the body value. The behavior was already encoded in `handle_post`
(the header check is skipped on `initialize`) but it was not stated in the code and there was
no regression test for the specific mismatch case.
MCP 2025-06-18 / 2025-11-25 only requires `MCP-Protocol-Version` on requests subsequent to
`initialize`, and the spec does not require the header and body to agree on the initial `initialize`.
All official MCP SDKs (TypeScript, Python, Rust, Java, Go, PHP) accept the mismatch and
treat the JSON-RPC body as authoritative.
## How Has This Been Tested?
Streamable HTTP tests: two new regression tests confirm that on an `initialize` POST
with a supported but mismatched `MCP-Protocol-Version` header, the server returns
HTTP 200 and the response's `result.protocolVersion` matches the JSON-RPC body value,
in both mismatch directions (header older / body newer, and header newer / body older).
## Breaking Changes
None. The behavior is unchanged: `initialize` requests with a mismatched `MCP-Protocol-Version` header
are still accepted, and the JSON-RPC body `params.protocolVersion` continues to drive negotiation.
This commit only adds an explanatory comment and regression tests.
Closes#351.
711f064088