Everything-server rejects valid _meta on stateful initialize

[lucarlig]

Problem

The TypeScript everything-server misclassifies a stateful initialize request as stateless whenever params._meta is present. It rejects a valid 2025-11-25 request with HTTP 400 and -32020 Missing MCP-Protocol-Version header.

InitializeRequestParams extends RequestParams in the 2025-11-25 schema, so _meta is allowed on initialize. MCP Python SDK 2.x currently emits _meta: {} there, which makes the conformance fixture unusable as an upstream server for that client.

Reproduction

Start the fixture:

npx tsx examples/servers/typescript/everything-server.ts

Then send:

curl -i -X POST http://127.0.0.1:3000/mcp \
  -H 'Accept: application/json, text/event-stream' \
  -H 'Content-Type: application/json' \
  --data '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"probe","version":"1"},"_meta":{}}}'

Observed:

{"jsonrpc":"2.0","id":1,"error":{"code":-32020,"message":"Missing MCP-Protocol-Version header"}}

The same request succeeds when _meta is omitted.

Suggested fix

Route initialize through the stateful transport when params.protocolVersion names a session-era protocol, independently of whether optional request metadata is present. Add an integration regression test that sends _meta: {} to the everything-server and asserts a successful initialize response.

[po-et]

Reproduced on main (7169291). The classification keys on whether _meta is present, but _meta is base-shape in every revision, so any session-era request that carries it lands on the 2026-07-28 path. Besides the -32020 you hit, the same line answers -32602 when the session-era header is present — same root cause, second symptom.

Sending a PR that keys on the header's revision plus the io.modelcontextprotocol/protocolVersion marker inside _meta instead. All 55 server scenarios still pass against the fixture, and the -32020/-32602 rejections the envelope is there to trigger still fire.