[Server] Parallel elicitations on one session get each other's answers #56

Two tool calls on one session that both elicit get elicitation/create with the same request id (1000). The client answers both, the answers overwrite each other in _mcp.responses.1000 and one call resumes with the other call's answer. The other one fails with -32603 ("Transport returned an unexpected payload; expected a Response or Error message.").

On top, an elicitation is sent on another call's SSE stream - all streams drain one session-wide outgoing queue.

Happens on every runtime (php -S, FPM, Apache, FrankenPHP classic & worker, LB + Redis), on main and v0.8.1. One elicitation per session across parallel sessions is fine.

bin/up.sh fpm worker
cd harness && uv run gauntlet run -t fpm,worker -s concurrency.parallel-elicitation

Cause:

• the request id counter is read-modify-written in the session: https://github.com/modelcontextprotocol/php-sdk/blob/9591ad292788f717e8785b282edc2292850a98d4/src/Server/Protocol.php#L447-L449
• one outgoing queue per session, any stream drains all of it: https://github.com/modelcontextprotocol/php-sdk/blob/9591ad292788f717e8785b282edc2292850a98d4/src/Server/Protocol.php#L538, https://github.com/modelcontextprotocol/php-sdk/blob/9591ad292788f717e8785b282edc2292850a98d4/src/Server/Protocol.php#L563

modelcontextprotocol#556 made each stream poll only its own awaited id - correct, but doesn't help once two streams await the same id.

Options: ids that can't collide across processes (random or per stream), and per-stream outgoing queues - or writing server requests inline like modelcontextprotocol#535 does for responses.