[Server][Transport] SEP-2322: Multi Round-Trip Requests on Streamable HTTP #342

Implements SEP-2322 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #334.

Spec summary

Introduces a transport mechanism for multi round-trip exchanges within a single logical request (e.g. server → client → server → client → ...). Replaces session-scoped server-initiated requests with a per-request "conversation". Schema additions correlate sub-requests/responses to a parent request.

PHP SDK changes

• src/Server/Transport/StreamableHttpTransport.php — POST handler must support multi-turn streams (keep connection open for nested round trips within a single logical request), separate from messages/listen.
• src/Server/Protocol.php — Fiber suspension currently relies on session-stored __pending_responses. Refactor to associate pending responses with the request rather than the session.
• src/JsonRpc/MessageFactory.php — new envelope fields for parent-request correlation.
• src/Client/Transport/HttpTransport.php and src/Client/Protocol.php — handle inline server requests on the response stream.
• New schemas in src/Schema/ to describe MRTR envelope.

Breaking change. Companion to SEP-2567 (sessionless) — they must land together.

Related

• SEP-2575 / SEP-2567 (umbrella [2026-07-28] Stateless Core overhaul #334)
• Conformance scenarios: modelcontextprotocol/conformance#188