[2026-07-28] Stateless Core overhaul

[chr-hertel]

Tracking issue for the MCP Spec 2026-07-28 release — Stateless Core overhaul milestone.

These four SEPs are tightly coupled and should land together to avoid inconsistent intermediate states. They constitute the biggest design impact of the 2026-07-28 release for the PHP SDK: the initialize/initialized handshake disappears, sessions stop being protocol-level, and Fiber-based server→client requests must be scoped per client request.

SEPs covered

SEP	Title	Spec PR
SEP-2575	Make MCP Stateless	#2575
SEP-2567	Sessionless MCP via Explicit State Handles	#2567
SEP-2322	Multi Round-Trip Requests	#2322
SEP-2260	Server requests scoped to client request	#2260

Sub-issues

• [Server][Client] SEP-2575: Stateless protocol (per-request version, server/discover, messages/listen) #340 — SEP-2575: Stateless protocol (per-request version, server/discover, messages/listen)
• [Server] SEP-2567: Remove session dependency for list endpoints; deprecate Mcp-Session-Id #341 — SEP-2567: Remove session dependency for list endpoints; deprecate Mcp-Session-Id
• [Server][Transport] SEP-2322: Multi Round-Trip Requests on Streamable HTTP #342 — SEP-2322: Multi Round-Trip Requests on Streamable HTTP
• [Server][Protocol] SEP-2260: Scope server-to-client requests to in-flight client requests #343 — SEP-2260: Scope server→client requests to in-flight client requests

Notes

• All four SEPs are merged into the spec repo as of this filing.
• Breaking changes — coordinate with deprecation policy (SEP-2596, see Milestone F umbrella [2026-07-28] Governance: deprecations, lifecycle, conformance, SDK tier #339).
• Touches: src/Server/Protocol.php, src/Server/Session/*, src/Server/Transport/StreamableHttpTransport.php, src/Client/Protocol.php, src/Schema/Request/InitializeRequest.php.