Implement SEP-2575: Make MCP Stateless

[felixweinberger]

This is a tracking issue for implementation of SEP-2575.

[mattzcarey]

Status — SEP-2575 (Make MCP Stateless)

Track: A (core, sequential spine) — biggest single piece of work in release-1 (~43 wire + 4 sdk + 4 guidance requirements). Tangled with SEP-2567 (#2185).
Spec: seps/2575-stateless-mcp.md · merged spec#2575 2026-05-11

What's done

• Nothing in main yet (no _meta.protocolVersion enforcement, no server/discover, no removal of initialize/logging.setLevel/etc.).
• Felix's exploratory closed PRs (re-stacked into [SEP-2663] refactor!: remove 2025-11 experimental tasks #2128–[SEP-2575][SEP-2567] 2026-06 stateless support: stdio + InMemory transports #2132):
  • feat(core): readMetaRequestScope lifts _meta into RequestEnv (SEP-2575) #2063 (closed) — readMetaRequestScope lifts _meta into RequestEnv
  • feat(server): server/discover handler; per-request clientCapabilities resolution (SEP-2575) #2064 (closed) — server/discover handler + per-request clientCapabilities
  • feat(client): MRTR retry loop, ClientTransport, version-gate notifications/initialized (SEP-2322/2575) #2065 (closed) — client MRTR retry loop + ClientTransport + version-gated notifications/initialized
  • feat(core): deprecate sessionId on base Transport/BaseContext; legacySessionId() helper (SEP-2567) #2060 (closed) — deprecate sessionId on base Transport / BaseContext

Open PR stack (Felix, v2-stateless label) — the active line of work

1. [SEP-2663] refactor!: remove 2025-11 experimental tasks #2128 — [SEP-2663] refactor!: remove 2025-11 experimental tasks (mechanical, base of the stack)
2. chore: sync schemas to spec @142b3c3c; segregate pre-2026 wire schemas #2129 — chore: sync schemas to spec @142b3c3c; segregate pre-2026 wire schemas (mechanical schema regen)
3. refactor(core): extract Dispatcher; Protocol composes it #2130 — refactor(core): extract Dispatcher; Protocol composes it (zero-Δ refactor)
4. [SEP-2575][SEP-2322][SEP-2567] 2026-06 stateless support over StreamableHTTP #2131 — [SEP-2575][SEP-2322][SEP-2567] 2026-06 stateless support over StreamableHTTP ← the substantive review (+4229/-1047)
5. [SEP-2575][SEP-2567] 2026-06 stateless support: stdio + InMemory transports #2132 — [SEP-2575][SEP-2567] 2026-06 stateless support: stdio + InMemory transports

Outstanding (acceptance criteria, summary)

• Every request _meta carries protocolVersion / clientInfo / clientCapabilities (io.modelcontextprotocol/* keys)
• MCP-Protocol-Version header MUST match body or 400
• Mandatory server/discover RPC
• Removed RPCs: logging/setLevel, resources/subscribe/unsubscribe, ping, roots/list, notifications/roots/list_changed
• GET endpoint removed (POST only); no batching; no Last-Event-ID resumability
• New UnsupportedProtocolVersionError (custom code, see fix spec#2716)
• subscriptions/listen (single long-lived POST stream, opt-in types, ack-first, io.modelcontextprotocol/subscriptionId in _meta)
• Per-request _meta.logLevel; servers MUST NOT emit notifications/message without it
• Dual-era stdio probe: server/discover first → fall back to initialize on -32601
• Per-message protocol-version routing (legacy / modern paths kept separate — see §7.1 of the spec page)
• Per-request auth (no auth-bypass through removal of initialize)

Related work in main

• fix(server): reject initialize with mismatched MCP-Protocol-Version header #2111 (fix(server): reject initialize with mismatched MCP-Protocol-Version header, by @dparikh79) — fixes legacy-path bug Streamable HTTP server accepts mismatched MCP-Protocol-Version header and body protocolVersion on initialize #2108. Useful as the wedge for header-vs-body version validation; will likely be subsumed by the modern dispatch path.

Post-85ec3771 clarifications to fold in

• spec#2716 — custom error code for UnsupportedProtocolVersionError
• spec#2791 — incorporates SEP-2596 / SEP-1865; clarifies Streamable HTTP
• spec#2776 (open) — consolidates back-compat rule + broadens stdio fallback
• spec#2770 (open) — clarifies stdio process lifetime

Test backlog

• [DRAFT] test(e2e): 2026-06 spec backlog — SEP-2260 + SEP-2575/2567 as expected-fail stubs #2180 (DRAFT) — SEP-2575 acceptance criteria as known-failure e2e stubs (currently 63 new requirements for 2260 + 2575/2567 combined)

Coordination

Per the track plan: this issue + #2185 are sequential and must land together. Two cross-track coordination moments:

1. Agreeing the transport seam when envelope enforcement starts (Track B is building 2243/headers/subscriptions against this contract).
2. The MRTR public API decision in Implement SEP-2322: Multi Round-Trip Requests #2187 (Track A also).

Dupes

#2181 is a duplicate of this issue — should be closed.