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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Implement SEP-2567: Sessionless MCP via Explicit State Handles","articleBody":"# SEP-2567: Sessionless MCP via Explicit State Handles — rust-sdk implementation\n\n**Spec PR:** https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2567\n**Track:** Specification · **Stage:** final · **Priority:** P0 · **Theme:** Transport Evolution and Scalability\n**Needs code changes:** Yes (Large)\n\n## Summary\nRemoves protocol-level sessions, replacing implicit session-scoped state with explicit,\nserver-minted **state handles**. Builds on SEP-1442 (stateless-by-default) but argues the opt-in\nstateful path should not exist at all.\n\nWhat changes:\n- No `session/create`, `session/destroy`, or `Mcp-Session-Id` header.\n- `tools/list` / `resources/list` / `prompts/list` **MUST NOT** depend on per-connection or prior-tool-call state (so list endpoints become cacheable at `(deployment, auth)` granularity).\n- Stateful workflows use `create_*() -\u003e handle` + threaded parameters (guidance, not a protocol construct).\n\n## Why this needs code changes in rust-sdk\nThe streamable HTTP transport mints and threads a session id today, but a disable path already\nexists:\n- Server: `StreamableHttpServerConfig.stateful_mode` (default `true`); when `false` the server \"disables sessions entirely (stateless mode)\" per the doc comment, backed by the `SessionManager`/`SessionStore` trait (`session.rs`, with `Local…`/`Never…` stores and `RestoreOutcome`/`SessionRestoreMarker`). The session id rides on `HEADER_SESSION_ID`.\n- Client: `streamable_http_client.rs` carries `session_id: Arc\u003cstr\u003e` through requests and SSE reconnects.\n- List handlers live behind `service/server.rs` + `handler/server/router.rs`.\n\nSo \"no sessions\" is partly reachable via `stateful_mode = false`. The remaining work is: making that\npath the default/clean one for `V_2026_07_28`, ensuring list endpoints are state-independent, and\nremoving the client's hard dependency on a session id.\n\n## Proposed work\n- [ ] Ensure the `stateful_mode = false` server path neither requires nor emits `HEADER_SESSION_ID`, and verify the `Never…`-store path is the one selected.\n- [ ] Audit `tools/list` / `resources/list` / `prompts/list` handlers (`handler/server/router.rs`) so results don't depend on per-connection or prior-call state in the sessionless path (precondition for the SEP-2549 caching story).\n- [ ] Make the client's `session_id` optional (`Option\u003cArc\u003cstr\u003e\u003e`) so it can operate without one when the negotiated version is `V_2026_07_28`; keep sending it for older negotiated versions.\n- [ ] Keep `HEADER_SESSION_ID` handling for back-compat behind older protocol versions.\n- [ ] Add an example demonstrating the `create_*() -\u003e handle` pattern (state handles are app-level, not a protocol construct).\n\n## Affected areas\n`transport/streamable_http_server/session.rs`, `transport/streamable_http_server/tower.rs`,\n`transport/streamable_http_client.rs`, `service/server.rs`, `handler/server/router.rs`.\n\n## Notes / risks\n- Overlaps heavily with SEP-2575. Treat as one transport-statelessness workstream; this SEP is the application-state half, 2575 is the handshake half.\n- Making client `session_id` optional touches several `Arc\u003cstr\u003e` call sites in `streamable_http_client.rs` (reconnect/get-stream paths) — mechanical but broad.\n\n---\n## Related existing issues\n- #863 — Expose negotiated `Mcp-Session-Id` (sessionless work changes how session identity is surfaced)\n- #526 — SEP-1442 Make MCP Stateless (same transport-statelessness workstream)\n","author":{"url":"https://github.com/alexhancock","@type":"Person","name":"alexhancock"},"datePublished":"2026-06-02T16:01:56.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/870/rust-sdk/issues/870"}