Implement SEP-2663: Tasks Extension #2189

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1. felixweinberger

   added this to 2026-07-28 Spec Implementationon May 29, 2026
2. github-project-automation

   moved this to Todo in 2026-07-28 Spec Implementationon May 29, 2026
3. mattzcarey commented on May 29, 2026

   

   mattzcarey

   on May 29, 2026

   Member

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   Status — SEP-2663 (Tasks Extension)

   Track: B — last in the dependency chain: needs #2188 (Extensions framework) and #2187 (MRTR) before this can really land.
   Spec: seps/2663-tasks-extension.md · merged spec#2663 2026-05-15

   What's done

   • main still carries the 2025-11 experimental tasks machinery (_taskManager, ExperimentalServerTasks, ExperimentalClientTasks, BaseContext.task, RequestOptions.task, processInbound*/processOutbound* interception). This is on its way out.
   • Closed exploration:
     • refactor(core)!: remove experimental tasks interception from Protocol (R0) #2004 (closed) — first attempt to remove experimental tasks interception from Protocol
     • refactor(core)!: remove experimental tasks interception from Protocol #2055 (closed) — second attempt; keeps storage interfaces (InMemoryTaskStore etc.)
     • feat(core,server,client)!: tasksPlugin() replaces 2025-11 interception; remove Experimental*Tasks classes (SEP-2663) #2066 (closed) — tasksPlugin({store}) as DispatchMiddleware registered via mcp.use(...); handles tasks/get|list|cancel|result; injects ctx.ext.task; taskContext(ctx) helper. Marks ExperimentalServerTasks.getTask/listTasks as @deprecated throw-stubs.

   Open PR

   • [SEP-2663] refactor!: remove 2025-11 experimental tasks #2128 — [SEP-2663] refactor!: remove 2025-11 experimental tasks (the v2-stateless stack base). Mechanical deletion (+272/-17461). Ships first so the rest of the stack doesn't have to preserve the old interception.

   Outstanding (acceptance criteria, summary)

   • New extension id io.modelcontextprotocol/tasks; capability negotiation via Implement SEP-2133: Extensions framework for MCP #2188
   • resultType: "task" on CreateTaskResult; server MUST NOT return it without the capability
   • Methods: tasks/get, tasks/update, tasks/cancel (poll-based; tasks/result removed → -32601; tasks/list removed)
   • Per-status response shapes: working / input_required / completed / cancelled / failed field requirements
   • Durable creation: tasks/get(id) resolves immediately after CreateTaskResult returned
   • notifications/cancelled MUST NOT cancel tasks; only tasks/cancel does (eventually-consistent, cooperative)
   • Tool-result isError: true → status completed (not failed); failed is for JSON-RPC errors
   • Legacy task param on CallToolRequest → silently ignored (not treated as opt-in)
   • tasks/get with invalid/unknown taskId → -32602
   • subscriptions/listen with taskIds filter without ext-tasks → -32003 with requiredCapabilities
   • Mcp-Name: <taskId> header on tasks/get/update/cancel over HTTP (overlap with SEP-2243 Implement SEP-2243: HTTP Standardization #2190)
   • Auth-binding evaluated per task request
   • Task IDs: 128-bit entropy / UUIDv4 default TaskStore
   • Client SHOULD deduplicate inputRequests keys across consecutive polls
   • Progress / log messages NOT delivered via subscriptions/listen for tasks

   Post-85ec3771 clarifications to fold in

   • spec#2756 (open) — specify -32003 error for missing client capability

   Related spec / open PRs to monitor

   • spec#2694 — Resumable Task Event Streams (post-release-1)
   • spec#2679 — Task streaming partial results (post-release-1)
   • spec#2669 — Task Interaction Methods (steer/pause/resume) (post-release-1)

   Coordination

   • Per the track plan: this is Track B's last item, picked up after MRTR (Implement SEP-2322: Multi Round-Trip Requests #2187) and Extensions (Implement SEP-2133: Extensions framework for MCP #2188) land.
   • The plugin shape (tasksPlugin via mcp.use(...) from feat(core,server,client)!: tasksPlugin() replaces 2025-11 interception; remove Experimental*Tasks classes (SEP-2663) #2066) is also the reference for how every other future extension will look — worth aligning on naming/API before merging.
4. felixweinberger

   assigned

   KKonstantinov

   on May 29, 2026
5. norika1207-lab commented on Jun 2, 2026

   

   norika1207-lab

   on Jun 2, 2026 · Hidden as spam

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6. felixweinberger

   moved this from Todo to In Progress in 2026-07-28 Spec Implementationon Jun 24, 2026
7. felixweinberger

   added

   triageQueued for automated analysis — bot will process and remove this labelQueued for automated analysis — bot will process and remove this label

   on Jul 9, 2026
8. ghost added

   enhancementRequest for a new feature that's not currently supportedRequest for a new feature that's not currently supported

   on Jul 9, 2026
9. ghost added

   needs decisionIssue is actionable, needs maintainer decision on whether to implementIssue is actionable, needs maintainer decision on whether to implement

   on Jul 9, 2026
10. ghost added

    P2Moderate issues affecting some users, edge cases, potentially valuable featureModerate issues affecting some users, edge cases, potentially valuable feature

    on Jul 9, 2026
11. ghost added

    v2Ideas, requests and plans for v2 of the SDK which will incorporate major changes and fixesIdeas, requests and plans for v2 of the SDK which will incorporate major changes and fixes

    on Jul 9, 2026
12. ghost removed

    triageQueued for automated analysis — bot will process and remove this labelQueued for automated analysis — bot will process and remove this label

    on Jul 9, 2026
13. howardjohn

    added a commit that references this issue on Jul 29, 2026

    mcp tasks support (#2745)

    ...

    f39387f
14. felixweinberger

    marked [v2] Tasks extension: tools/call rejects CreateTaskResult but accepts an omitted discriminator as complete #2637 as a duplicate of this issue on Sep 28, 2026
15. felixweinberger

    marked subscriptions/listen cannot carry extension notifications (blocks notifications/tasks) #2569 as a duplicate of this issue on Sep 28, 2026