[Major-5] Migrate tasks to official extension #5683

Part of #5677.

Blocked by

• [Major-1] Dual-stack stateless core protocol #5679 — modern stateless lifecycle, server/discover, and per-request client metadata.
• [Major-4] Multi Round-Trip Requests (MRTR) and required resultType #5682 — the shared MRTR-aware response model and required resultType.
• [Minor-6] Extensions capability field + OpenTelemetry _meta propagation #5689 — extension capability negotiation and advertisement.

Protocol sources

• MCP 2026-07-28 core
• Tasks extension
• SEP-2663
• 2026-07-28 schema

The dated core schema is authoritative for core error codes: Missing Required Client Capability is -32021. Do not use the stale -32003 value currently shown in the Tasks extension draft.

Required behavior

• Tasks are the io.modelcontextprotocol/tasks extension, not a core capability.
• The client declares the extension in _meta.io.modelcontextprotocol/clientCapabilities.extensions on every relevant request; the server advertises it through server/discover.
• Task creation is server-directed, but a server MUST NOT return CreateTaskResult to a request that omitted the extension capability. If task support is required, return -32021.
• Only tools/call currently supports task-augmented execution.
• CreateTaskResult uses resultType: "task"; lifecycle acknowledgements and tasks/get use resultType: "complete".
• A task must be durably resolvable through tasks/get before its CreateTaskResult is returned.
• The client drives polling and honors pollIntervalMs; the gateway performs no polling loop.
• Lifecycle methods are tasks/get, tasks/update, and tasks/cancel. tasks/list and tasks/result do not exist.
• Streamable HTTP lifecycle requests use Mcp-Method and Mcp-Name: <taskId>.

Gateway work

• Advertise Tasks through the extension capability and enforce the per-request client declaration.
• Preserve the complete/MRTR/task-aware tools/call result union.
• Rewrite upstream task IDs to opaque backend-scoped handles before returning them downstream.
• Proxy each tasks/get, tasks/update, and tasks/cancel request to exactly one backend; restore the upstream task ID at that boundary.
• Preserve statuses, timestamps, TTL, changing polling interval, input requests/responses, final result/error payloads, and cooperative cancellation.
• Rewrite Mcp-Name from the exposed handle downstream to the upstream task ID upstream.
• Return -32602 for invalid or nonexistent task IDs and -32021 for missing per-request Tasks capability.
• Keep tasks/list, tasks/result, MCP sessions, and legacy SSE out of the modern dataplane path.

Implementation split

• [CF-DATAPLANE][TASKS]: Add stateless backend-scoped task handles #6185 — stateless backend-scoped task handles.
• [CF-DATAPLANE][TASKS]: Forward server-directed task results from tools/call #6187 — server-directed task results from tools/call.
• [CF-DATAPLANE][TASKS]: Proxy tasks/get, tasks/update, and tasks/cancel #6186 — tasks/get, tasks/update, and tasks/cancel proxying.
• [TESTING][CF-DATAPLANE][TASKS]: Add modern Tasks extension conformance coverage #6188 — modern Tasks extension conformance coverage.

Acceptance criteria

• Tasks are advertised only through io.modelcontextprotocol/tasks in server/discover.
• A request without the per-request Tasks capability never receives resultType: "task"; missing capability returns -32021.
• A task-capable tools/call preserves either its standard result, MRTR result, or CreateTaskResult.
• tasks/get, tasks/update, and tasks/cancel route through opaque stable handles without session affinity.
• Task fields, status-specific payloads, resultType, and routing headers are preserved at both boundaries.
• tasks/list and tasks/result return method-not-found on the modern path.
• Applicable official Tasks extension conformance scenarios pass.

Out of scope

• Python control-plane task handling and legacy protocol translation.
• MCP 2025-11-25 Tasks compatibility, legacy initialize, sessions, or SSE.
• Gateway-managed polling or a blocking tasks/result adapter.
• Optional notifications/tasks delivery unless separately scoped.