Analyzed the official SEP-1686 specification against both the MCP SDK's draft implementation (PR #1645) and FastMCP's current shims. Made corrections to match the spec exactly.

**Key changes:**

1. **Removed `error` field** - Spec only defines `statusMessage` for error details. Changed all handlers to use `statusMessage` instead of separate `error` field.

2. **Removed non-spec status values** - Spec defines exactly 5 statuses: `working`, `input_required`, `completed`, `failed`, `cancelled`. Removed FastMCP's `"submitted"` and `"unknown"` extensions.

3. **Non-existent tasks raise errors** - Aligned with SDK behavior: `tasks/get` for non-existent/deleted tasks raises `ValueError` (JSON-RPC error) instead of returning synthetic `status="unknown"`.

4. **Test updates** - Fixed 12+ tests expecting removed statuses. Changed assertions to expect JSON-RPC errors for not-found scenarios.

**What stayed the same:**

- Client already sends both `task=` (spec-compliant) and `_meta=` (SDK compatibility)
- Server monkeypatches work correctly for request params
- `createdAt` as ISO 8601 string matches spec (SDK uses datetime but serializes same)
- `ttl` field naming confirmed correct in both spec and SDK

All 3270 tests passing. FastMCP is now fully aligned with SEP-1686 final specification.

Related: modelcontextprotocol/python-sdk#1645

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Co-Authored-By: Claude <noreply@anthropic.com>