feat(tasks): add task streaming with partial result notifications

[morozow]

Reference implementation for SEP-2679 — Task Streaming via Partial Results for Long-Running Operations.

Referenced SEP: modelcontextprotocol/modelcontextprotocol#2679

Motivation

SEP-1686 (Tasks) introduced a "call-now, fetch-later" mechanism for long-running MCP operations but has no standard way to deliver incremental content during execution. This forces agent-to-agent calls to block for 10–30s with no feedback, degrades IDE streaming UX to a spinner, and adds polling overhead.

This PR implements notifications/tasks/partial — a new JSON-RPC notification that enables servers to push incremental content chunks to clients while a task is in progress, extending the existing Tasks infrastructure without transport-level changes.

Changes

Core package (packages/core/)

File	Change
src/types/schemas.ts	TaskPartialNotificationParamsSchema, TaskPartialNotificationSchema; extended ServerTasksCapabilitySchema and ClientTasksCapabilitySchema with streaming.partial; extended ToolExecutionSchema with streamPartial; added to ServerNotificationSchema union
src/types/types.ts	Inferred TaskPartialNotificationParams, TaskPartialNotification types
src/types/spec.types.ts	Hand-written conformance types; updated ServerCapabilities, ClientCapabilities, ToolExecution, ServerNotification
src/types/specTypeSchema.ts	Schema registry entries
test/spec.types.test.ts	Type conformance assertions
test/types/taskPartialNotification.test.ts	Schema validation + property-based round-trip tests (fast-check)

Server package (packages/server/)

File	Change
src/experimental/tasks/mcpServer.ts	sendTaskPartial(taskId, content, seq) with Zod validation, terminal status check, capability gating; createPartialEmitter(taskId) with auto-incrementing seq; auto capability declaration in registerToolTask when streamPartial: true
test/server/taskPartialNotification.test.ts	Unit tests for sendTaskPartial, createPartialEmitter, capability declaration + property-based seq auto-increment test

Client package (packages/client/)

File	Change
src/experimental/tasks/client.ts	subscribeTaskPartials(taskId, handler) with cleanup function; notification handler for notifications/tasks/partial; seq-based ordering, duplicate detection, gap warning
test/client/taskPartialNotification.test.ts	Subscription, routing, seq ordering tests + property-based routing and dedup tests

Integration tests (test/integration/)

File	Change
test/experimental/tasks/taskPartialStreaming.test.ts	Full lifecycle via InMemoryTransport; edge cases (capability mismatch, terminal status, seq gaps)

Examples

File	Description
examples/server/src/simpleTaskStreaming.ts	Server simulating LLM token generation with partial results
examples/client/src/simpleTaskStreamingClient.ts	Client consuming streaming partials and retrieving canonical result

Design Decisions

• sendTaskPartial and createPartialEmitter on ExperimentalMcpServerTasks — keeps experimental surface isolated, follows existing registerToolTask pattern
• createPartialEmitter(taskId) returns a standalone function — compatible with the fire-and-forget ToolTaskHandler.createTask pattern where background work runs after the handler returns
• Presence-based capability ("partial": {}) — matches the existing pattern used by list, cancel, requests in task capabilities
• Zod-based validation in sendTaskPartial — uses TaskPartialNotificationParamsSchema.safeParse() for consistent error messages
• Client seq tracking per subscription — scoped to each subscribeTaskPartials call, resets on re-subscribe

Test Results

packages/core:    22 files,  587 tests ✓
packages/server:   7 files,   83 tests ✓
packages/client:  11 files,  378 tests ✓
test/integration: 17 files,  433 tests ✓
typecheck: all packages clean ✓

Zero regressions against existing tests. Property-based tests run 100 iterations each via fast-check.

Checklist

• Builds without errors (pnpm build:all)
• All existing tests pass
• New unit tests added for all new functionality
• Property-based tests for correctness properties
• Integration tests for end-to-end lifecycle
• Type conformance tests (Zod-inferred ↔ spec.types.ts)
• Typecheck clean across all packages
• Examples included with wire-format traces
• No transport-level changes required
• Backward compatible — additive changes only

AI Disclosure

This PR was developed with AI assistance (Claude via Kiro IDE). AI was used for codebase analysis, implementation of types/schemas/server/client APIs, and test generation. All architectural decisions (API placement, emitter pattern, capability format, seq ordering semantics) and protocol design were made by the human author.

[changeset-bot]

🦋 Changeset detected

Latest commit: d581c0f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@modelcontextprotocol/core	Minor
@modelcontextprotocol/server	Minor
@modelcontextprotocol/client	Minor
@modelcontextprotocol/node	Major
@modelcontextprotocol/express	Major
@modelcontextprotocol/fastify	Major
@modelcontextprotocol/hono	Major

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commit: d581c0f

[stomde]

Good job merge STOMDE