[Proposal] SEP-2663: Tasks Extension architecture & migration design (#817) #1003

Proposal: SEP-2663 Tasks Extension Architecture & Migration Design (#817)

Motivation & Context

Tracking implementation of SEP-2663 in kotlin-sdk for the 2026-07-28 MCP specification release (aligned with umbrella issue #842).

The experimental tasks mechanism specified in 2025-11-25 served as an initial asynchronous execution mechanism for tool calls. Real-world experience surfaced critical structural issues:

1. Fragile Handshake: Method-level capabilities (tasks.requests.tools.call) combined with per-tool execution.taskSupport required clients to prime cache with tools/list before issuing task requests, preventing isomorphic request handling.
2. The tasks/result Blocking Trap: Forcing clients to open a long-lived blocking stream on tasks/result to receive out-of-band events conflicts with SEP-2260 (prohibiting unsolicited server-to-client requests) and modern streamable transports.
3. Undefinable Scoping under SEP-2567: With sessions removed from the core specification, server-wide tasks/list cannot be safely scoped or authorized without bespoke perimeter isolation, risking data leakage across unrelated callers.
4. No Client-Hosted Tasks: Under SEP-2260, client-hosted sampling/elicitation tasks would require unsolicited polling from server to client, making client-hosted tasks inexpressible.

SEP-2663 formally moves Tasks into an official MCP Extension under the SEP-2133 Extensions Framework (io.modelcontextprotocol/tasks), streamlining the polling lifecycle to tasks/get, tasks/update, and tasks/cancel, while directly embedding final results in the Task object.

This proposal outlines the Kotlin Multiplatform architecture to migrate kotlin-sdk from legacy experimental tasks to the SEP-2663 Tasks Extension across kotlin-sdk-core, kotlin-sdk-server, and kotlin-sdk-client.

Proposed Architecture & Component Design

1. Extensions Framework Registration (kotlin-sdk-core)

Under the SEP-2133 Extensions Framework:

• Define the official extension identifier:

  public object TasksExtension {
      public const val EXTENSION_ID: String = "io.modelcontextprotocol/tasks"
      public const val EXTENSION_VERSION: String = "1.0.0"
  }

• Client and server capability declarations register support via extensions:

  // In ClientCapabilities / ServerCapabilities
  val extensions: Map<String, JsonElement>? = mapOf(
      TasksExtension.EXTENSION_ID to buildJsonObject {
          put("version", TasksExtension.EXTENSION_VERSION)
      }
  )

• Server-directed task creation: When the client advertises TasksExtension.EXTENSION_ID, the server may return a task handle for any tool execution without requiring per-tool flags.

2. Wire Data Models & Discriminators (kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/tasks.kt)

Migrate and expand tasks.kt:

• Task Status Enumeration:

  @Serializable
  public enum class TaskStatus {
      @SerialName("working") WORKING,
      @SerialName("input_required") INPUT_REQUIRED,
      @SerialName("completed") COMPLETED,
      @SerialName("failed") FAILED,
      @SerialName("cancelled") CANCELLED
  }

• Redesigned Task Entity:
  Final payloads and errors are carried directly within the task representation, eliminating the separate tasks/result stream:

  @Serializable
  public data class Task(
      val taskId: String,
      val status: TaskStatus,
      val statusMessage: String? = null,
      val createdAt: String,
      val lastUpdatedAt: String,
      val ttl: Long? = null,
      val pollInterval: Long? = null,
      val result: JsonElement? = null,
      val error: JsonRpcError? = null,
      val inputRequests: List<ServerRequest>? = null,
      @SerialName("_meta")
      override val meta: JsonObject? = null,
  ) : WithMeta

• Polymorphic Tool Call Response:
  tools/call results can return either standard content or a task handle (resultType: "task"):

  @Serializable
  public sealed interface CallToolResponse : ClientResult {
      public val meta: JsonObject?
  }
  
  @Serializable
  @SerialName("call")
  public data class CallToolResult(
      val content: List<ContentBlock> = emptyList(),
      val isError: Boolean = false,
      @SerialName("_meta")
      override val meta: JsonObject? = null,
  ) : CallToolResponse
  
  @Serializable
  @SerialName("task")
  public data class CallToolTaskResult(
      val task: Task,
      @SerialName("_meta")
      override val meta: JsonObject? = null,
  ) : CallToolResponse

• Lifecycle Methods:
  • tasks/get: Retain GetTaskRequest / GetTaskResult for status polling.
  • tasks/update: Add UpdateTaskRequest(taskId, inputResponses) allowing clients to respond to multi round-trip requests (SEP-2322) when task status is input_required.
  • tasks/cancel: Retain CancelTaskRequest / CancelTaskResult.
  • Deprecation / Removal:
    • Mark legacy GetTaskPayloadRequest (tasks/result) as @Deprecated(message = "Removed in SEP-2663; result is now embedded in Task", level = DeprecationLevel.WARNING).
    • Mark legacy ListTasksRequest (tasks/list) as @Deprecated(message = "Removed in SEP-2663 per SEP-2567 sessionless scoping", level = DeprecationLevel.WARNING).

3. Server Task Management & Execution (kotlin-sdk-server)

• Task Runner & Storage SPI:
  Define a thread-safe task store interface with coroutine job lifecycle management:

  public interface TaskStore {
      public suspend fun get(taskId: String): Task?
      public suspend fun save(task: Task)
      public suspend fun update(taskId: String, block: (Task) -> Task): Task
      public suspend fun cancel(taskId: String): Boolean
  }
  
  public class InMemoryTaskStore(
      private val clock: Clock = Clock.System
  ) : TaskStore { ... }

• Server Handler Integration:
  Allow tool handlers to yield execution to a background task:

  public suspend fun ServerSession.launchTask(
      pollInterval: Long = 1000L,
      ttl: Long? = 3600_000L,
      block: suspend (TaskExecutionContext) -> JsonElement
  ): CallToolTaskResult

  The context automatically coordinates coroutine cancellation with tasks/cancel and stores intermediate status messages.

4. Client Polling & Ergonomics (kotlin-sdk-client)

• High-Level Polling Utility:
  Provide an ergonomic suspend extension on ClientSession:

  public suspend fun ClientSession.awaitTaskCompletion(
      taskId: String,
      timeout: Duration = 5.minutes,
      pollInterval: Duration? = null,
      onInputRequired: (suspend (Task, List<ServerRequest>) -> List<ClientResult>)? = null
  ): Task

  • Automatically respects server's pollInterval from Task.pollInterval if specified.
  • Automatically dispatches tasks/update when input_required is observed and a handler is provided.
  • Times out cleanly with TimeoutCancellationException.

5. Conformance & Verification Plan

• Unit Tests (kotlin-sdk-core):
  • Serialization / deserialization of polymorphic CallToolResponse (CallToolResult vs CallToolTaskResult).
  • Wire validation of tasks/update and updated Task model containing result and inputRequests.
• Functional Tests (kotlin-sdk-server & kotlin-sdk-client):
  • End-to-end task execution: client initiates tool call, receives task handle, polls tasks/get until COMPLETED, and accesses embedded result.
  • Multi round-trip task update: working -> input_required -> tasks/update -> completed.
  • Cancellation test: client issues tasks/cancel, server cancels underlying Kotlin coroutine Job, task transitions to CANCELLED.
  • TTL eviction: verified using virtual time.
• Official Conformance Suite:
  • Validate against Feat/tasks mrtr extension conformance#262 scenarios for SEP-2663.

Next Steps

Upon review and alignment on this migration design:

1. Land core data models (Task, CallToolResponse, UpdateTaskRequest) in kotlin-sdk-core.
2. Implement TaskStore and server-side task execution runners in kotlin-sdk-server.
3. Provide high-level awaitTaskCompletion client helpers in kotlin-sdk-client.
4. Validate against the official conformance test suite.

AI assistance disclosure: AI was used to discover this opportunity and draft the change or text. The submission was checked against the prepared artifact and recorded verification evidence.