feat(task): route server-originated tasks/* to ClientHandler (SEP-1686)

Tasks are bidirectional per SEP-1686: either party can be the requestor
or the receiver. The ServerHandler side is already wired for client→server
task flow (tools/call augmentation). This patch mirrors the same wiring
on the client side so servers that initiate task-augmented requests
(notably sampling/createMessage and elicitation/create) can follow up
with tasks/get, tasks/list, tasks/result, and tasks/cancel directed at
the client.

Changes, purely additive:

* ServerRequest: add GetTaskInfoRequest | ListTasksRequest |
  GetTaskResultRequest | CancelTaskRequest variants. Add a
  ServerRequest::method() accessor mirroring ClientRequest::method().
  Update the variant_extension! invocation so the existing
  GetExtensions / GetMeta impls cover the new variants.
* ClientResult: add ListTasksResult | GetTaskResult |
  GetTaskPayloadResult | CancelTaskResult response variants.
  GetTaskPayloadResult retains its existing custom Deserialize-fails
  behavior, so payload responses are still observed on the wire as
  CustomResult (matching the server-side pattern).
* ClientHandler: add list_tasks, get_task_info, get_task_result,
  and cancel_task methods with default -32601 Method-not-found impls,
  mirroring the server-side signatures. Propagate via the Box/Arc
  wrapper macro. Dispatch all four from the handle_request match.

This unblocks clients that want to advertise
capabilities.tasks.requests.sampling.createMessage,
capabilities.tasks.requests.elicitation.create, or the client-side
tasks.list / tasks.cancel capabilities: previously, servers had no
way to reach the client's task methods through the typed request enum,
and such capabilities couldn't be honored end-to-end.

Tests: new test_task_client_receiver.rs exercises a full bidirectional
roundtrip for each of the four methods (server → client RPC →
ClientHandler → response → server), plus a default-impl test that
confirms the unit () client returns -32601 for tasks/get. Existing
message-schema golden files regenerated to include the new ServerRequest
and ClientResult variants; no other tests affected.

Related: #528, #536 (which added the server-side half of SEP-1686).

Author: TheUnderdev

crates/rmcp/Cargo.toml

@@ -283,6 +283,11 @@ name = "test_custom_request"
 required-features = ["server", "client"]
 path = "tests/test_custom_request.rs"
 
+[[test]]
+name = "test_task_client_receiver"
+required-features = ["server", "client"]
+path = "tests/test_task_client_receiver.rs"
+
 [[test]]
 name = "test_prompt_macros"
 required-features = ["server", "client"]

crates/rmcp/src/handler/client.rs

@@ -30,6 +30,22 @@ impl<H: ClientHandler> Service<RoleClient> for H {
                 .create_elicitation(request.params, context)
                 .await
                 .map(ClientResult::CreateElicitationResult),
+            ServerRequest::ListTasksRequest(request) => self
+                .list_tasks(request.params, context)
+                .await
+                .map(ClientResult::ListTasksResult),
+            ServerRequest::GetTaskInfoRequest(request) => self
+                .get_task_info(request.params, context)
+                .await
+                .map(ClientResult::GetTaskResult),
+            ServerRequest::GetTaskResultRequest(request) => self
+                .get_task_result(request.params, context)
+                .await
+                .map(ClientResult::GetTaskPayloadResult),
+            ServerRequest::CancelTaskRequest(request) => self
+                .cancel_task(request.params, context)
+                .await
+                .map(ClientResult::CancelTaskResult),
             ServerRequest::CustomRequest(request) => self
                 .on_custom_request(request, context)
                 .await
@@ -191,6 +207,68 @@ pub trait ClientHandler: Sized + Send + Sync + 'static {
         )))
     }
 
+    /// Handle a `tasks/list` request from a server. Only relevant when the
+    /// client is also a task *receiver* (e.g. it accepted a task-augmented
+    /// `sampling/createMessage` or `elicitation/create` request).
+    ///
+    /// # Default Behavior
+    /// Returns `-32601` (Method not found). Clients that advertise
+    /// `capabilities.tasks.list` must override this.
+    fn list_tasks(
+        &self,
+        request: Option<PaginatedRequestParams>,
+        context: RequestContext<RoleClient>,
+    ) -> impl Future<Output = Result<ListTasksResult, McpError>> + MaybeSendFuture + '_ {
+        let _ = (request, context);
+        std::future::ready(Err(McpError::method_not_found::<ListTasksMethod>()))
+    }
+
+    /// Handle a `tasks/get` request from a server. Only relevant when the
+    /// client is also a task *receiver* (e.g. it accepted a task-augmented
+    /// `sampling/createMessage` or `elicitation/create` request).
+    ///
+    /// # Default Behavior
+    /// Returns `-32601` (Method not found). Clients that advertise
+    /// `capabilities.tasks.requests.sampling.createMessage` or
+    /// `capabilities.tasks.requests.elicitation.create` must override this.
+    fn get_task_info(
+        &self,
+        request: GetTaskInfoParams,
+        context: RequestContext<RoleClient>,
+    ) -> impl Future<Output = Result<GetTaskResult, McpError>> + MaybeSendFuture + '_ {
+        let _ = (request, context);
+        std::future::ready(Err(McpError::method_not_found::<GetTaskInfoMethod>()))
+    }
+
+    /// Handle a `tasks/result` request from a server. Only relevant when
+    /// the client is also a task *receiver*.
+    ///
+    /// # Default Behavior
+    /// Returns `-32601` (Method not found).
+    fn get_task_result(
+        &self,
+        request: GetTaskResultParams,
+        context: RequestContext<RoleClient>,
+    ) -> impl Future<Output = Result<GetTaskPayloadResult, McpError>> + MaybeSendFuture + '_ {
+        let _ = (request, context);
+        std::future::ready(Err(McpError::method_not_found::<GetTaskResultMethod>()))
+    }
+
+    /// Handle a `tasks/cancel` request from a server. Only relevant when
+    /// the client is also a task *receiver*.
+    ///
+    /// # Default Behavior
+    /// Returns `-32601` (Method not found). Clients that advertise
+    /// `capabilities.tasks.cancel` must override this.
+    fn cancel_task(
+        &self,
+        request: CancelTaskParams,
+        context: RequestContext<RoleClient>,
+    ) -> impl Future<Output = Result<CancelTaskResult, McpError>> + MaybeSendFuture + '_ {
+        let _ = (request, context);
+        std::future::ready(Err(McpError::method_not_found::<CancelTaskMethod>()))
+    }
+
     fn on_cancelled(
         &self,
         params: CancelledNotificationParam,
@@ -310,6 +388,38 @@ macro_rules! impl_client_handler_for_wrapper {
                 (**self).on_custom_request(request, context)
             }
 
+            fn list_tasks(
+                &self,
+                request: Option<PaginatedRequestParams>,
+                context: RequestContext<RoleClient>,
+            ) -> impl Future<Output = Result<ListTasksResult, McpError>> + MaybeSendFuture + '_ {
+                (**self).list_tasks(request, context)
+            }
+
+            fn get_task_info(
+                &self,
+                request: GetTaskInfoParams,
+                context: RequestContext<RoleClient>,
+            ) -> impl Future<Output = Result<GetTaskResult, McpError>> + MaybeSendFuture + '_ {
+                (**self).get_task_info(request, context)
+            }
+
+            fn get_task_result(
+                &self,
+                request: GetTaskResultParams,
+                context: RequestContext<RoleClient>,
+            ) -> impl Future<Output = Result<GetTaskPayloadResult, McpError>> + MaybeSendFuture + '_ {
+                (**self).get_task_result(request, context)
+            }
+
+            fn cancel_task(
+                &self,
+                request: CancelTaskParams,
+                context: RequestContext<RoleClient>,
+            ) -> impl Future<Output = Result<CancelTaskResult, McpError>> + MaybeSendFuture + '_ {
+                (**self).cancel_task(request, context)
+            }
+
             fn on_cancelled(
                 &self,
                 params: CancelledNotificationParam,

crates/rmcp/src/model.rs

@@ -3323,6 +3323,10 @@ ts_union!(
     box CreateMessageResult
     | ListRootsResult
     | CreateElicitationResult
+    | ListTasksResult
+    | GetTaskResult
+    | GetTaskPayloadResult
+    | CancelTaskResult
     | EmptyResult
     | CustomResult;
 );
@@ -3341,9 +3345,29 @@ ts_union!(
     | CreateMessageRequest
     | ListRootsRequest
     | CreateElicitationRequest
+    | GetTaskInfoRequest
+    | ListTasksRequest
+    | GetTaskResultRequest
+    | CancelTaskRequest
     | CustomRequest;
 );
 
+impl ServerRequest {
+    pub fn method(&self) -> &str {
+        match &self {
+            ServerRequest::PingRequest(r) => r.method.as_str(),
+            ServerRequest::CreateMessageRequest(r) => r.method.as_str(),
+            ServerRequest::ListRootsRequest(r) => r.method.as_str(),
+            ServerRequest::CreateElicitationRequest(r) => r.method.as_str(),
+            ServerRequest::GetTaskInfoRequest(r) => r.method.as_str(),
+            ServerRequest::ListTasksRequest(r) => r.method.as_str(),
+            ServerRequest::GetTaskResultRequest(r) => r.method.as_str(),
+            ServerRequest::CancelTaskRequest(r) => r.method.as_str(),
+            ServerRequest::CustomRequest(r) => r.method.as_str(),
+        }
+    }
+}
+
 ts_union!(
     export type ServerNotification =
     | CancelledNotification

crates/rmcp/src/model/meta.rs

@@ -166,6 +166,10 @@ variant_extension! {
         ListRootsRequest
         CreateElicitationRequest
         CustomRequest
+        GetTaskInfoRequest
+        ListTasksRequest
+        GetTaskResultRequest
+        CancelTaskRequest
     }
 }