mcp: fix cancellation for HTTP transport

In #202, I added the checkRequest helper to validate incoming requests,
and invoked it in the stremable transports to preemptively reject
invalid HTTP requests, so that a jsonrpc error could be translated to an
HTTP error.

However, this introduced a bug: since cancellation was handled in the
jsonrpc2 layer, we never had to validate it in the mcp layer, and
therefore never added methodInfo. As a result, it was reported as an
invalid request in the http layer.

Add a test, and a fix. The simplest fix was to create stubs that are
placeholders for cancellation.

This was discovered in the course of investigating #285.

Author: findleyr

mcp/client.go

@@ -305,6 +305,7 @@ var clientMethodInfos = map[string]methodInfo{
 	methodPing:                      newClientMethodInfo(clientSessionMethod((*ClientSession).ping), missingParamsOK),
 	methodListRoots:                 newClientMethodInfo(clientMethod((*Client).listRoots), missingParamsOK),
 	methodCreateMessage:             newClientMethodInfo(clientMethod((*Client).createMessage), 0),
+	notificationCancelled:           newClientMethodInfo(clientSessionMethod((*ClientSession).cancel), notification|missingParamsOK),
 	notificationToolListChanged:     newClientMethodInfo(clientMethod((*Client).callToolChangedHandler), notification|missingParamsOK),
 	notificationPromptListChanged:   newClientMethodInfo(clientMethod((*Client).callPromptChangedHandler), notification|missingParamsOK),
 	notificationResourceListChanged: newClientMethodInfo(clientMethod((*Client).callResourceChangedHandler), notification|missingParamsOK),
@@ -344,6 +345,15 @@ func (*ClientSession) ping(context.Context, *PingParams) (*emptyResult, error) {
 	return &emptyResult{}, nil
 }
 
+// cancel is a placeholder: cancellation is handled the jsonrpc2 package.
+//
+// It should never be invoked in practice because cancellation is preempted,
+// but having its signature here facilitates the construction of methodInfo
+// that can be used to validate incoming cancellation notifications.
+func (*ClientSession) cancel(context.Context, *CancelledParams) (Result, error) {
+	return nil, nil
+}
+
 func newClientRequest[P Params](cs *ClientSession, params P) *ClientRequest[P] {
 	return &ClientRequest[P]{Session: cs, Params: params}
 }

mcp/mcp_test.go

@@ -646,8 +646,7 @@ func TestCancellation(t *testing.T) {
 		start     = make(chan struct{})
 		cancelled = make(chan struct{}, 1) // don't block the request
 	)
-
-	slowRequest := func(ctx context.Context, req *ServerRequest[*CallToolParamsFor[map[string]any]]) (*CallToolResult, error) {
+	slowRequest := func(ctx context.Context, req *ServerRequest[*CallToolParams]) (*CallToolResult, error) {
 		start <- struct{}{}
 		select {
 		case <-ctx.Done():
@@ -658,7 +657,7 @@ func TestCancellation(t *testing.T) {
 		return nil, nil
 	}
 	_, cs := basicConnection(t, func(s *Server) {
-		s.AddTool(&Tool{Name: "slow", InputSchema: &jsonschema.Schema{}}, slowRequest)
+		AddTool(s, &Tool{Name: "slow"}, slowRequest)
 	})
 	defer cs.Close()
 

mcp/server.go

@@ -760,6 +760,7 @@ var serverMethodInfos = map[string]methodInfo{
 	methodSetLevel:               newServerMethodInfo(serverSessionMethod((*ServerSession).setLevel), 0),
 	methodSubscribe:              newServerMethodInfo(serverMethod((*Server).subscribe), 0),
 	methodUnsubscribe:            newServerMethodInfo(serverMethod((*Server).unsubscribe), 0),
+	notificationCancelled:        newServerMethodInfo(serverSessionMethod((*ServerSession).cancel), notification|missingParamsOK),
 	notificationInitialized:      newServerMethodInfo(serverSessionMethod((*ServerSession).initialized), notification|missingParamsOK),
 	notificationRootsListChanged: newServerMethodInfo(serverMethod((*Server).callRootsListChangedHandler), notification|missingParamsOK),
 	notificationProgress:         newServerMethodInfo(serverSessionMethod((*ServerSession).callProgressNotificationHandler), notification),
@@ -838,6 +839,15 @@ func (ss *ServerSession) ping(context.Context, *PingParams) (*emptyResult, error
 	return &emptyResult{}, nil
 }
 
+// cancel is a placeholder: cancellation is handled the jsonrpc2 package.
+//
+// It should never be invoked in practice because cancellation is preempted,
+// but having its signature here facilitates the construction of methodInfo
+// that can be used to validate incoming cancellation notifications.
+func (ss *ServerSession) cancel(context.Context, *CancelledParams) (Result, error) {
+	return nil, nil
+}
+
 func (ss *ServerSession) setLevel(_ context.Context, params *SetLevelParams) (*emptyResult, error) {
 	ss.updateState(func(state *ServerSessionState) {
 		state.LogLevel = params.Level

mcp/streamable.go

@@ -115,12 +115,12 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 		return
 	}
 
-	var session *StreamableServerTransport
+	var transport *StreamableServerTransport
 	if id := req.Header.Get(sessionIDHeader); id != "" {
 		h.mu.Lock()
-		session, _ = h.transports[id]
+		transport, _ = h.transports[id]
 		h.mu.Unlock()
-		if session == nil {
+		if transport == nil {
 			http.Error(w, "session not found", http.StatusNotFound)
 			return
 		}
@@ -129,22 +129,22 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 	// TODO(rfindley): simplify the locking so that each request has only one
 	// critical section.
 	if req.Method == http.MethodDelete {
-		if session == nil {
+		if transport == nil {
 			// => Mcp-Session-Id was not set; else we'd have returned NotFound above.
 			http.Error(w, "DELETE requires an Mcp-Session-Id header", http.StatusBadRequest)
 			return
 		}
 		h.mu.Lock()
-		delete(h.transports, session.SessionID)
+		delete(h.transports, transport.SessionID)
 		h.mu.Unlock()
-		session.connection.Close()
+		transport.connection.Close()
 		w.WriteHeader(http.StatusNoContent)
 		return
 	}
 
 	switch req.Method {
 	case http.MethodPost, http.MethodGet:
-		if req.Method == http.MethodGet && session == nil {
+		if req.Method == http.MethodGet && transport == nil {
 			http.Error(w, "GET requires an active session", http.StatusMethodNotAllowed)
 			return
 		}
@@ -154,7 +154,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 		return
 	}
 
-	if session == nil {
+	if transport == nil {
 		server := h.getServer(req)
 		if server == nil {
 			// The getServer argument to NewStreamableHTTPHandler returned nil.
@@ -191,10 +191,10 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			h.transports[s.SessionID] = s
 			h.mu.Unlock()
 		}
-		session = s
+		transport = s
 	}
 
-	session.ServeHTTP(w, req)
+	transport.ServeHTTP(w, req)
 }
 
 // StreamableServerTransportOptions configures the stramable server transport.

mcp/streamable_test.go

@@ -37,9 +37,26 @@ func TestStreamableTransports(t *testing.T) {
 
 	for _, useJSON := range []bool{false, true} {
 		t.Run(fmt.Sprintf("JSONResponse=%v", useJSON), func(t *testing.T) {
-			// 1. Create a server with a simple "greet" tool.
+			// Create a server with some simple tools.
 			server := NewServer(testImpl, nil)
 			AddTool(server, &Tool{Name: "greet", Description: "say hi"}, sayHi)
+			// The "hang" tool checks that context cancellation is propagated.
+			// It hangs until the context is cancelled.
+			var (
+				start     = make(chan struct{})
+				cancelled = make(chan struct{}, 1) // don't block the request
+			)
+			hang := func(ctx context.Context, req *ServerRequest[*CallToolParams]) (*CallToolResult, error) {
+				start <- struct{}{}
+				select {
+				case <-ctx.Done():
+					cancelled <- struct{}{}
+				case <-time.After(5 * time.Second):
+					return nil, nil
+				}
+				return nil, nil
+			}
+			AddTool(server, &Tool{Name: "hang"}, hang)
 			AddTool(server, &Tool{Name: "sample"}, func(ctx context.Context, req *ServerRequest[*CallToolParams]) (*CallToolResult, error) {
 				// Test that we can make sampling requests during tool handling.
 				//
@@ -60,7 +77,7 @@ func TestStreamableTransports(t *testing.T) {
 				return &CallToolResultFor[any]{}, nil
 			})
 
-			// 2. Start an httptest.Server with the StreamableHTTPHandler, wrapped in a
+			// Start an httptest.Server with the StreamableHTTPHandler, wrapped in a
 			// cookie-checking middleware.
 			handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, &StreamableHTTPOptions{
 				jsonResponse: useJSON,
@@ -84,7 +101,7 @@ func TestStreamableTransports(t *testing.T) {
 			}))
 			defer httpServer.Close()
 
-			// 3. Create a client and connect it to the server using our StreamableClientTransport.
+			// Create a client and connect it to the server using our StreamableClientTransport.
 			// Check that all requests honor a custom client.
 			jar, err := cookiejar.New(nil)
 			if err != nil {
@@ -117,10 +134,13 @@ func TestStreamableTransports(t *testing.T) {
 			if g, w := session.mcpConn.(*streamableClientConn).initializedResult.ProtocolVersion, latestProtocolVersion; g != w {
 				t.Fatalf("got protocol version %q, want %q", g, w)
 			}
-			// 4. The client calls the "greet" tool.
+
+			// Verify the behavior of various tools.
+
+			// The "greet" tool should just work.
 			params := &CallToolParams{
 				Name:      "greet",
-				Arguments: map[string]any{"name": "streamy"},
+				Arguments: map[string]any{"name": "foo"},
 			}
 			got, err := session.CallTool(ctx, params)
 			if err != nil {
@@ -132,19 +152,26 @@ func TestStreamableTransports(t *testing.T) {
 			if g, w := lastHeader.Get(protocolVersionHeader), latestProtocolVersion; g != w {
 				t.Errorf("got protocol version header %q, want %q", g, w)
 			}
-
-			// 5. Verify that the correct response is received.
 			want := &CallToolResult{
-				Content: []Content{
-					&TextContent{Text: "hi streamy"},
-				},
+				Content: []Content{&TextContent{Text: "hi foo"}},
 			}
 			if diff := cmp.Diff(want, got); diff != "" {
 				t.Errorf("CallTool() returned unexpected content (-want +got):\n%s", diff)
 			}
 
-			// 6. Run the "sampling" tool and verify that the streamable server can
-			// call tools.
+			// The "hang" tool should be cancellable.
+			ctx2, cancel := context.WithCancel(context.Background())
+			go session.CallTool(ctx2, &CallToolParams{Name: "hang"})
+			<-start
+			cancel()
+			select {
+			case <-cancelled:
+			case <-time.After(5 * time.Second):
+				t.Fatal("timeout waiting for cancellation")
+			}
+
+			// The "sampling" tool should be able to issue sampling requests during
+			// tool operation.
 			result, err := session.CallTool(ctx, &CallToolParams{
 				Name:      "sample",
 				Arguments: map[string]any{},

mcp/transport.go

@@ -171,7 +171,7 @@ type canceller struct {
 
 // Preempt implements [jsonrpc2.Preempter].
 func (c *canceller) Preempt(ctx context.Context, req *jsonrpc.Request) (result any, err error) {
-	if req.Method == "notifications/cancelled" {
+	if req.Method == notificationCancelled {
 		var params CancelledParams
 		if err := json.Unmarshal(req.Params, &params); err != nil {
 			return nil, err