mcp: make NotificationSubscriptions a mandatory field (#1050)

Fixes #1037

mcp/client.go

@@ -330,7 +330,9 @@ func (c *Client) Connect(ctx context.Context, t Transport, opts *ClientSessionOp
 				if hc, ok := cs.mcpConn.(clientConnection); ok {
 					hc.sessionUpdated(cs.state)
 				}
-				subscribeParams := &SubscriptionsListenParams{}
+				subscribeParams := &SubscriptionsListenParams{
+					Notifications: &NotificationSubscriptions{},
+				}
 				if c.opts.ToolListChangedHandler != nil {
 					subscribeParams.Notifications.ToolsListChanged = true
 				}
@@ -1393,7 +1395,7 @@ func (cs *ClientSession) Subscribe(ctx context.Context, params *SubscribeParams)
 	}
 
 	return cs.subscriptionsListen(listenCtx, &SubscriptionsListenParams{
-		Notifications: NotificationSubscriptions{
+		Notifications: &NotificationSubscriptions{
 			ResourceSubscriptions: []string{uri},
 		},
 	})

mcp/mcp_test.go

@@ -2652,6 +2652,48 @@ func TestSubscriptionsListen_NoHandlersNoListen(t *testing.T) {
 	}
 }
 
+// TestSubscriptionsListen_MissingNotifications verifies that a
+// subscriptions/listen request without the required "notifications" field
+// is rejected with an invalid params error.
+func TestSubscriptionsListen_MissingNotifications(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	server := newSubListenServer()
+	_, st := NewInMemoryTransports()
+	ss, err := server.Connect(ctx, st, nil)
+	if err != nil {
+		t.Fatalf("server connect: %v", err)
+	}
+	defer ss.Close()
+
+	// Invoke the server handler directly with a nil Notifications field,
+	// simulating a request whose params omit the required field on the wire.
+	// The client-side subscriptionsListen does not await the RPC response
+	// (the call's lifetime is the notification stream), so we assert the
+	// server-side behavior at the handler level.
+	id, err := jsonrpc.MakeID("test-1")
+	if err != nil {
+		t.Fatalf("MakeID: %v", err)
+	}
+	reqCtx := context.WithValue(ctx, idContextKey{}, id)
+	req := &SubscriptionsListenRequest{
+		Session: ss,
+		Params:  &SubscriptionsListenParams{}, // Notifications is nil
+	}
+	_, err = server.subscriptionsListen(reqCtx, req)
+	if err == nil {
+		t.Fatal("expected error for missing notifications field, got nil")
+	}
+	var jerr *jsonrpc.Error
+	if !errors.As(err, &jerr) {
+		t.Fatalf("expected *jsonrpc.Error, got %T: %v", err, err)
+	}
+	if jerr.Code != jsonrpc.CodeInvalidParams {
+		t.Errorf("error code = %d, want %d", jerr.Code, jsonrpc.CodeInvalidParams)
+	}
+}
+
 // resourceSubServer builds a server that advertises resource subscriptions
 // and records every Subscribe/Unsubscribe handler invocation through chans.
 func resourceSubServer(t *testing.T, subCh, unsubCh chan string) *Server {

mcp/protocol.go

@@ -2063,7 +2063,7 @@ type SubscriptionsListenParams struct {
 	Meta `json:"_meta,omitempty"`
 	// Notifications declares which notification types the client wants to
 	// receive on this stream.
-	Notifications NotificationSubscriptions `json:"notifications"`
+	Notifications *NotificationSubscriptions `json:"notifications"`
 }
 
 func (x *SubscriptionsListenParams) isParams()   {}

mcp/server.go

@@ -1169,6 +1169,10 @@ func (s *Server) subscriptionsListen(ctx context.Context, req *SubscriptionsList
 		return nil, fmt.Errorf("%w: subscriptions/listen requires a request ID", jsonrpc2.ErrInvalidRequest)
 	}
 
+	if req.Params.Notifications == nil {
+		return nil, fmt.Errorf("%w: missing required 'notifications' field", jsonrpc2.ErrInvalidParams)
+	}
+
 	allowed := s.allowedSubscriptions(req.Params.Notifications)
 	s.mu.Lock()
 	if allowed.ToolsListChanged {
@@ -1217,11 +1221,14 @@ func (s *Server) subscriptionsListen(ctx context.Context, req *SubscriptionsList
 		return nil, fmt.Errorf("sending subscriptions/acknowledged: %w", err)
 	}
 
-	<-ctx.Done()
+	// If there are any active subscriptions, we block until the context is cancelled. Otherwise, we return immediately.
+	if len(allowed.ResourceSubscriptions) > 0 || allowed.ToolsListChanged || allowed.PromptsListChanged || allowed.ResourcesListChanged {
+		<-ctx.Done()
+	}
 	return &emptyResult{}, nil
 }
 
-func (s *Server) allowedSubscriptions(want NotificationSubscriptions) NotificationSubscriptions {
+func (s *Server) allowedSubscriptions(want *NotificationSubscriptions) NotificationSubscriptions {
 	caps := s.capabilities()
 	agreed := NotificationSubscriptions{}
 	if want.ToolsListChanged && caps.Tools != nil && caps.Tools.ListChanged {