mcp: add return type to subscriptions/listen (#1088)

This PR adds the expected return type to `subscriptions/listen` rpc

Author: guglielmo-san

mcp/client.go

@@ -1443,7 +1443,7 @@ func (cs *ClientSession) cancelAllResourceSubscriptions() {
 // usual handlers registered in [ClientOptions].
 func (cs *ClientSession) subscriptionsListen(ctx context.Context, params *SubscriptionsListenParams) error {
 	params = injectRequestMeta(cs, params)
-	_, err := handleSend[*emptyResult](ctx, methodSubscriptionsListen, newClientRequest(cs, orZero[Params](params)))
+	_, err := handleSend[*SubscriptionsListenResult](ctx, methodSubscriptionsListen, newClientRequest(cs, orZero[Params](params)))
 	return err
 }
 

mcp/protocol.go

@@ -2109,6 +2109,18 @@ type SubscriptionsAcknowledgedParams struct {
 func (x *SubscriptionsAcknowledgedParams) isParams()   {}
 func (x *SubscriptionsAcknowledgedParams) isNil() bool { return x == nil }
 
+// SubscriptionsListenResult is the response to a "subscriptions/listen"
+// request, signalling that the subscription has ended gracefully (for example,
+// during server shutdown). Because the listen stream is long-lived, this
+// result is sent only when the server tears the subscription down; an abrupt
+// transport close carries no response.
+type SubscriptionsListenResult struct {
+	completeResultWithType
+	Meta `json:"_meta"`
+}
+
+func (*SubscriptionsListenResult) isResult() {}
+
 // TODO(jba): add CompleteRequest and related types.
 
 // A request from the server to elicit additional information from the user via the client.

mcp/server.go

@@ -1180,7 +1180,7 @@ func (s *Server) unsubscribe(ctx context.Context, req *UnsubscribeRequest) (*emp
 	return &emptyResult{}, nil
 }
 
-func (s *Server) subscriptionsListen(ctx context.Context, req *SubscriptionsListenRequest) (*emptyResult, error) {
+func (s *Server) subscriptionsListen(ctx context.Context, req *SubscriptionsListenRequest) (*SubscriptionsListenResult, error) {
 	requestID, ok := ctx.Value(idContextKey{}).(jsonrpc.ID)
 	if !ok || !requestID.IsValid() {
 		return nil, fmt.Errorf("%w: subscriptions/listen requires a request ID", jsonrpc2.ErrInvalidRequest)
@@ -1242,7 +1242,9 @@ func (s *Server) subscriptionsListen(ctx context.Context, req *SubscriptionsList
 	if len(allowed.ResourceSubscriptions) > 0 || allowed.ToolsListChanged || allowed.PromptsListChanged || allowed.ResourcesListChanged {
 		<-ctx.Done()
 	}
-	return &emptyResult{}, nil
+	return &SubscriptionsListenResult{
+		Meta: Meta{MetaKeySubscriptionID: requestID.Raw()},
+	}, nil
 }
 
 func (s *Server) allowedSubscriptions(want *NotificationSubscriptions) NotificationSubscriptions {