fix(streamable_http): include MCP method in client JSON-RPC error messages

server/streamable_http.go

@@ -959,7 +959,17 @@ func (s *StreamableHTTPServer) handleSamplingResponse(w HTTPResponseWriter, r *H
 		if err := json.Unmarshal(responseMessage.Error, &jsonrpcError); err != nil {
 			response.err = fmt.Errorf("failed to parse error: %v", err)
 		} else {
-			response.err = fmt.Errorf("sampling error %d: %s", jsonrpcError.Code, jsonrpcError.Message)
+			method := mcp.MethodSamplingCreateMessage
+			if sessionInterface, ok := s.activeSessions.Load(sessionID); ok {
+				if session, ok := sessionInterface.(*streamableHttpSession); ok {
+					if pendingInterface, exists := session.samplingRequests.Load(requestID); exists {
+						if pending, ok := pendingInterface.(pendingClientRequest); ok {
+							method = pending.method
+						}
+					}
+				}
+			}
+			response.err = fmt.Errorf("%s error %d: %s", method, jsonrpcError.Code, jsonrpcError.Message)
 		}
 	} else if responseMessage.Result != nil {
 		// Store the result to be unmarshaled later
@@ -997,18 +1007,20 @@ func (s *StreamableHTTPServer) deliverSamplingResponse(w HTTPResponseWriter, ses
 	}
 
 	// Look up the dedicated response channel for this specific request
-	responseChannelInterface, exists := session.samplingRequests.Load(response.requestID)
+	pendingInterface, exists := session.samplingRequests.Load(response.requestID)
 	if !exists {
 		writeHTTPError(w, "No pending sampling request found for the given request ID", http.StatusBadRequest)
 		return fmt.Errorf("no pending request found for session %s, request %d", sessionID, response.requestID)
 	}
 
-	responseChan, ok := responseChannelInterface.(chan samplingResponseItem)
+	pending, ok := pendingInterface.(pendingClientRequest)
 	if !ok {
 		writeHTTPError(w, "Failed to deliver response", http.StatusInternalServerError)
-		return fmt.Errorf("invalid response channel type for session %s, request %d", sessionID, response.requestID)
+		return fmt.Errorf("invalid pending request type for session %s, request %d", sessionID, response.requestID)
 	}
 
+	responseChan := pending.response
+
 	// Attempt to deliver the response with timeout to prevent indefinite blocking
 	select {
 	case responseChan <- response:
@@ -1270,6 +1282,11 @@ type samplingRequestItem struct {
 	response  chan samplingResponseItem
 }
 
+type pendingClientRequest struct {
+	response chan samplingResponseItem
+	method   mcp.MCPMethod
+}
+
 type samplingResponseItem struct {
 	requestID int64
 	result    json.RawMessage
@@ -1410,7 +1427,10 @@ func (s *streamableHttpSession) RequestSampling(ctx context.Context, request mcp
 	}
 
 	// Store the pending request
-	s.samplingRequests.Store(requestID, responseChan)
+	s.samplingRequests.Store(requestID, pendingClientRequest{
+		response: responseChan,
+		method:   mcp.MethodSamplingCreateMessage,
+	})
 	defer s.samplingRequests.Delete(requestID)
 
 	// Send the sampling request via the channel (non-blocking)
@@ -1467,7 +1487,10 @@ func (s *streamableHttpSession) ListRoots(ctx context.Context, request mcp.ListR
 	}
 
 	// Store the pending request
-	s.samplingRequests.Store(requestID, responseChan)
+	s.samplingRequests.Store(requestID, pendingClientRequest{
+		response: responseChan,
+		method:   mcp.MethodListRoots,
+	})
 	defer s.samplingRequests.Delete(requestID)
 
 	// Send the list roots request via the channel (non-blocking)
@@ -1512,7 +1535,10 @@ func (s *streamableHttpSession) RequestElicitation(ctx context.Context, request
 	}
 
 	// Store the pending request
-	s.samplingRequests.Store(requestID, responseChan)
+	s.samplingRequests.Store(requestID, pendingClientRequest{
+		response: responseChan,
+		method:   mcp.MethodElicitationCreate,
+	})
 	defer s.samplingRequests.Delete(requestID)
 
 	// Send the sampling request via the channel (non-blocking)

server/streamable_http_sampling_test.go

@@ -214,3 +214,68 @@ func TestStreamableHTTPServer_SamplingQueueFull(t *testing.T) {
 		t.Errorf("Expected queue full error, got: %v", err)
 	}
 }
+
+// TestStreamableHTTPServer_ClientRequestErrorIncludesMethod verifies JSON-RPC client
+// errors include the originating MCP method (e.g. elicitation/create vs sampling/createMessage).
+func TestStreamableHTTPServer_ClientRequestErrorIncludesMethod(t *testing.T) {
+	mcpServer := NewMCPServer("test-server", "1.0.0")
+	mcpServer.EnableSampling()
+
+	httpServer := NewStreamableHTTPServer(mcpServer, WithStateLess(true))
+	testServer := httptest.NewServer(httpServer)
+	defer testServer.Close()
+
+	sessionID := "test-session-elicitation-error"
+	session := newStreamableHttpSession(sessionID, httpServer.sessionTools, httpServer.sessionResources, httpServer.sessionResourceTemplates, httpServer.sessionLogLevels)
+	httpServer.activeSessions.Store(sessionID, session)
+
+	requestID := int64(99)
+	responseChan := make(chan samplingResponseItem, 1)
+	session.samplingRequests.Store(requestID, pendingClientRequest{
+		response: responseChan,
+		method:   mcp.MethodElicitationCreate,
+	})
+
+	body := map[string]any{
+		"jsonrpc": "2.0",
+		"id":      requestID,
+		"error": map[string]any{
+			"code":    -32601,
+			"message": "Method not found",
+		},
+	}
+	payload, err := json.Marshal(body)
+	if err != nil {
+		t.Fatalf("Failed to marshal body: %v", err)
+	}
+
+	req, err := http.NewRequest("POST", testServer.URL, bytes.NewReader(payload))
+	if err != nil {
+		t.Fatalf("Failed to create request: %v", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Mcp-Session-Id", sessionID)
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Failed to send request: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusAccepted {
+		t.Fatalf("Expected status %d, got %d", http.StatusAccepted, resp.StatusCode)
+	}
+
+	select {
+	case response := <-responseChan:
+		if response.err == nil {
+			t.Fatal("Expected error response, got nil")
+		}
+		expected := "elicitation/create error -32601: Method not found"
+		if response.err.Error() != expected {
+			t.Errorf("Expected error %q, got %q", expected, response.err.Error())
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("Timed out waiting for elicitation error response")
+	}
+}