Add duplicate tool warning log and test logger for MCPServer

server/server.go

@@ -11,6 +11,7 @@ import (
 	"sync"
 
 	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/mark3labs/mcp-go/util"
 )
 
 // resourceEntry holds both a resource and its handler
@@ -160,6 +161,7 @@ type MCPServer struct {
 	paginationLimit        *int
 	sessions               sync.Map
 	hooks                  *Hooks
+	logger                 util.Logger
 }
 
 // WithPaginationLimit sets the pagination limit for the server.
@@ -281,6 +283,12 @@ func WithLogging() ServerOption {
 	}
 }
 
+func WithMCPLogger(logger util.Logger) ServerOption {
+	return func(s *MCPServer) {
+		s.logger = logger
+	}
+}
+
 // WithInstructions sets the server instructions for the client returned in the initialize response
 func WithInstructions(instructions string) ServerOption {
 	return func(s *MCPServer) {
@@ -308,6 +316,7 @@ func NewMCPServer(
 			prompts:   nil,
 			logging:   nil,
 		},
+		logger: util.DefaultLogger(),
 	}
 
 	for _, opt := range opts {
@@ -474,6 +483,9 @@ func (s *MCPServer) AddTools(tools ...ServerTool) {
 
 	s.toolsMu.Lock()
 	for _, entry := range tools {
+		if _, exists := s.tools[entry.Tool.Name]; exists {
+			s.logger.Infof("Tool with name %q already exists. Overwriting.", entry.Tool.Name)
+		}
 		s.tools[entry.Tool.Name] = entry
 	}
 	s.toolsMu.Unlock()

server/server_race_test.go

@@ -3,6 +3,7 @@ package server
 import (
 	"context"
 	"fmt"
+	"math/rand"
 	"sync"
 	"testing"
 	"time"
@@ -56,7 +57,7 @@ func TestRaceConditions(t *testing.T) {
 	})
 
 	runConcurrentOperation(&wg, testDuration, "add-tools", func() {
-		name := fmt.Sprintf("tool-%d", time.Now().UnixNano())
+		name := fmt.Sprintf("tool-%d-%d", time.Now().UnixNano(), rand.Int())
 		srv.AddTool(mcp.Tool{
 			Name:        name,
 			Description: "Test tool",

server/server_test.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"bytes"
 	"context"
 	"encoding/base64"
 	"encoding/json"
@@ -12,6 +13,7 @@ import (
 	"time"
 
 	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/mark3labs/mcp-go/testutil"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -342,11 +344,50 @@ func TestMCPServer_Tools(t *testing.T) {
 				assert.Equal(t, "test-tool-2", tools[1].Name)
 			},
 		},
+		{
+			name: "AddTools overwrites tool with same name",
+			action: func(t *testing.T, server *MCPServer, notificationChannel chan mcp.JSONRPCNotification) {
+				err := server.RegisterSession(context.TODO(), &fakeSession{
+					sessionID:           "test",
+					notificationChannel: notificationChannel,
+					initialized:         true,
+				})
+				require.NoError(t, err)
+				server.AddTool(
+					mcp.NewTool("test-tool-dup"),
+					func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+						return &mcp.CallToolResult{}, nil
+					},
+				)
+				// Add same tool name with different handler or data to test overwrite
+				server.AddTool(
+					mcp.NewTool("test-tool-dup"),
+					func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+						return &mcp.CallToolResult{}, nil
+					},
+				)
+			},
+			expectedNotifications: 2, // one per AddTool with active session
+			validate: func(t *testing.T, notifications []mcp.JSONRPCNotification, toolsList mcp.JSONRPCMessage) {
+				// Both adds must have triggered notifications
+				assert.Equal(t, mcp.MethodNotificationToolsListChanged, notifications[0].Method)
+				assert.Equal(t, mcp.MethodNotificationToolsListChanged, notifications[1].Method)
+
+				tools := toolsList.(mcp.JSONRPCResponse).Result.(mcp.ListToolsResult).Tools
+				assert.Len(t, tools, 1, "Expected only one tool after overwrite")
+				assert.Equal(t, "test-tool-dup", tools[0].Name)
+			},
+		},
+
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			ctx := context.Background()
-			server := NewMCPServer("test-server", "1.0.0", WithToolCapabilities(true))
+
+			var buf bytes.Buffer
+			logger := &testutil.TestLogger{Buf: &buf}
+
+			server := NewMCPServer("test-server", "1.0.0", WithToolCapabilities(true), WithMCPLogger(logger))
 			_ = server.HandleMessage(ctx, []byte(`{
 				"jsonrpc": "2.0",
 				"id": 1,
@@ -373,6 +414,11 @@ func TestMCPServer_Tools(t *testing.T) {
 				"method": "tools/list"
 			}`))
 			tt.validate(t, notifications, toolsList)
+
+			if tt.name == "AddTools overwrites tool with same name" {
+				logOutput := buf.String()
+				assert.Contains(t, logOutput, "already exists")
+		}
 		})
 	}
 }

testutil/testlogger.go

@@ -0,0 +1,18 @@
+package testutil
+
+import (
+	"bytes"
+	"fmt"
+)
+
+type TestLogger struct {
+	Buf *bytes.Buffer
+}
+
+func (l *TestLogger) Infof(format string, v ...any) {
+	fmt.Fprintf(l.Buf, "INFO: "+format, v...)
+}
+
+func (l *TestLogger) Errorf(format string, v ...any) {
+	fmt.Fprintf(l.Buf, "ERROR: "+format, v...)
+}