Fix concurrent writes bug in intent capture

Author: jboolean

contrib/mark3labs/mcp-go/intent_capture.go

@@ -7,6 +7,7 @@ package mcpgo
 
 import (
 	"context"
+	"maps"
 	"slices"
 
 	instrmcp "github.com/DataDog/dd-trace-go/v2/instrumentation/mcp"
@@ -35,6 +36,9 @@ func injectTelemetryListToolsHook(ctx context.Context, id any, message *mcp.List
 		return
 	}
 
+	// The server reuses tools across requests. Slices and nested objects are cloned to avoid concurrent writes.
+	result.Tools = slices.Clone(result.Tools)
+
 	for i := range result.Tools {
 		t := &result.Tools[i]
 
@@ -48,14 +52,16 @@ func injectTelemetryListToolsHook(ctx context.Context, id any, message *mcp.List
 		}
 		if t.InputSchema.Properties == nil {
 			t.InputSchema.Properties = map[string]any{}
+		} else {
+			t.InputSchema.Properties = maps.Clone(t.InputSchema.Properties)
 		}
 
 		// Insert/overwrite the telemetry property
 		t.InputSchema.Properties[instrmcp.TelemetryKey] = telemetrySchema()
 
 		// Mark telemetry as required (idempotent)
 		if !slices.Contains(t.InputSchema.Required, instrmcp.TelemetryKey) {
-			t.InputSchema.Required = append(t.InputSchema.Required, instrmcp.TelemetryKey)
+			t.InputSchema.Required = append(slices.Clone(t.InputSchema.Required), instrmcp.TelemetryKey)
 		}
 	}
 }

contrib/mark3labs/mcp-go/intent_capture_test.go

@@ -91,6 +91,41 @@ func TestIntentCapture(t *testing.T) {
 	assert.Equal(t, "test intent description", toolSpan.Meta["intent"])
 }
 
+func TestIntentCaptureConcurrentListTools(t *testing.T) {
+	tt := testTracer(t)
+	defer tt.Stop()
+
+	srv := server.NewMCPServer("test-server", "1.0.0", WithMCPServerTracing(&TracingConfig{IntentCaptureEnabled: true}))
+
+	calcTool := mcp.NewTool("calculator",
+		mcp.WithDescription("A simple calculator"),
+		mcp.WithString("operation", mcp.Required(), mcp.Description("The operation to perform")),
+		mcp.WithNumber("x", mcp.Required(), mcp.Description("First number")),
+		mcp.WithNumber("y", mcp.Required(), mcp.Description("Second number")))
+
+	srv.AddTool(calcTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+		return mcp.NewToolResultText(`{"result":8}`), nil
+	})
+
+	ctx := context.Background()
+
+	const numGoroutines = 10
+	done := make(chan struct{})
+
+	for i := 0; i < numGoroutines; i++ {
+		go func() {
+			defer func() { done <- struct{}{} }()
+			for j := 0; j < 100; j++ {
+				srv.HandleMessage(ctx, []byte(`{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}`))
+			}
+		}()
+	}
+
+	for i := 0; i < numGoroutines; i++ {
+		<-done
+	}
+}
+
 func mustMarshal(v interface{}) []byte {
 	b, _ := json.Marshal(v)
 	return b