mcp/tool: duplicate tools should not error

mcp/mcp_test.go

@@ -938,4 +938,30 @@ func TestKeepAliveFailure(t *testing.T) {
 	t.Errorf("expected connection to be closed by keepalive, but it wasn't. Last error: %v", err)
 }
 
+func TestAddTool_DuplicateNoPanicAndNoDuplicate(t *testing.T) {
+	// Adding the same tool pointer twice should not panic and should not
+	// produce duplicates in the server's tool list.
+	_, cs := basicConnection(t, func(s *Server) {
+		tool := &Tool{Name: "dup", InputSchema: &jsonschema.Schema{}}
+		s.AddTool(tool, nopHandler)
+		s.AddTool(tool, nopHandler)
+	})
+	defer cs.Close()
+
+	ctx := context.Background()
+	res, err := cs.ListTools(ctx, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var count int
+	for _, tt := range res.Tools {
+		if tt.Name == "dup" {
+			count++
+		}
+	}
+	if count != 1 {
+		t.Fatalf("expected exactly one 'dup' tool, got %d", count)
+	}
+}
+
 var testImpl = &Implementation{Name: "test", Version: "v1.0.0"}

mcp/tool.go

@@ -97,11 +97,31 @@ func setSchema[T any](sfield **jsonschema.Schema, rfield **jsonschema.Resolved)
 	var err error
 	if *sfield == nil {
 		*sfield, err = jsonschema.For[T](nil)
+		if err != nil {
+			return err
+		}
 	}
+	// Resolve operates with internal state and cannot be called twice on the same
+	// Schema instance. Deep-copy the schema before resolving to avoid mutating the
+	// original (which may be reused when adding the same tool again).
+	cloned, err := func(orig *jsonschema.Schema) (*jsonschema.Schema, error) {
+		if orig == nil {
+			return nil, nil
+		}
+		b, err := json.Marshal(orig)
+		if err != nil {
+			return nil, err
+		}
+		var cp jsonschema.Schema
+		if err := json.Unmarshal(b, &cp); err != nil {
+			return nil, err
+		}
+		return &cp, nil
+	}(*sfield)
 	if err != nil {
 		return err
 	}
-	*rfield, err = (*sfield).Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
+	*rfield, err = cloned.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
 	return err
 }