diff --git a/internal/server/common_test.go b/internal/server/common_test.go
index 54109ac467..a56839cce0 100644
--- a/internal/server/common_test.go
+++ b/internal/server/common_test.go
@@ -30,6 +30,7 @@ import (
 	"github.com/googleapis/genai-toolbox/internal/server/resources"
 	"github.com/googleapis/genai-toolbox/internal/telemetry"
 	"github.com/googleapis/genai-toolbox/internal/tools"
+	"github.com/googleapis/genai-toolbox/internal/util"
 	"github.com/googleapis/genai-toolbox/internal/util/parameters"
 )
 
@@ -51,7 +52,7 @@ type MockTool struct {
 	requiresClientAuthrorization bool
 }
 
-func (t MockTool) Invoke(context.Context, tools.SourceProvider, parameters.ParamValues, tools.AccessToken) (any, error) {
+func (t MockTool) Invoke(context.Context, tools.SourceProvider, parameters.ParamValues, tools.AccessToken) (any, util.ToolboxError) {
 	mock := []any{t.Name}
 	return mock, nil
 }