diff --git a/src/agents/system-prompt-config.test.ts b/src/agents/system-prompt-config.test.ts
index 83334c466a5..16dcec04ab5 100644
--- a/src/agents/system-prompt-config.test.ts
+++ b/src/agents/system-prompt-config.test.ts
@@ -1,10 +1,14 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import type { OpenClawConfig } from "../config/types.openclaw.js";
 import {
   buildConfiguredAgentSystemPrompt,
   resolveAgentSystemPromptConfig,
 } from "./system-prompt-config.js";
 
+vi.mock("../tts/tts.js", () => ({
+  buildTtsSystemPromptHint: vi.fn(() => undefined),
+}));
+
 describe("resolveAgentSystemPromptConfig", () => {
   it("defaults sub-agent delegation mode to suggest", () => {
     expect(resolveAgentSystemPromptConfig({ config: {} }).subagentDelegationMode).toBe("suggest");