diff --git a/src/infra/heartbeat-runner.returns-default-unset.test.ts b/src/infra/heartbeat-runner.returns-default-unset.test.ts
index 2ac6a8be0f3..2d02550eba3 100644
--- a/src/infra/heartbeat-runner.returns-default-unset.test.ts
+++ b/src/infra/heartbeat-runner.returns-default-unset.test.ts
@@ -1018,6 +1018,7 @@ describe("runHeartbeatOnce", () => {
     reason?: "interval" | "wake";
     queueCronEvent?: boolean;
     replyText?: string;
+    cfgOverrides?: Partial<OpenClawConfig>;
   }) {
     const tmpDir = await createCaseDir("openclaw-hb");
     const storePath = path.join(tmpDir, "sessions.json");
@@ -1041,7 +1042,7 @@ describe("runHeartbeatOnce", () => {
       await fs.mkdir(path.join(workspaceDir, "HEARTBEAT.md"), { recursive: true });
     }
 
-    const cfg: OpenClawConfig = {
+    const cfgBase: OpenClawConfig = {
       agents: {
         defaults: {
           workspace: workspaceDir,
@@ -1051,6 +1052,9 @@ describe("runHeartbeatOnce", () => {
       channels: { whatsapp: { allowFrom: ["*"] } },
       session: { store: storePath },
     };
+    const cfg: OpenClawConfig = params.cfgOverrides
+      ? { ...cfgBase, ...params.cfgOverrides }
+      : cfgBase;
     const sessionKey = resolveMainSessionKey(cfg);
     await fs.writeFile(
       storePath,
@@ -1083,7 +1087,7 @@ describe("runHeartbeatOnce", () => {
     return { res, replySpy, sendWhatsApp, workspaceDir };
   }
 
-  it("adds explicit workspace HEARTBEAT.md path guidance to heartbeat prompts", async () => {
+  it("adds explicit workspace HEARTBEAT.md path guidance to default heartbeat prompts", async () => {
     const { res, replySpy, sendWhatsApp, workspaceDir } = await runHeartbeatFileScenario({
       fileState: "actionable",
       reason: "interval",
@@ -1102,6 +1106,35 @@ describe("runHeartbeatOnce", () => {
     }
   });
 
+  it("does not mutate a custom heartbeat prompt", async () => {
+    const customPrompt =
+      "Read HEARTBEAT.md if it exists (workspace context). Use the system prompt only.";
+    const { res, replySpy } = await runHeartbeatFileScenario({
+      fileState: "actionable",
+      reason: "interval",
+      replyText: "Checked logs and PRs",
+      cfgOverrides: {
+        agents: {
+          defaults: {
+            heartbeat: {
+              prompt: customPrompt,
+            },
+          },
+        },
+      },
+    });
+    try {
+      expect(res.status).toBe("ran");
+      expect(replySpy).toHaveBeenCalledTimes(1);
+      const calledCtx = replySpy.mock.calls[0]?.[0] as { Body?: string };
+      expect(calledCtx.Body).toContain(customPrompt);
+      expect(calledCtx.Body).not.toContain("Do not read docs/heartbeat.md.");
+      expect(calledCtx.Body).not.toContain("use workspace file");
+    } finally {
+      replySpy.mockRestore();
+    }
+  });
+
   it("applies HEARTBEAT.md gating rules across file states and triggers", async () => {
     const cases: Array<{
       name: string;
diff --git a/src/infra/heartbeat-runner.ts b/src/infra/heartbeat-runner.ts
index c3c58d34c1e..8bac3aadf8d 100644
--- a/src/infra/heartbeat-runner.ts
+++ b/src/infra/heartbeat-runner.ts
@@ -560,7 +560,15 @@ type HeartbeatPromptResolution = {
   hasCronEvents: boolean;
 };
 
-function appendHeartbeatWorkspacePathHint(prompt: string, workspaceDir: string): string {
+function appendHeartbeatWorkspacePathHint(params: {
+  prompt: string;
+  workspaceDir: string;
+  shouldAppend: boolean;
+}): string {
+  const { prompt, workspaceDir, shouldAppend } = params;
+  if (!shouldAppend) {
+    return prompt;
+  }
   if (!/heartbeat\.md/i.test(prompt)) {
     return prompt;
   }
@@ -597,7 +605,16 @@ function resolveHeartbeatRunPrompt(params: {
     : hasCronEvents
       ? buildCronEventPrompt(cronEvents, { deliverToUser: params.canRelayToUser })
       : resolveHeartbeatPrompt(params.cfg, params.heartbeat);
-  const prompt = appendHeartbeatWorkspacePathHint(basePrompt, params.workspaceDir);
+  const hasExplicitPrompt = Boolean(
+    params.heartbeat?.prompt?.trim() || params.cfg.agents?.defaults?.heartbeat?.prompt?.trim(),
+  );
+  const prompt = appendHeartbeatWorkspacePathHint({
+    prompt: basePrompt,
+    workspaceDir: params.workspaceDir,
+    // Only append the hint when using the built-in default heartbeat prompt.
+    // If the user configured a custom prompt, do not mutate it.
+    shouldAppend: !hasExplicitPrompt && !hasExecCompletion && !hasCronEvents,
+  });
 
   return { prompt, hasExecCompletion, hasCronEvents };
 }