diff --git a/src/infra/heartbeat-runner-config.ts b/src/infra/heartbeat-runner-config.ts
index 6fc2cef44742..50c158b6a7ec 100644
--- a/src/infra/heartbeat-runner-config.ts
+++ b/src/infra/heartbeat-runner-config.ts
@@ -52,7 +52,10 @@ export function resolveHeartbeatTimeoutOverrideSeconds(
     typeof agentDefaultTimeoutSeconds === "number" &&
     Number.isFinite(agentDefaultTimeoutSeconds)
   ) {
-    return Math.max(1, Math.floor(agentDefaultTimeoutSeconds));
+    // Preserve the unlimited sentinel consumed by resolveAgentTimeoutMs.
+    return agentDefaultTimeoutSeconds === 0
+      ? 0
+      : Math.max(1, Math.floor(agentDefaultTimeoutSeconds));
   }
   // The wake dispatcher awaits heartbeat turns serially. Keep unset heartbeat
   // timeouts tied to the cadence instead of the 48h built-in agent default.
diff --git a/src/infra/heartbeat-runner.model-override.test.ts b/src/infra/heartbeat-runner.model-override.test.ts
index a3c83d5ece11..120899ff7621 100644
--- a/src/infra/heartbeat-runner.model-override.test.ts
+++ b/src/infra/heartbeat-runner.model-override.test.ts
@@ -227,13 +227,16 @@ describe("runHeartbeatOnce – heartbeat model override", () => {
     });
   });
 
-  it("preserves explicit default agent timeout for heartbeat runs", async () => {
-    const replyOpts = await runDefaultsHeartbeat({ defaultTimeoutSeconds: 60, every: "30m" });
-    expectReplyOptions(replyOpts, {
-      isHeartbeat: true,
-      timeoutOverrideSeconds: 60,
-    });
-  });
+  it.each([0, 60])(
+    "preserves explicit default agent timeout %d for heartbeat runs",
+    async (defaultTimeoutSeconds) => {
+      const replyOpts = await runDefaultsHeartbeat({ defaultTimeoutSeconds, every: "30m" });
+      expectReplyOptions(replyOpts, {
+        isHeartbeat: true,
+        timeoutOverrideSeconds: defaultTimeoutSeconds,
+      });
+    },
+  );
 
   it("passes bootstrapContextMode when heartbeat lightContext is enabled", async () => {
     const replyOpts = await runDefaultsHeartbeat({ lightContext: true });