diff --git a/server/src/__tests__/heartbeat-workspace-session.test.ts b/server/src/__tests__/heartbeat-workspace-session.test.ts
index 5a1498f2..17718055 100644
--- a/server/src/__tests__/heartbeat-workspace-session.test.ts
+++ b/server/src/__tests__/heartbeat-workspace-session.test.ts
@@ -6,6 +6,7 @@ import {
   applyPersistedExecutionWorkspaceConfig,
   buildRealizedExecutionWorkspaceFromPersisted,
   buildExplicitResumeSessionOverride,
+  deriveTaskKeyWithHeartbeatFallback,
   formatRuntimeWorkspaceWarningLog,
   prioritizeProjectWorkspaceCandidatesForRun,
   parseSessionCompactionPolicy,
@@ -328,6 +329,34 @@ describe("shouldResetTaskSessionForWake", () => {
   });
 });
 
+describe("deriveTaskKeyWithHeartbeatFallback", () => {
+  it("returns explicit taskKey when present", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ taskKey: "issue-123" }, null)).toBe("issue-123");
+  });
+
+  it("returns explicit issueId when no taskKey", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ issueId: "issue-456" }, null)).toBe("issue-456");
+  });
+
+  it("returns __heartbeat__ for timer wakes with no explicit key", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ wakeSource: "timer" }, null)).toBe("__heartbeat__");
+  });
+
+  it("prefers explicit key over heartbeat fallback even on timer wakes", () => {
+    expect(
+      deriveTaskKeyWithHeartbeatFallback({ wakeSource: "timer", taskKey: "issue-789" }, null),
+    ).toBe("issue-789");
+  });
+
+  it("returns null for non-timer wakes with no explicit key", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({ wakeSource: "on_demand" }, null)).toBeNull();
+  });
+
+  it("returns null for empty context", () => {
+    expect(deriveTaskKeyWithHeartbeatFallback({}, null)).toBeNull();
+  });
+});
+
 describe("buildExplicitResumeSessionOverride", () => {
   it("reuses saved task session params when they belong to the selected failed run", () => {
     const result = buildExplicitResumeSessionOverride({
diff --git a/server/src/services/heartbeat.ts b/server/src/services/heartbeat.ts
index 168db86b..18093598 100644
--- a/server/src/services/heartbeat.ts
+++ b/server/src/services/heartbeat.ts
@@ -607,6 +607,14 @@ function parseIssueAssigneeAdapterOverrides(
   };
 }
 
+/**
+ * Synthetic task key for timer/heartbeat wakes that have no issue context.
+ * This allows timer wakes to participate in the `agentTaskSessions` system
+ * and benefit from robust session resume, instead of relying solely on the
+ * simpler `agentRuntimeState.sessionId` fallback.
+ */
+const HEARTBEAT_TASK_KEY = "__heartbeat__";
+
 function deriveTaskKey(
   contextSnapshot: Record<string, unknown> | null | undefined,
   payload: Record<string, unknown> | null | undefined,
@@ -622,6 +630,28 @@ function deriveTaskKey(
   );
 }
 
+/**
+ * Extended task key derivation that falls back to a stable synthetic key
+ * for timer/heartbeat wakes. This ensures timer wakes can resume their
+ * previous session via `agentTaskSessions` instead of starting fresh.
+ *
+ * The synthetic key is only used when:
+ * - No explicit task/issue key exists in the context
+ * - The wake source is "timer" (scheduled heartbeat)
+ */
+export function deriveTaskKeyWithHeartbeatFallback(
+  contextSnapshot: Record<string, unknown> | null | undefined,
+  payload: Record<string, unknown> | null | undefined,
+) {
+  const explicit = deriveTaskKey(contextSnapshot, payload);
+  if (explicit) return explicit;
+
+  const wakeSource = readNonEmptyString(contextSnapshot?.wakeSource);
+  if (wakeSource === "timer") return HEARTBEAT_TASK_KEY;
+
+  return null;
+}
+
 export function shouldResetTaskSessionForWake(
   contextSnapshot: Record<string, unknown> | null | undefined,
 ) {
@@ -1595,7 +1625,7 @@ export function heartbeatService(db: Db) {
   ) {
     const contextSnapshot = parseObject(run.contextSnapshot);
     const issueId = readNonEmptyString(contextSnapshot.issueId);
-    const taskKey = deriveTaskKey(contextSnapshot, null);
+    const taskKey = deriveTaskKeyWithHeartbeatFallback(contextSnapshot, null);
     const sessionBefore = await resolveSessionBeforeForWakeup(agent, taskKey);
     const retryContextSnapshot = {
       ...contextSnapshot,
@@ -2050,7 +2080,7 @@ export function heartbeatService(db: Db) {
 
     const runtime = await ensureRuntimeState(agent);
     const context = parseObject(run.contextSnapshot);
-    const taskKey = deriveTaskKey(context, null);
+    const taskKey = deriveTaskKeyWithHeartbeatFallback(context, null);
     const sessionCodec = getAdapterSessionCodec(agent.adapterType);
     const issueId = readNonEmptyString(context.issueId);
     const issueContext = issueId