diff --git a/.changeset/fn-8778-planning-lock-receiver.md b/.changeset/fn-8778-planning-lock-receiver.md
new file mode 100644
index 0000000000..8bde094ee4
--- /dev/null
+++ b/.changeset/fn-8778-planning-lock-receiver.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Prevent planning finalization from crashing after PROMPT.md writes.
+category: fix
+dev: Preserves the TaskStore receiver when invoking the planning lifecycle lock.
diff --git a/packages/engine/src/__tests__/triage.test.ts b/packages/engine/src/__tests__/triage.test.ts
index 9619445f38..fcdfb156ee 100644
--- a/packages/engine/src/__tests__/triage.test.ts
+++ b/packages/engine/src/__tests__/triage.test.ts
@@ -3278,9 +3278,23 @@ describe("specified triage recovery", () => {
   it("fences a stale finalizer after dependency re-seed commits before lifecycle-lock acquisition", async () => {
     const stalePlannerSnapshot = createTriageTask({ status: "planning" });
     const store = createMockStore({
-      withPlanningLifecycleLock: vi.fn(async (_id, callback) => callback()),
       getTask: vi.fn().mockResolvedValue({ ...stalePlannerSnapshot, status: "needs-replan" }),
     });
+    /*
+     * FNXC:PlanningDependencyReseed 2026-08-04-08:04:
+     * Finalization may feature-detect the lifecycle method, but it must invoke
+     * that extracted method with TaskStore as its receiver. The production
+     * method reads instance state (`asyncLayer`); an arrow-function mock hid the
+     * unbound-call crash that stranded FN-8778 after every successful prompt write.
+     */
+    store.withPlanningLifecycleLock = vi.fn(async function (
+      this: TaskStore,
+      _id: string,
+      callback: () => Promise<unknown>,
+    ) {
+      expect(this).toBe(store);
+      return await callback();
+    }) as typeof store.withPlanningLifecycleLock;
     const processor = new TriageProcessor(store, rootDir);
 
     await (processor as unknown as {
diff --git a/packages/engine/src/triage.ts b/packages/engine/src/triage.ts
index 5f2fdaa9fa..23026ee887 100644
--- a/packages/engine/src/triage.ts
+++ b/packages/engine/src/triage.ts
@@ -4138,7 +4138,7 @@ export class TriageProcessor {
     */
     try {
       return lifecycleLock
-        ? await lifecycleLock(task.id, finalizeUnderLock)
+        ? await this.store.withPlanningLifecycleLock(task.id, finalizeUnderLock)
         : await finalizeUnderLock();
     } finally {
       this.finalizing.delete(task.id);