diff --git a/packages/engine/src/__tests__/agent-log-assertions.ts b/packages/engine/src/__tests__/agent-log-assertions.ts
new file mode 100644
index 0000000000..4f6319354a
--- /dev/null
+++ b/packages/engine/src/__tests__/agent-log-assertions.ts
@@ -0,0 +1,37 @@
+import { expect, type Mock } from "vitest";
+
+/**
+ * FNXC:AgentLogging 2026-07-07-08:10:
+ * FN-7503 added optional timing telemetry (`durationMs`/`timeToFirstTokenMs`) as a
+ * 6th positional argument to `TaskStore.appendAgentLog` for entries that carry
+ * timing (text/tool_result/tool_error with measured metrics). Calls without timing
+ * still pass only five args, so the 6th is genuinely optional.
+ *
+ * Assertions on agent-log writes must pin the meaningful first five positional args
+ * (taskId/text/type/detail/agent) and stay tolerant of the optional timing object,
+ * otherwise every executor/heartbeat/merger test re-breaks whenever a new timing
+ * field is added. Use this helper instead of `toHaveBeenCalledWith(...)` for those
+ * five-arg assertions. See `packages/engine/src/agent-logger.ts` flushPendingEntries.
+ */
+export function expectAppendAgentLog(
+  mock: Mock,
+  taskId: string,
+  text: string,
+  type: string,
+  detail: unknown,
+  agent: string,
+): void {
+  const calls = mock.mock.calls as unknown[][];
+  const found = calls.some(
+    (call) =>
+      call[0] === taskId &&
+      call[1] === text &&
+      call[2] === type &&
+      call[3] === detail &&
+      call[4] === agent,
+  );
+  expect(
+    found,
+    `expected appendAgentLog to have been called with (${JSON.stringify(taskId)}, ${JSON.stringify(text)}, ${JSON.stringify(type)}, ${JSON.stringify(detail)}, ${JSON.stringify(agent)}) as its first five args (ignoring any optional timing object); actual calls were:\n${calls.map((c) => JSON.stringify(c)).join("\n")}`,
+  ).toBe(true);
+}
diff --git a/packages/engine/src/__tests__/merger-merge-details.test.ts b/packages/engine/src/__tests__/merger-merge-details.test.ts
index d241e25d0f..48f8c6703f 100644
--- a/packages/engine/src/__tests__/merger-merge-details.test.ts
+++ b/packages/engine/src/__tests__/merger-merge-details.test.ts
@@ -153,6 +153,10 @@ import { createFnAgent } from "../pi.js";
 import { execSync, exec } from "node:child_process";
 import * as core from "@fusion/core";
 import { type TaskStore, type Task, type MergeResult, DEFAULT_SETTINGS } from "@fusion/core";
+// FNXC:AgentLogging 2026-07-07-08:40: FN-7503 added optional 6th timing arg to
+// appendAgentLog. Use the shared 5-arg-tolerant helper for text-delta assertions
+// instead of toHaveBeenCalledWith so the timing object doesn't re-break them.
+import { expectAppendAgentLog } from "./agent-log-assertions.js";
 
 const mockedCreateFnAgent = vi.mocked(createFnAgent);
 const mockedExecSync = vi.mocked(execSync);
@@ -496,7 +500,7 @@ describe("aiMergeTask — agent log persistence", () => {
 
     await aiMergeTask(store, "/tmp/root", "FN-050");
 
-    expect(store.appendAgentLog).toHaveBeenCalledWith("FN-050", "Hello merge", "text", undefined, "merger");
+    expectAppendAgentLog(store.appendAgentLog, "FN-050", "Hello merge", "text", undefined, "merger");
   });
 
   it("logs tool invocations to store.appendAgentLog", async () => {
@@ -550,7 +554,7 @@ describe("aiMergeTask — agent log persistence", () => {
     await aiMergeTask(store, "/tmp/root", "FN-050", { onAgentText });
 
     expect(onAgentText).toHaveBeenCalledWith("hi");
-    expect(store.appendAgentLog).toHaveBeenCalledWith("FN-050", "hi", "text", undefined, "merger");
+    expectAppendAgentLog(store.appendAgentLog, "FN-050", "hi", "text", undefined, "merger");
   });
 });