diff --git a/packages/server/src/server/agent/agent-timeline-store.test.ts b/packages/server/src/server/agent/agent-timeline-store.test.ts
index 4baaf89edb..5d3c836e65 100644
--- a/packages/server/src/server/agent/agent-timeline-store.test.ts
+++ b/packages/server/src/server/agent/agent-timeline-store.test.ts
@@ -1,4 +1,6 @@
-import { measureMainThreadWork } from "../../test-utils/main-thread-work.js";
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import { fileURLToPath } from "node:url";
 import { describe, expect, it } from "vitest";
 import { selectReferenceProjectedTimelinePage } from "./test-utils/timeline-page-reference.js";
 import type { AgentTimelineItem } from "./agent-sdk-types.js";
@@ -251,23 +253,52 @@ it("includes transitive tool updates in a contiguous projected tail", () => {
   expect(tail).toMatchObject({ startSeq: 1, endSeq: 6, hasOlder: false });
 });
 
+async function measureTimelineTail(wholeTurns: boolean) {
+  const fixture = fileURLToPath(
+    new URL("./test-utils/timeline-responsiveness.ts", import.meta.url),
+  );
+  const { stdout, stderr } = await promisify(execFile)(
+    process.execPath,
+    ["--expose-gc", "--import", "tsx", fixture, String(wholeTurns)],
+    { timeout: 15_000 },
+  );
+  process.stderr.write(stderr);
+  return JSON.parse(stdout) as {
+    result: ReturnType<InMemoryAgentTimelineStore["fetch"]>;
+    blockingMs: number;
+  };
+}
+
+it("excludes unrelated test-runner CPU from the timeline fetch budget", async () => {
+  let unrelatedWork: ReturnType<typeof setImmediate>;
+  const burnCpu = () => {
+    const threadCpuUsage: (previous?: NodeJS.CpuUsage) => NodeJS.CpuUsage = Reflect.get(
+      process,
+      "threadCpuUsage",
+    );
+    const start = threadCpuUsage();
+    let elapsed: NodeJS.CpuUsage;
+    do {
+      elapsed = threadCpuUsage(start);
+    } while (elapsed.user + elapsed.system < 80_000);
+    unrelatedWork = setImmediate(burnCpu);
+  };
+  unrelatedWork = setImmediate(burnCpu);
+  try {
+    const { result: tail, blockingMs } = await measureTimelineTail(false);
+    expect(tail.rows).toHaveLength(200);
+    expect(tail.startSeq).toBe(99_801);
+    expect(tail.endSeq).toBe(100_000);
+    expect(blockingMs).toBeLessThan(50);
+  } finally {
+    clearImmediate(unrelatedWork);
+  }
+});
+
 it.each([false, true])(
   "serves tail 200 from 100,000 retained rows without stalling the loop (wholeTurns=%s)",
   async (wholeTurns) => {
-    const store = new InMemoryAgentTimelineStore();
-    store.initialize("large");
-    for (let index = 0; index < 100_000; index++) {
-      store.append("large", { type: "user_message", text: String(index) });
-    }
-    const { result: tail, blockingMs } = await measureMainThreadWork(
-      `timeline tail main-thread work (wholeTurns=${wholeTurns})`,
-      () => {
-        let page = store.fetch("large", { limit: 200, wholeTurns });
-        for (let repeat = 1; repeat < 8; repeat++)
-          page = store.fetch("large", { limit: 200, wholeTurns });
-        return page;
-      },
-    );
+    const { result: tail, blockingMs } = await measureTimelineTail(wholeTurns);
     expect(tail.rows).toHaveLength(200);
     expect(tail.startSeq).toBe(99_801);
     expect(tail.endSeq).toBe(100_000);
diff --git a/packages/server/src/server/agent/test-utils/timeline-responsiveness.ts b/packages/server/src/server/agent/test-utils/timeline-responsiveness.ts
new file mode 100644
index 0000000000..bfe5ee59c4
--- /dev/null
+++ b/packages/server/src/server/agent/test-utils/timeline-responsiveness.ts
@@ -0,0 +1,25 @@
+import { setImmediate as nextTurn } from "node:timers/promises";
+import { measureMainThreadWork } from "../../../test-utils/main-thread-work.js";
+import { InMemoryAgentTimelineStore } from "../agent-timeline-store.js";
+
+// Measure a cold fetch on its own thread, without Vitest reporting or callbacks.
+const wholeTurns = process.argv[2] === "true";
+const store = new InMemoryAgentTimelineStore();
+store.initialize("large");
+for (let index = 0; index < 100_000; index++) {
+  store.append("large", { type: "user_message", text: String(index) });
+}
+// Finish fixture allocation and collect its temporary garbage before measuring.
+globalThis.gc!();
+await nextTurn();
+const measured = await measureMainThreadWork(
+  `timeline tail main-thread work (wholeTurns=${wholeTurns})`,
+  () => {
+    let page = store.fetch("large", { limit: 200, wholeTurns });
+    for (let repeat = 1; repeat < 8; repeat++)
+      page = store.fetch("large", { limit: 200, wholeTurns });
+    return page;
+  },
+);
+
+process.stdout.write(JSON.stringify(measured));