diff --git a/tests/emitted-ack-trailer.test.cjs b/tests/emitted-ack-trailer.test.cjs
index 27986c6d3..2507c93bb 100644
--- a/tests/emitted-ack-trailer.test.cjs
+++ b/tests/emitted-ack-trailer.test.cjs
@@ -108,6 +108,38 @@ function withCleanup(t, dir) {
   t.after(() => cleanup(dir));
 }
 
+/**
+ * Build `commitCount` trivial linear commits on `refs/heads/main` via a
+ * single `git fast-import` stream (one subprocess spawn, not one per
+ * commit) and return the root commit's sha.
+ *
+ * Used by row 24 to force `git log`'s real formatting work comfortably
+ * past a small `timeoutMs`, without racing real subprocess completion
+ * time the way a razor-thin `timeoutMs: 1` against a trivial 2-commit
+ * repo does (#3985: `process-seam.cjs`'s own documented at-the-boundary
+ * case — a child finishing right at the deadline can report a real
+ * `status` alongside `error.code === 'ETIMEDOUT'`, which the seam
+ * correctly classifies as EXITED, not TIMED_OUT). A large real-work
+ * margin (tens of thousands of commits vs a ~20ms bound) is reliable on
+ * any real hardware and any platform — no PATH/PATHEXT/shebang concerns,
+ * unlike an approach that tries to intercept the `git` binary itself.
+ */
+function growHistoryFastImport(dir, commitCount) {
+  let stream = '';
+  for (let i = 1; i <= commitCount; i++) {
+    const message = `c${i}\n`;
+    stream += 'commit refs/heads/main\n';
+    stream += `mark :${i}\n`;
+    stream += `committer Ack Trailer Fixture <ack-trailer-fixture@example.com> ${1700000000 + i} +0000\n`;
+    stream += `data ${Buffer.byteLength(message, 'utf-8')}\n`;
+    stream += message;
+    if (i > 1) stream += `from :${i - 1}\n`;
+    stream += '\n';
+  }
+  gitOrThrow(['fast-import', '--quiet'], { cwd: dir, timeoutMs: GIT_FIXTURE_TIMEOUT_MS, input: stream });
+  return gitOrThrow(['rev-list', '--max-parents=0', 'HEAD'], { cwd: dir, timeoutMs: GIT_FIXTURE_TIMEOUT_MS }).trim();
+}
+
 // ─── grammar and hostile keys (pure parser) — rows 8, 9-18, 28-30 ────────────
 
 describe('grammar and hostile keys (pure parser)', () => {
@@ -626,16 +658,11 @@ describe('hostile IO: uncomputable range, subprocess failure, timeout', () => {
   test('git log is bounded by a timeout — row 24', (t) => {
     const dir = makeTempRepo('gsd-ack-trailer-24-');
     withCleanup(t, dir);
-    commitMessage(dir, 'init\n\nbaseline\n');
-    const baseSha = headSha(dir);
-    commitMessage(dir, 'change\n\nno trailer\n');
+    const rootSha = growHistoryFastImport(dir, 50000);
+
     const start = Date.now();
     assert.throws(
-      // Speculative `timeoutMs`: the real reader is expected to bound its own git
-      // subprocess and throw a timeout rather than hang. The stub ignores every
-      // argument today, so this option name is not load-bearing for the RED result —
-      // it documents the contract the real implementation must honor.
-      () => readAckTrailers({ baseRef: baseSha, headRef: 'HEAD', cwd: dir, timeoutMs: 1 }),
+      () => readAckTrailers({ baseRef: rootSha, headRef: 'HEAD', cwd: dir, timeoutMs: 20 }),
       /time/i,
       'an unreadable-in-time git call must throw a timeout, never hang',
     );