diff --git a/packages/engine/src/services/chunkEncoder.test.ts b/packages/engine/src/services/chunkEncoder.test.ts
index 80491e966..f0ce7adab 100644
--- a/packages/engine/src/services/chunkEncoder.test.ts
+++ b/packages/engine/src/services/chunkEncoder.test.ts
@@ -373,6 +373,28 @@ describe("encodeFramesChunkedConcat ffmpegEncodeTimeout", () => {
 });
 
 describe("muxVideoWithAudio audio codec handling", () => {
+  it("caps copied audio to the encoded video duration", async () => {
+    const { spawn, calls } = createSpawnSpy();
+    vi.resetModules();
+    vi.doMock("child_process", () => ({ spawn }));
+
+    const { muxVideoWithAudio } = await import("./chunkEncoder.js");
+    const muxPromise = muxVideoWithAudio(
+      "/tmp/video-only.mp4",
+      "/tmp/audio.duration-normalized.m4a",
+      "/tmp/output.mp4",
+      undefined,
+      { audioCodec: "aac", durationSeconds: 16.066667 },
+      { num: 30, den: 1 },
+    );
+
+    await flushMuxCodecResolution();
+    expect(calls[0]!.args).toContain("-t");
+    expect(calls[0]!.args).toContain("16.066667");
+    emitClose(calls[0]!.proc, 0);
+    await expect(muxPromise).resolves.toMatchObject({ success: true });
+  });
+
   it("copies HyperFrames AAC sidecars into MP4 instead of re-encoding", async () => {
     const { spawn, calls } = createSpawnSpy();
     vi.resetModules();