fix(render): scope M4A priming preservation to trims

packages/engine/src/services/chunkEncoder.test.ts

@@ -418,7 +418,7 @@ describe("muxVideoWithAudio audio codec handling", () => {
     });
   });
 
-  it("preserves M4A priming edit lists instead of shifting copied video", async () => {
+  it("keeps negative-timestamp repair for an M4A without a known priming edit list", async () => {
     const { spawn, calls } = createSpawnSpy();
     vi.resetModules();
     vi.doMock("child_process", () => ({ spawn }));
@@ -433,6 +433,30 @@ describe("muxVideoWithAudio audio codec handling", () => {
       { num: 30, den: 1 },
     );
 
+    await flushMuxCodecResolution();
+    expect(calls).toHaveLength(1);
+    expect(calls[0]!.args).toContain("copy");
+    expect(calls[0]!.args).toContain("-avoid_negative_ts");
+
+    emitClose(calls[0]!.proc, 0);
+    await expect(muxPromise).resolves.toMatchObject({ success: true });
+  });
+
+  it("preserves a known M4A priming edit list instead of shifting copied video", 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", preserveAudioPrimingEditList: true },
+      { num: 30, den: 1 },
+    );
+
     await flushMuxCodecResolution();
     expect(calls).toHaveLength(1);
     expect(calls[0]!.args).toContain("copy");

packages/engine/src/services/chunkEncoder.ts

@@ -77,6 +77,8 @@ export interface MuxVideoWithAudioOptions extends Partial<
    * depend on the file extension alone.
    */
   audioCodec?: "aac";
+  /** Preserve a priming edit list known to have been created by AAC re-encoding. */
+  preserveAudioPrimingEditList?: boolean;
 }
 
 async function shouldCopyAacSidecar(
@@ -691,7 +693,7 @@ export async function muxVideoWithAudio(
     }
   }
   const copiesContainerizedAac =
-    !isWebm && shouldCopyAudio && extname(audioPath).toLowerCase() === ".m4a";
+    !isWebm && shouldCopyAudio && config?.preserveAudioPrimingEditList === true;
   // PTS bases can diverge during mux and reintroduce negative DTS. See
   // buildEncoderArgs for the full reasoning on why that breaks playback.
   // A freshly encoded M4A is the exception: its edit list already hides the

packages/producer/src/services/distributed/assemble.ts

@@ -290,6 +290,7 @@ export async function assemble(
 
     // ── 3. Audio: pad-or-trim then mux ────────────────────────────────────
     let audioForMux: string | null = null;
+    let preserveAudioPrimingEditList = false;
     if (audioPath !== null && existsSync(audioPath)) {
       const paddedAudioPath = join(workDir, "audio-padded.m4a");
       const padTrimResult = await padOrTrimAudioToVideoFrameCount({
@@ -302,6 +303,7 @@ export async function assemble(
         throw new Error(`[assemble] audio pad/trim failed: ${padTrimResult.error}`);
       }
       audioForMux = paddedAudioPath;
+      preserveAudioPrimingEditList = padTrimResult.operation === "trim";
       log.info("[assemble] audio normalized for mux", {
         operation: padTrimResult.operation,
         targetDurationSeconds: padTrimResult.targetDurationSeconds,
@@ -321,7 +323,7 @@ export async function assemble(
         audioForMux,
         muxOutputPath,
         abortSignal,
-        { audioCodec: "aac" },
+        { audioCodec: "aac", preserveAudioPrimingEditList },
         { num: plan.dimensions.fpsNum, den: plan.dimensions.fpsDen },
       );
       if (!muxResult.success) {

packages/producer/src/services/render/stages/assembleStage.test.ts

@@ -69,7 +69,7 @@ describe("runAssembleStage audio duration parity", () => {
       "/tmp/audio.duration-normalized.m4a",
       "/tmp/output.mp4",
       undefined,
-      { audioCodec: "aac" },
+      { audioCodec: "aac", preserveAudioPrimingEditList: true },
       { num: 30, den: 1 },
     );
   });

packages/producer/src/services/render/stages/assembleStage.ts

@@ -76,7 +76,10 @@ export async function runAssembleStage(input: AssembleStageInput): Promise<Assem
       normalizeResult.outputPath,
       outputPath,
       abortSignal,
-      { audioCodec: "aac" },
+      {
+        audioCodec: "aac",
+        preserveAudioPrimingEditList: normalizeResult.operation === "trim",
+      },
       job.config.fps,
     );
     assertNotAborted();