diff --git a/packages/producer/src/regression-harness.ts b/packages/producer/src/regression-harness.ts
index a540fc31e..776d14145 100644
--- a/packages/producer/src/regression-harness.ts
+++ b/packages/producer/src/regression-harness.ts
@@ -266,7 +266,14 @@ function validateMetadata(meta: unknown): TestMetadata {
       "meta.json: 'renderConfig.codec' must be 'h264' or 'h265' (or omit for the format's default)",
     );
   }
-  if (rc.codec !== undefined && rc.format !== undefined && rc.format !== "mp4") {
+  // Normalize the implicit default before comparing so a fixture that
+  // omits `format` (which defaults to "mp4" everywhere downstream) doesn't
+  // get accidentally treated as "format is missing, so codec is illegal."
+  // The previous formulation `rc.format !== undefined && rc.format !== "mp4"`
+  // worked but relied on the reader knowing the default; this reads the
+  // intent more directly.
+  const effectiveFormat = (rc.format as string | undefined) ?? "mp4";
+  if (rc.codec !== undefined && effectiveFormat !== "mp4") {
     throw new Error(
       `meta.json: 'renderConfig.codec' is only valid for format='mp4' (got format=${JSON.stringify(
         rc.format,
diff --git a/packages/producer/tests/distributed/mp4-h265-sdr/src/index.html b/packages/producer/tests/distributed/mp4-h265-sdr/src/index.html
index 710a6ead3..ce638a515 100644
--- a/packages/producer/tests/distributed/mp4-h265-sdr/src/index.html
+++ b/packages/producer/tests/distributed/mp4-h265-sdr/src/index.html
@@ -84,9 +84,21 @@
         <circle cx="24" cy="24" r="18" fill="none" stroke="#6366f1" stroke-width="4" />
         <circle cx="24" cy="24" r="6" fill="#6366f1" />
       </svg>
+      <!--
+        No audio element on purpose: AAC frame quantization pads a 2-second
+        silent track past 2.0s of container time, which extends format.duration
+        past nb_frames / fps and trips the harness PSNR sampler at the very
+        last checkpoint. The chunk-boundary contracts this fixture pins are
+        video-only; omitting audio keeps container duration == 2.0s exactly.
+        (Same rationale as mp4-h264-sdr.)
+      -->
     </div>
 
     <script>
+      // Same chunk-seam stressors as the mp4-h264-sdr sibling: crossfade
+      // straddles the frame-30 seam (0.9-1.1s = frames 27-33); continuous
+      // icon rotation makes per-frame angle a pure function of seek-position
+      // so virtual-clock drift surfaces as discontinuity at frame 15/30/45.
       const tl = gsap.timeline({ paused: true });
       window.__timelines = window.__timelines || {};
       window.__timelines["main-comp"] = tl;