diff --git a/templates/clips/app/components/player/video-player.tsx b/templates/clips/app/components/player/video-player.tsx
index 74c3767640..8664d4755f 100644
--- a/templates/clips/app/components/player/video-player.tsx
+++ b/templates/clips/app/components/player/video-player.tsx
@@ -277,9 +277,18 @@ export const VideoPlayer = forwardRef<VideoPlayerHandle, VideoPlayerProps>(
     // without this guard that effect would treat the two URLs as the "same
     // resource" and immediately revert our retry before `.load()` completes.
     const recoveringFromErrorRef = useRef(false);
+    const prevMseModeRef = useRef("");
+    // Render-phase mirrors of currentMs / isPlaying so the MSE-fallback effect
+    // below can read the pre-failure values. By the time effects run, React has
+    // already committed the new <video src>, causing the browser to reset
+    // currentTime -> 0 and paused -> true, making the element values useless.
+    const currentMsRef = useRef(startMs ?? 0);
+    const isPlayingRef = useRef(false);
     const [activeVideoSrc, setActiveVideoSrc] = useState(resolvedVideoSrc);
     const [isPlaying, setIsPlaying] = useState(false);
     const [currentMs, setCurrentMs] = useState(startMs ?? 0);
+    currentMsRef.current = currentMs;
+    isPlayingRef.current = isPlaying;
     const [loomStartMs, setLoomStartMs] = useState<number | null>(null);
     const [volume, setVolume] = useState(1);
     // Autoplaying players (e.g. the Slack unfurl embed, `?autoplay=1`) must
@@ -392,6 +401,45 @@ export const VideoPlayer = forwardRef<VideoPlayerHandle, VideoPlayerProps>(
         ? undefined
         : activeVideoSrc;
 
+    // When the MSE pipeline breaks mid-stream (premature 416 or
+    // ERR_CONTENT_LENGTH_MISMATCH after the backing GCS object is replaced by
+    // the compressed version), the loader calls onFatal which flips mse.mode to
+    // "native". This effect detects that transition and:
+    //  1. saves the playback position via currentMsRef (v.currentTime is already 0)
+    //  2. cache-busts activeVideoSrc so the native <video> path fetches fresh
+    //     headers rather than a proxy-cached content-length from the old object
+    //  3. if the user was playing, arms playAttemptPendingRef so the existing
+    //     retryPendingPlay call in onLoadedData resumes without user interaction
+    useEffect(() => {
+      const prev = prevMseModeRef.current;
+      prevMseModeRef.current = mse.mode;
+      if (prev !== "mse" || mse.mode !== "native") return;
+
+      const wasPlaying = isPlayingRef.current;
+      const posMs = currentMsRef.current > 0 ? currentMsRef.current : null;
+      if (posMs != null) resumeAfterReloadMsRef.current = posMs;
+
+      if (activeVideoSrc) {
+        recoveringFromErrorRef.current = true;
+        setActiveVideoSrc(
+          setUrlSearchParam(activeVideoSrc, "cb", String(Date.now())),
+        );
+      }
+
+      if (wasPlaying) {
+        const nextId = playAttemptIdRef.current + 1;
+        playAttemptIdRef.current = nextId;
+        playAttemptPendingRef.current = true;
+        setIsPlayPending(true);
+        setIsBuffering(true);
+      } else {
+        setIsPlaying(false);
+        setIsBuffering(false);
+      }
+      setIsPreparing(true);
+      setCanPlay(false);
+    }, [mse.mode, activeVideoSrc]);
+
     useEffect(() => {
       if (!resolvedVideoSrc) {
         setActiveVideoSrc(undefined);
diff --git a/templates/clips/app/lib/fmp4.test.ts b/templates/clips/app/lib/fmp4.test.ts
index 355705557a..8f7baea887 100644
--- a/templates/clips/app/lib/fmp4.test.ts
+++ b/templates/clips/app/lib/fmp4.test.ts
@@ -57,6 +57,19 @@ describe("isFragmentedMp4Head", () => {
     expect(isFragmentedMp4Head(new Uint8Array([...ftyp, ...moov]))).toBe(false);
   });
 
+  it("returns false when classic mp4 binary data contains the bytes 'mvex' inside mvhd", () => {
+    // A regular MP4 whose mvhd payload happens to contain the byte sequence
+    // 0x6d766578 ("mvex") — a false positive the old raw-scan code would hit.
+    const ftyp = box("ftyp", [
+      ...Array.from(enc.encode("isom")),
+      ...u32(0),
+      ...Array.from(enc.encode("mp42")),
+    ]);
+    const mvexBytes = Array.from(enc.encode("mvex")); // 6d 76 65 78
+    const moov = box("moov", box("mvhd", [...u32(0), ...mvexBytes, ...u32(0)]));
+    expect(isFragmentedMp4Head(new Uint8Array([...ftyp, ...moov]))).toBe(false);
+  });
+
   it("returns false when it is not an mp4", () => {
     expect(
       isFragmentedMp4Head(new Uint8Array(enc.encode("not an mp4 file"))),
diff --git a/templates/clips/app/lib/fmp4.ts b/templates/clips/app/lib/fmp4.ts
index f62b18ab94..e3fa0a5eb4 100644
--- a/templates/clips/app/lib/fmp4.ts
+++ b/templates/clips/app/lib/fmp4.ts
@@ -208,14 +208,36 @@ export function findMoofOffset(bytes: Uint8Array): number {
  * True when the head of an MP4 shows the fragmented shape: the `hlsf` brand in
  * `ftyp`, or an `mvex` box inside `moov` (present only in fragmented files).
  * `headBytes` should be the first few KB of the file.
+ *
+ * Both checks walk the ISO BMFF box structure rather than scanning raw bytes.
+ * Numeric fields inside moov children (timestamps, matrix values, codec config)
+ * can accidentally contain the byte sequences "mvex" or "hlsf", producing false
+ * positives if we just scan the whole buffer.
  */
 export function isFragmentedMp4Head(headBytes: Uint8Array): boolean {
   if (headBytes.byteLength < 8) return false;
-  // Must look like an MP4 at all.
+
   if (indexOfAscii(headBytes, "ftyp") !== 4) return false;
-  if (indexOfAscii(headBytes, "hlsf") !== -1) return true;
-  if (indexOfAscii(headBytes, "mvex") !== -1) return true;
-  return false;
+
+  const ftypSize = readU32(headBytes, 0);
+  if (ftypSize < 12) return false;
+  const ftypEnd = Math.min(ftypSize, headBytes.byteLength);
+
+  // Offset 12 is minor_version (uint32), not a brand — start compatible brands at 16.
+  if (ftypEnd >= 12 && readType(headBytes, 8) === "hlsf") return true;
+  for (let i = 16; i + 4 <= ftypEnd; i += 4) {
+    if (readType(headBytes, i) === "hlsf") return true;
+  }
+
+  const boxes = readTopLevelBoxes(headBytes);
+  const moov = boxes.find((b) => b.type === "moov");
+  if (!moov || moov.size === 0) return false;
+  const moovPayloadEnd = Math.min(moov.start + moov.size, headBytes.byteLength);
+  const moovPayload = headBytes.subarray(
+    moov.start + moov.headerSize,
+    moovPayloadEnd,
+  );
+  return readTopLevelBoxes(moovPayload).some((b) => b.type === "mvex");
 }
 
 /** Cache detection by URL identity so we sniff each asset only once. */
diff --git a/templates/clips/app/lib/mse-video-loader.ts b/templates/clips/app/lib/mse-video-loader.ts
index 2242677c9e..72f800a7cb 100644
--- a/templates/clips/app/lib/mse-video-loader.ts
+++ b/templates/clips/app/lib/mse-video-loader.ts
@@ -555,11 +555,22 @@ export class MseVideoLoader {
       headers: { Range: `bytes=${start}-${end}` },
       signal: controller.signal,
     });
-    // 416 means we asked past the end — treat as a clean end-of-stream.
-    if (res.status === 416) return { bytes: new Uint8Array(0), eof: true };
+    if (res.status === 416) {
+      // A 416 at an offset before the known file total means the backing object
+      // was replaced with a smaller compressed version while we were streaming.
+      // Throw so runPump's catch calls fail() -> onFatal -> native path recovery.
+      // When totalKnown is false (cross-origin CDN hides Content-Range), we
+      // cannot tell premature from real EOF so we keep the old safe-EOF behaviour.
+      if (this.totalKnown && start < this.totalBytes) {
+        throw new Error("Range 416 before known EOF: backing file replaced");
+      }
+      return { bytes: new Uint8Array(0), eof: true };
+    }
     if (!res.ok) {
       throw new Error(`Range request failed: ${res.status}`);
     }
+    // If the backing file shrank mid-response (ERR_CONTENT_LENGTH_MISMATCH),
+    // arrayBuffer() throws here, which also routes through fail() -> onFatal.
     const buffer = await res.arrayBuffer();
     const bytes = new Uint8Array(buffer);