diff --git a/frontend/src/features/optimize/OptimizeModal.tsx b/frontend/src/features/optimize/OptimizeModal.tsx
index 0d38881..86e1088 100644
--- a/frontend/src/features/optimize/OptimizeModal.tsx
+++ b/frontend/src/features/optimize/OptimizeModal.tsx
@@ -68,23 +68,36 @@ function formatEta(ms: number): string {
   return `${minutes} min left`;
 }
 
+interface MediaInfo {
+  width: number;
+  height: number;
+  duration: number | null;
+}
+
 function PreviewPanel({
   title,
   url,
   isVideo,
   size,
-  dimensions,
   note,
   state,
+  onMetadata,
 }: {
   title: string;
   url: string | null;
   isVideo: boolean;
   size: number | null;
-  dimensions: { width: number | null; height: number | null } | null;
   note?: string | null;
   state: "original" | "result" | "empty";
+  onMetadata?: (info: MediaInfo) => void;
 }) {
+  const [info, setInfo] = useState<MediaInfo | null>(null);
+
+  function report(next: MediaInfo) {
+    setInfo(next);
+    onMetadata?.(next);
+  }
+
   return (
     <section className="flex flex-col overflow-hidden rounded-lg border border-ctp-surface0 bg-ctp-base">
       <div className="flex items-center justify-between border-b border-ctp-surface0 px-3 py-1.5">
@@ -99,18 +112,38 @@ function PreviewPanel({
         {url && state !== "empty" ? (
           isVideo ? (
             <video
+              key={url}
               src={url}
               controls
               loop
               muted
               preload="metadata"
               className="max-h-56 w-full object-contain"
+              onLoadedMetadata={(event) => {
+                const element = event.currentTarget;
+                report({
+                  width: element.videoWidth,
+                  height: element.videoHeight,
+                  duration: Number.isFinite(element.duration)
+                    ? element.duration
+                    : null,
+                });
+              }}
             />
           ) : (
             <img
+              key={url}
               src={url}
               alt={title}
               className="max-h-56 w-full object-contain"
+              onLoad={(event) => {
+                const element = event.currentTarget;
+                report({
+                  width: element.naturalWidth,
+                  height: element.naturalHeight,
+                  duration: null,
+                });
+              }}
             />
           )
         ) : (
@@ -122,8 +155,10 @@ function PreviewPanel({
         )}
       </div>
       <div className="px-3 py-2 text-[11px] text-ctp-overlay0">
-        {dimensions?.width && dimensions?.height
-          ? `${dimensions.width}×${dimensions.height}`
+        {info
+          ? `${info.width}×${info.height}${
+              info.duration !== null ? ` · ${info.duration.toFixed(1)} s` : ""
+            }`
           : "…"}
         {note ? ` · ${note}` : ""}
       </div>
@@ -150,6 +185,8 @@ export function OptimizeModal({
   const [uploadPercent, setUploadPercent] = useState(0);
   const [applyError, setApplyError] = useState<string | null>(null);
   const [applied, setApplied] = useState(false);
+  const [originalInfo, setOriginalInfo] = useState<MediaInfo | null>(null);
+  const [resultInfo, setResultInfo] = useState<MediaInfo | null>(null);
 
   const codecQuery = useQuery({
     queryKey: ["optimize-codecs"],
@@ -204,10 +241,16 @@ export function OptimizeModal({
   const hardwareAvailable =
     codecQuery.data?.some((entry) => entry.supported && entry.hardware) ??
     false;
+  const truncated =
+    isVideo &&
+    originalInfo?.duration != null &&
+    resultInfo?.duration != null &&
+    resultInfo.duration + 1 < originalInfo.duration;
 
   function process() {
     setApplied(false);
     setApplyError(null);
+    setResultInfo(null);
     run({
       url: item.raw_url,
       filename: item.filename,
@@ -257,24 +300,29 @@ export function OptimizeModal({
                 url={item.raw_url}
                 isVideo={isVideo}
                 size={originalSize}
-                dimensions={{ width: null, height: null }}
                 state="original"
+                onMetadata={setOriginalInfo}
               />
               <PreviewPanel
                 title="Processed"
                 url={result?.url ?? null}
                 isVideo={isVideo}
                 size={resultSize}
-                dimensions={
-                  result
-                    ? { width: result.width, height: result.height }
-                    : null
-                }
                 note={result?.note ?? null}
                 state={result ? "result" : "empty"}
+                onMetadata={setResultInfo}
               />
             </div>
 
+            {truncated ? (
+              <p className="rounded-md border border-ctp-red/40 bg-ctp-red/10 px-3 py-2 text-xs text-ctp-red">
+                The processed video is shorter than the original (
+                {resultInfo?.duration?.toFixed(1)} s vs{" "}
+                {originalInfo?.duration?.toFixed(1)} s) — encoding may have
+                stopped early. Don't apply it.
+              </p>
+            ) : null}
+
             {status === "running" ? (
               <div className="rounded-md border border-ctp-surface1 bg-ctp-base p-3">
                 <div className="flex items-center gap-3">
diff --git a/frontend/src/features/optimize/video.ts b/frontend/src/features/optimize/video.ts
index 820c0ef..d1587f5 100644
--- a/frontend/src/features/optimize/video.ts
+++ b/frontend/src/features/optimize/video.ts
@@ -68,11 +68,11 @@ function assembleBlob(chunks: CollectedChunk[], type: string): Blob {
   if (chunks.length === 0) {
     throw new Error("Encoding produced no output.");
   }
-  const sorted = [...chunks].sort(
-    (a, b) => a.position - b.position || a.sequence - b.sequence,
-  );
+  // The muxer may rewrite earlier ranges (header patching), so apply writes
+  // in arrival order — the newest write wins per byte range — and only then
+  // lay the segments out by position.
+  const ordered = [...chunks].sort((a, b) => a.sequence - b.sequence);
 
-  // Overwrite ranges so the newest write for each byte range survives.
   interface Segment {
     start: number;
     end: number;
@@ -80,7 +80,7 @@ function assembleBlob(chunks: CollectedChunk[], type: string): Blob {
     blob: Blob;
   }
   let segments: Segment[] = [];
-  for (const chunk of sorted) {
+  for (const chunk of ordered) {
     const start = chunk.position;
     const end = start + chunk.blob.size;
     const next: Segment[] = [];