diff --git a/src/reels/render.rs b/src/reels/render.rs
index ca39515..9643309 100644
--- a/src/reels/render.rs
+++ b/src/reels/render.rs
@@ -159,7 +159,9 @@ pub fn build_segment_args(
 
 /// Build the concat-demuxer args that join rendered segments losslessly.
 /// `+faststart` moves the moov atom up front so the reel streams immediately
-/// on the mobile client.
+/// on the mobile client. The output muxer is forced with `-f mp4` because we
+/// write to a `.tmp` path (atomic publish) whose extension ffmpeg can't map to
+/// a format on its own.
 pub fn build_concat_args(list_path: &str, out_path: &str) -> Vec<String> {
     [
         "-y",
@@ -173,6 +175,8 @@ pub fn build_concat_args(list_path: &str, out_path: &str) -> Vec<String> {
         "copy",
         "-movflags",
         "+faststart",
+        "-f",
+        "mp4",
         out_path,
     ]
     .iter()
@@ -317,12 +321,19 @@ mod tests {
     }
 
     #[test]
-    fn concat_args_stream_copy_with_faststart() {
-        let args = build_concat_args("/tmp/list.txt", "/out.mp4");
+    fn concat_args_stream_copy_with_faststart_and_forced_muxer() {
+        // Output goes to a .tmp path, so the muxer must be forced — ffmpeg
+        // can't infer mp4 from the extension (the bug this guards against).
+        let args = build_concat_args("/tmp/list.txt", "/out.mp4.tmp");
         let joined = args.join(" ");
         assert!(joined.contains("-f concat -safe 0 -i /tmp/list.txt"));
         assert!(joined.contains("-c copy"));
         assert!(joined.contains("+faststart"));
+        assert!(joined.contains("-f mp4"));
+        // The forced muxer must come before the output path.
+        let f_mp4 = args.windows(2).position(|w| w == ["-f", "mp4"]).unwrap();
+        let out = args.iter().position(|a| a == "/out.mp4.tmp").unwrap();
+        assert!(f_mp4 < out);
     }
 
     #[test]