diff --git a/server.py b/server.py
index 023b976..09fa2df 100644
--- a/server.py
+++ b/server.py
@@ -364,6 +364,67 @@ async def get_cached(content_hash: str):
     return FileResponse(cache_path)
 
 
+@app.get("/cache/{content_hash}/mp4")
+async def get_cached_mp4(content_hash: str):
+    """Get cached content as MP4 (transcodes MKV on first request, caches result)."""
+    cache_path = CACHE_DIR / content_hash
+    mp4_path = CACHE_DIR / f"{content_hash}.mp4"
+
+    if not cache_path.exists():
+        raise HTTPException(404, f"Content {content_hash} not in cache")
+
+    # If MP4 already cached, serve it
+    if mp4_path.exists():
+        return FileResponse(mp4_path, media_type="video/mp4")
+
+    # Check if source is already MP4
+    media_type = detect_media_type(cache_path)
+    if media_type != "video":
+        raise HTTPException(400, "Content is not a video")
+
+    # Check if already MP4 format
+    import subprocess
+    try:
+        result = subprocess.run(
+            ["ffprobe", "-v", "error", "-select_streams", "v:0",
+             "-show_entries", "format=format_name", "-of", "csv=p=0", str(cache_path)],
+            capture_output=True, text=True, timeout=10
+        )
+        if "mp4" in result.stdout.lower() or "mov" in result.stdout.lower():
+            # Already MP4-compatible, just serve original
+            return FileResponse(cache_path, media_type="video/mp4")
+    except Exception:
+        pass  # Continue with transcoding
+
+    # Transcode to MP4 (H.264 + AAC)
+    transcode_path = CACHE_DIR / f"{content_hash}.transcoding.mp4"
+    try:
+        result = subprocess.run(
+            ["ffmpeg", "-y", "-i", str(cache_path),
+             "-c:v", "libx264", "-preset", "fast", "-crf", "23",
+             "-c:a", "aac", "-b:a", "128k",
+             "-movflags", "+faststart",
+             str(transcode_path)],
+            capture_output=True, text=True, timeout=600  # 10 min timeout
+        )
+        if result.returncode != 0:
+            raise HTTPException(500, f"Transcoding failed: {result.stderr[:200]}")
+
+        # Move to final location
+        transcode_path.rename(mp4_path)
+
+    except subprocess.TimeoutExpired:
+        if transcode_path.exists():
+            transcode_path.unlink()
+        raise HTTPException(500, "Transcoding timed out")
+    except Exception as e:
+        if transcode_path.exists():
+            transcode_path.unlink()
+        raise HTTPException(500, f"Transcoding failed: {e}")
+
+    return FileResponse(mp4_path, media_type="video/mp4")
+
+
 @app.get("/ui/cache/{content_hash}", response_class=HTMLResponse)
 async def ui_cache_view(content_hash: str, request: Request):
     """View cached content with appropriate display."""
@@ -432,7 +493,8 @@ async def ui_cache_view(content_hash: str, request: Request):
 """
 
     if media_type == "video":
-        html += f'<video src="/cache/{content_hash}" controls autoplay muted loop playsinline style="max-width:100%;max-height:500px;"></video>'
+        video_src = video_src_for_request(content_hash, request)
+        html += f'<video src="{video_src}" controls autoplay muted loop playsinline style="max-width:100%;max-height:500px;"></video>'
     elif media_type == "image":
         html += f'<img src="/cache/{content_hash}" alt="{content_hash}" style="max-width:100%;max-height:500px;">'
     else:
@@ -962,6 +1024,19 @@ async def delete_collection(name: str, username: str = Depends(get_required_user
     raise HTTPException(404, "Collection not found")
 
 
+def is_ios_request(request: Request) -> bool:
+    """Check if request is from iOS device."""
+    ua = request.headers.get("user-agent", "").lower()
+    return "iphone" in ua or "ipad" in ua
+
+
+def video_src_for_request(content_hash: str, request: Request) -> str:
+    """Get video src URL, using MP4 endpoint for iOS."""
+    if is_ios_request(request):
+        return f"/cache/{content_hash}/mp4"
+    return f"/cache/{content_hash}"
+
+
 def detect_media_type(cache_path: Path) -> str:
     """Detect if file is image or video based on magic bytes."""
     with open(cache_path, "rb") as f:
@@ -1627,7 +1702,8 @@ async def ui_detail_page(run_id: str, request: Request):
                 <div class="media-container">
             '''
             if input_media_type == "video":
-                html += f'<video src="/cache/{input_hash}" controls muted loop playsinline></video>'
+                input_video_src = video_src_for_request(input_hash, request)
+                html += f'<video src="{input_video_src}" controls muted loop playsinline></video>'
             elif input_media_type == "image":
                 html += f'<img src="/cache/{input_hash}" alt="input">'
             html += '</div></div>'
@@ -1641,7 +1717,8 @@ async def ui_detail_page(run_id: str, request: Request):
                 <div class="media-container">
             '''
             if output_media_type == "video":
-                html += f'<video src="/cache/{output_hash}" controls autoplay muted loop playsinline></video>'
+                output_video_src = video_src_for_request(output_hash, request)
+                html += f'<video src="{output_video_src}" controls autoplay muted loop playsinline></video>'
             elif output_media_type == "image":
                 html += f'<img src="/cache/{output_hash}" alt="output">'
             html += '</div></div>'