How to Fix Mastodon ‘Could Not Process Video’ on Upload

When you upload a video to Mastodon and see the error message “Could Not Process Video,” the upload fails and the video never appears in your post. This error typically occurs because the video file does not meet Mastodon’s strict media requirements for format, size, resolution, or duration. This article explains the exact technical reasons behind the error and provides step-by-step methods to fix it on both the web interface and mobile apps.

Why Mastodon Rejects Your Video Upload

Mastodon uses the FFmpeg library to process uploaded videos. When FFmpeg fails to decode, re-encode, or extract metadata from a video file, the server returns the “Could Not Process Video” error. The most common root causes include:

• Unsupported container format: Mastodon only supports MP4, WebM, and MOV containers. AVI, MKV, FLV, WMV, and other formats cause FFmpeg to fail during remuxing.
• Wrong video codec: The video must use H.264 or VP9. HEVC H.265, AV1, MPEG-4, and other codecs are not supported by the default FFmpeg configuration on most Mastodon instances.
• Wrong audio codec: Only AAC and Opus audio codecs are accepted. MP3, Vorbis, FLAC, or PCM audio cause the processing pipeline to abort.
• File size exceeds server limit: Each Mastodon instance sets its own maximum upload size. The default is 40 MB for video, but some instances allow up to 100 MB. If your file is larger, the upload is rejected before processing begins.
• Resolution or duration too high: Mastodon clips videos to 1920×1080 pixels and 40 minutes maximum. Higher resolutions like 4K or longer durations trigger a processing failure.
• Corrupted or incomplete file: A video that was partially downloaded or has a broken header cannot be parsed by FFmpeg and produces the error.

Steps to Fix the Video Upload Error

Follow the steps below in order. Start with the simplest fix and proceed to more advanced methods only if needed.

Method 1: Convert the Video to a Supported Format Using a Desktop Tool

1. Install HandBrake
   Download and install HandBrake from handbrake.fr. This free tool converts video to Mastodon-compatible settings.
2. Open your video file
   Launch HandBrake and click Open Source. Select the video that failed to upload.
3. Set output format to MP4
   In the Summary tab, set Format to MP4. Do not use MKV or WebM unless you are certain the receiver supports them.
4. Choose H.264 video codec
   Under the Video tab, set Video Encoder to H.264 x264. This is the most compatible codec for Mastodon.
5. Set audio codec to AAC
   Under the Audio tab, change the Codec to AAC. Remove any additional audio tracks.
6. Limit resolution to 1920×1080
   Under the Dimensions tab, set Resolution Limit to 1920×1080. Enable Keep Aspect Ratio.
7. Verify file size
   Check the estimated file size shown in the Summary tab. If it exceeds 40 MB, reduce the Quality slider to a lower RF value (higher compression) or shorten the video.
8. Export and upload
   Click Start Encode. After conversion, upload the new MP4 file to Mastodon.

Method 2: Use an Online Converter If You Cannot Install Software

1. Open CloudConvert
   Go to cloudconvert.com in your browser. No account is required for small files.
2. Upload the failing video
   Click Select File and choose your video. CloudConvert supports most input formats.
3. Set output to MP4
   Under Convert to, select Video and then MP4.
4. Adjust advanced options
   Click the wrench icon next to the output format. Set Video Codec to H.264 and Audio Codec to AAC. Set Max Width to 1920 and Max Height to 1080.
5. Start conversion
   Click Convert. Wait for the process to finish, then download the new file.
6. Upload the converted video to Mastodon
   Return to Mastodon and attach the converted MP4. The error should no longer appear.

Method 3: Reduce File Size by Trimming or Compressing the Original Video

1. Open the video in a video editor
   Use the Photos app on Windows 10 or Windows 11, or any free editor like Shotcut.
2. Trim the video to under 40 minutes
   Cut out unnecessary sections. Mastodon does not support videos longer than 40 minutes.
3. Reduce resolution to 1080p
   In the editor’s export settings, set the resolution to 1920×1080 or lower. Remove 4K or 8K resolution.
4. Lower the bitrate
   In the export settings, set the video bitrate to 8 Mbps or lower. This significantly reduces file size.
5. Export as MP4 with H.264
   Choose the MP4 format with H.264 video and AAC audio. Save the file.
6. Check file size in File Explorer
   Right-click the exported file and select Properties. Confirm the size is below your instance’s limit (usually 40 MB).
7. Upload the compressed video
   Post the new file to Mastodon. The error should be resolved.

If the Error Persists After Conversion

If you have converted the video to a supported format and the error still appears, the problem may be on the server side. Try these additional checks.

Check Your Instance’s Upload Limit

Each Mastodon instance sets its own maximum file size. To check yours, go to Preferences > Administration > Server Settings. Look for the Upload limit field. The default is 40 MB for video. If your file is under this limit, the issue is not file size.

Test with a Short, Small Video Clip

Create a 10-second MP4 video using your phone’s camera or a screen recorder. Upload it to Mastodon. If this test video uploads successfully, the problem is specific to your original file. If the test also fails, the Mastodon instance may have a server-side processing issue. Contact your instance admin.

Clear Browser Cache or Try a Different Browser

Corrupted browser cache can interfere with uploads. Clear your browser cache and cookies, then restart the browser. Alternatively, try uploading from a different browser or from the Mastodon mobile app for Android or iOS.

Item	Supported	Not Supported
Container format	MP4, WebM, MOV	AVI, MKV, FLV, WMV, OGG
Video codec	H.264, VP9	HEVC H.265, AV1, MPEG-4, VP8
Audio codec	AAC, Opus	MP3, Vorbis, FLAC, PCM, WMA
Maximum resolution	1920×1080 pixels	3840×2160 4K or higher
Maximum duration	40 minutes	Over 40 minutes
Maximum file size	40 MB default	Over instance limit

After converting your video to MP4 with H.264 and AAC, keeping resolution at 1080p, and verifying the file size is under 40 MB, the “Could Not Process Video” error on Mastodon should be resolved. If you continue to see the error, contact your instance administrator to check the server’s FFmpeg configuration or upload limit settings. As an advanced tip, you can also try using the command-line tool FFmpeg directly with the command ffmpeg -i input.mp4 -c:v libx264 -c:a aac -vf scale=1920:1080 output.mp4 to ensure full control over encoding parameters.