Twilio error 11751 means the media file you attached to your MMS message exceeds the size limit that Twilio enforces before attempting delivery to the carrier. This is a Twilio-side validation error, meaning the message is rejected at the API call stage before it even reaches the carrier network. The error code 11751 is distinct from 30019, which is a carrier-level size rejection: 11751 is caught by Twilio itself, making it faster to diagnose because you can see it immediately in your API response without waiting for a StatusCallback.
What Causes This Error
Twilio enforces a 5 MB total media size limit per MMS message across all attached media files combined. A single high-resolution photograph from a modern smartphone camera is typically 3 to 8 MB in its native format, easily exceeding this limit when attached directly without compression. Video files are the most common cause for enterprise customers who attempt to send short promotional clips or product demos via MMS without pre-processing the video to a web-optimized format. Multiple media attachments where each individual file is under 5 MB but their combined total exceeds the limit is a third cause, particularly in applications that allow users to attach multiple images to a single message. GIF files created by screen recording or animation export tools are frequently oversized, as animation-optimized GIFs can grow to 10 to 20 MB for even short clips.
How to Fix It Step by Step
Measure the file size of every media file you plan to attach before making the API call by checking the Content-Length of the media URL or the file size in bytes in your application code, and reject any file above 4 MB to keep a buffer below Twilio's 5 MB limit and account for HTTP overhead. For images, convert to JPEG format at 80 percent quality and resize to a maximum of 1200 pixels on the longest side: this process typically reduces a 6 MB PNG to under 300 KB without visible quality loss for SMS screen sizes. For video, use FFmpeg to transcode to H.264 MP4 at 480p resolution with a target bitrate of 600 kbps, which produces files under 2 MB for clips up to 30 seconds. For GIFs, either convert to short MP4 (which is smaller at the same quality) or use a GIF optimization tool like gifsicle with the option --lossy=80 to reduce file size before uploading the URL to Twilio.
How to Prevent It from Recurring
Add a media size validation function to your MMS send wrapper that performs an HTTP HEAD request to retrieve the Content-Length header of each MediaUrl before constructing the Twilio API call, and throws an application error with a descriptive message if the combined size exceeds 4 MB. Implement a media preprocessing pipeline for user-uploaded content: any image or video submitted by a user that will be used in an MMS should pass through an automatic resize and compression step before the URL is stored in your database, ensuring that by the time the URL reaches your Twilio API call it is already within safe limits. Set maximum file size limits in your file upload endpoints, for example using multer's limits option in Node.js or Django's DATA_UPLOAD_MAX_MEMORY_SIZE setting, to refuse oversized uploads at the ingestion point rather than discovering the problem at send time. Document the size limits for each media type you send (images, video, audio, GIF) in your internal API documentation so that every developer who works with the messaging code understands the constraints without needing to look them up.
When to Call a Specialist
If your media files are under the 5 MB limit but you are still receiving 11751, the issue may be related to the Content-Type header served by your media host, which Twilio validates alongside the file size, or a redirecting URL that Twilio's media fetcher cannot follow correctly. A specialist can test your media URL against Twilio's media fetching infrastructure and identify whether the problem is the file size, the content type, URL accessibility from Twilio's IP ranges, or HTTP redirect behavior. For high-volume MMS operations where you are sending thousands of media messages per day, a specialist can design and implement a media preprocessing service that handles compression, format conversion, CDN hosting, and URL management automatically, removing the risk of 11751 at the infrastructure level. Recurring 11751 errors on otherwise valid media files almost always point to a hosting or URL configuration issue that benefits from systematic external testing.
Conclusion
Error 11751 is a file size validation failure that is resolved by compressing or resizing your media before attaching it to your MMS API call. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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