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LMArena Bridge supports multimodal requests by automatically handling image uploads on your behalf. When you send a message containing a base64-encoded image to a vision-capable model, the bridge validates the image, uploads it to LMArena’s R2 storage, and includes a signed URL in the request — all transparently through the standard OpenAI message format.

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Supported formats and limits

PropertyValue
Maximum file size10 MB (MAX_IMAGE_SIZE_BYTES)
Supported MIME typesimage/png, image/jpeg, image/gif, image/webp, image/svg+xml

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How to send an image

Pass images as base64-encoded data URLs inside the content array of a user message, following the OpenAI vision format. 
{
  "model": "gpt-4o",
  "messages": [
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "What's in this image?"
        },
        {
          "type": "image_url",
          "image_url": {
            "url": "data:image/png;base64,..."
          }
        }
      ]
    }
  ]
}

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What the bridge does automatically

When the bridge receives a message with an image, it performs these steps in order:
1

Validate MIME type and size

The bridge checks that the image’s MIME type is in SUPPORTED_IMAGE_MIME_TYPES and that the file does not exceed MAX_IMAGE_SIZE_BYTES (10 MB). Invalid images are rejected before any upload is attempted.
2

Check the MD5 cache

The bridge computes an MD5 hash of the image bytes and checks an in-memory cache. If a valid (non-expired) signed URL exists for that hash, the cached URL is reused and no upload occurs.
3

Upload to LMArena R2 storage

If the image is not cached, the bridge requests a pre-signed upload URL from LMArena, uploads the raw bytes to R2 storage, and then requests a signed download URL.
4

Cache the result

The returned signed URL and its expiry are stored in the in-memory cache keyed by MD5 hash. Subsequent requests with the same image bytes skip the upload until the URL expires.
5

Include the signed URL in the LMArena request

The signed download URL is substituted into the request payload sent to LMArena, replacing the original base64 data URL.

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MD5 caching

The bridge caches uploaded images by their MD5 hash to avoid redundant uploads. The cache holds up to 1,000 entries; when full, the oldest 100 entries are evicted. A cached entry is considered valid as long as its signed URL has not expired (with a 60-second margin).
Sending the same image multiple times within a session will only result in a single upload, provided the signed URL has not expired.

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Error handling

The bridge includes built-in resilience for image upload failures:
  • Upload failures are logged with contextual debug information.
  • Timeout handling is applied to both the upload request (60 s) and the signed URL request (30 s).
  • HTTP errors from R2 storage are caught and logged; the request fails gracefully rather than crashing.
If next_action_upload or next_action_signed_url are not present in config.json, image uploads will fail immediately. Refresh these values from the LMArena dashboard to restore image support.

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