Summary

The native vision fast path inlines screenshots as base64 into the tool-result envelope without checking pixel dimensions. Anthropic rejects any image >8000 px on either axis with a non-retryable 400. Once such an image is in the message history, every subsequent request fails identically — session is permanently bricked.

Reproduction

1. Anthropic vision model as main.
2. Agent calls browser_vision or vision_analyze on a page producing a screenshot >8000 px tall or wide (long articles, wide dashboards).
3. Next API call:

400 invalid_request_error: messages.N.content.M.image.source.base64.data:
  At least one of the image dimensions exceed max allowed size: 8000 pixels

4. Every retry hits the same error. Recovery requires /undo or editing the session JSON.

Root cause

tools/vision_tools.py validates byte size (20 MB cap) but not pixel dimensions before inlining. A low-byte image (e.g. 10000×800 solid-colour screenshot) passes the byte check but violates Anthropic's pixel cap.

Frequency

Hit twice in ~24 hours on independent sessions during normal browsing.