[BUG] API Error 400: "Image format image/png not supported" — Claude Code auto-generated screenshots rejected

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

During a coding session, Claude Code automatically generates/captures PNG screenshots and sends them to the API. The API rejects them with error 400: "Image format image/png not supported". This happens without any user action — Claude Code itself initiates the image capture. The error breaks the current request. This is different from known MIME mismatch issues (#11936) — here the API says the PNG format itself is not supported in this context.

What Should Happen?

Claude Code should either use a supported image format for the given API endpoint, or gracefully handle the rejection without breaking the session — e.g., skip the image and continue with text-only context.

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.1.content.19.image.source.base64.data: Image format image/png not supported"},"request_id":"req_011CafcHTPge4dZUXQZFY1v6"}

Steps to Reproduce

Start a Claude Code session
Work on a task that causes Claude Code to auto-capture screenshots (e.g., [opisz krótko co robiłaś — np. "reviewing UI components" / "working with MCP tools"])
Claude Code automatically sends a PNG image to the API
Error appears: "Image format image/png not supported"

Reoeatded in 3 other approaches, repeating constantly

Note: The error occurs spontaneously — no manual image paste or attachment by the user.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Version 1.5354.0 (9a9e3d)

Platform

Other

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_