[BUG]API Error: 400 messages: text content blocks must be non-empty

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?

When submitting an image with no accompanying text in Claude Code's chat,
the API throws: "400 messages: text content blocks must be non-empty"

A fallback text block should be added when only an image is submitted.

Steps to reproduce:

1. Open Claude Code chat
2. Attach an image without typing any text
3. Submit

Expected: Request succeeds with a default prompt
Actual: API Error 400 - text content blocks must be non-empty

What Should Happen?

API Error 400 when sending image with no text in chat

Error Messages/Logs

API Error: 400 messages: text content blocks must be non-empty

Steps to Reproduce

please help

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Opus

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

<img width="1460" height="723" alt="Image" src="https://github.com/user-attachments/assets/2d1fd15f-0321-407a-8f96-565d306725f7" />