[BUG] Claude Code Cloud: API 400 "cache_control cannot be set for empty text blocks" bricks sessions unrecoverably

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?

Claude Code Cloud sessions are crashing with a 400 from the Anthropic
Messages API and become permanently unusable once they do. The agent
is constructing request payloads that contain a text content block with
empty text and a cache_control marker attached to it, which the API
correctly rejects.

<img width="1352" height="287" alt="Image" src="https://github.com/user-attachments/assets/8ab1ac23-1064-405c-b4d0-5c2d012e5911" />
<img width="1302" height="713" alt="Image" src="https://github.com/user-attachments/assets/f8ec8db2-13f8-401b-ae9e-5e7033753e03" />

What Should Happen?

Claude should work and continue coding...

Error Messages/Logs

API Error: 400 messages.248.content.3.text: cache_control cannot be set for empty text blocks
API Error: 400 messages.28.content.2.text: cache_control cannot be set for empty text blocks
API Error: 400 messages: text content blocks must be non-empty

Steps to Reproduce

I don't know how to reproduce, happend in two different sessions, one long (>200 messages) and one brand new (~25 messages). I think it happend when i pasted a screenshot.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude 1.7196.0 (2dbd78) 2026-05-12T05:34:40.000Z

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_