[BUG] Session crashes with API 400 errors after very short session - cache_control on empty text blocks

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?

After a very short session (only a few messages), Claude Code becomes completely unusable and returns repeated API 400 errors. The session cannot be continued even after clicking "Try again" or resuming the session.
Three distinct errors appeared in sequence:
Error 1:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.48.content.3.text: cache_control cannot be set for empty text blocks"},"request_id":"req_011Cac7YqWzBPSvKV2qdzjSZ"}
Error 2:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac7jsf2hxnRYKm1Sv8HA"}
Error 3 (after "Resumed session"):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac9WxydYfKGoUGdUeV8r"}
The session was very short — not a long context issue. Claude Code appears to be generating malformed message history with empty text blocks that have cache_control set on them, which the API rejects. Resuming the session does not fix it. The only workaround is starting a brand new session and losing all prior work.
This is costing paying users money and time.

What Should Happen?

After a very short session (only a few messages), Claude Code becomes completely unusable and returns repeated API 400 errors. The session cannot be continued even after clicking "Try again" or resuming the session.
Three distinct errors appeared in sequence:
Error 1:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.48.content.3.text: cache_control cannot be set for empty text blocks"},"request_id":"req_011Cac7YqWzBPSvKV2qdzjSZ"}
Error 2:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac7jsf2hxnRYKm1Sv8HA"}
Error 3 (after "Resumed session"):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac9WxydYfKGoUGdUeV8r"}
The session was very short — not a long context issue. Claude Code appears to be generating malformed message history with empty text blocks that have cache_control set on them, which the API rejects. Resuming the session does not fix it. The only workaround is starting a brand new session and losing all prior work.
This is costing paying users money and time.

Error Messages/Logs

After a very short session (only a few messages), Claude Code becomes completely unusable and returns repeated API 400 errors. The session cannot be continued even after clicking "Try again" or resuming the session.
Three distinct errors appeared in sequence:
Error 1:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.48.content.3.text: cache_control cannot be set for empty text blocks"},"request_id":"req_011Cac7YqWzBPSvKV2qdzjSZ"}
Error 2:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac7jsf2hxnRYKm1Sv8HA"}
Error 3 (after "Resumed session"):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac9WxydYfKGoUGdUeV8r"}
The session was very short — not a long context issue. Claude Code appears to be generating malformed message history with empty text blocks that have cache_control set on them, which the API rejects. Resuming the session does not fix it. The only workaround is starting a brand new session and losing all prior work.
This is costing paying users money and time.

Steps to Reproduce

After a very short session (only a few messages), Claude Code becomes completely unusable and returns repeated API 400 errors. The session cannot be continued even after clicking "Try again" or resuming the session.
Three distinct errors appeared in sequence:
Error 1:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.48.content.3.text: cache_control cannot be set for empty text blocks"},"request_id":"req_011Cac7YqWzBPSvKV2qdzjSZ"}
Error 2:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac7jsf2hxnRYKm1Sv8HA"}
Error 3 (after "Resumed session"):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: text content blocks must be non-empty"},"request_id":"req_011Cac9WxydYfKGoUGdUeV8r"}
The session was very short — not a long context issue. Claude Code appears to be generating malformed message history with empty text blocks that have cache_control set on them, which the API rejects. Resuming the session does not fix it. The only workaround is starting a brand new session and losing all prior work.
This is costing paying users money and time.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Sonnet 4.6

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_