API Error 400: "user messages must have non-empty content" breaks long conversations

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 long conversation (~332+ messages), I started getting a persistent API Error 400 that made the conversation completely unrecoverable. Every subsequent message returned the same error.

What Should Happen?

Long conversations should not break. If a message slot becomes empty due to context compression or other internal processing, the API should handle it gracefully rather than returning an unrecoverable error.

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.332: user messages must have non-empty content"},"request_id":"req_011CZPSH4nwewodHGx9vezpn"}

Steps to Reproduce

Have a long conversation with 300+ messages
At some point the conversation breaks with this error
Every new message after that returns the same error
No way to recover — must start a new conversation

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.81

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_