[BUG] Switching from Sonnet back to Opus after "prompt too long" error leaves session in broken state

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 accidentally switching from Opus to Sonnet mid-session, Sonnet triggered a "Prompt is too long" error. Switching back to Opus did not recover the session — it remained broken. /compact was unavailable, and opening a new session did not resolve the issue.

What Should Happen?

Switching back to Opus (which supports a larger context window) should clear the error state and allow the session to resume normally.

Error Messages/Logs

Steps to Reproduce

Start a long Claude Code session using Opus with a large codebase context
Accidentally switch model to Sonnet (e.g. via /model)
Sonnet returns "Prompt is too long" error (Sonnet has a smaller context window)
Switch back to Opus
Session remains broken — cannot send messages, /compact is unavailable
Opening a new session does not resolve the issue

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.34

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_