[BUG] Can't background sessions

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 creating a new session not from Agents View in the CLI after starting work I try to /background it will only work maybe 1 out of 10 times.

As you can see by the screenshot, there be messages sent MANY!

<img width="1444" height="451" alt="Image" src="https://github.com/user-attachments/assets/c80eb42c-013a-444e-8705-5c5543b96677" />

What Should Happen?

Should background as advertised.

I think it's a new session issue, if I leave, then resume -w -r etc then it seems to let me do it the second time.

Error Messages/Logs

Steps to Reproduce

Create new claude code session claude -w treename
Send message / start work
/bg
Fails to bg

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.141

Claude Code Version

2.1.14{2,3,4}

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_