statusLine stops rendering under cmux after updating from 2.1.263 (blank, no error)

Hi team, thanks for Claude Code. Filing a regression I hit this morning after an auto-update.

What happens

My statusLine went completely blank in every new session. No error, no partial line, just nothing where the HUD normally sits. It had been working fine the day before.

The key finding

The statusLine command itself is healthy. I pointed statusLine at a wrapper that logs every invocation and always prints a fallback line, then opened fresh sessions. The log showed the command firing on every refresh (hundreds of invocations), each exiting 0 with valid single-line output. Yet nothing rendered. So Claude Code is running the command and getting good output, but the line is not making it onto the screen.

Rolling the claude binary back to 2.1.263 fixed it instantly:

• Broken: 2.1.266 (and I suspect 2.1.265, which landed the night before)
• Working: 2.1.263

Flipping ~/.local/bin/claude back to 2.1.263 and opening a new session brought the HUD straight back. Flipping forward to 2.1.266 makes it blank again.

Why I think it is worth a look

The command works, the output is valid, and the only variable that changed between working and broken was the Claude Code version. That points to a change in how the statusLine is emitted or drawn (for example alternate-screen handling, cursor positioning, or the final row) that a PTY host does not display.

Environment

• macOS 15.5 (Darwin 25.5.0), Apple Silicon
• Terminal host: cmux 0.64.22 (com.cmuxterm.app), TERM=xterm-ghostty
• Claude Code broken on 2.1.266, works on 2.1.263
• statusLine type: command (tested with two different commands, same result)

Repro

1. Run Claude Code 2.1.266 inside cmux with any statusLine command configured.
2. Open a fresh session. The statusLine is blank.
3. Point statusLine.command at a script that logs to a file and always echoes a fixed string. Confirm the log fills up (command is firing, exit 0) while the screen stays blank.
4. ln -sfn ~/.local/share/claude/versions/2.1.263 ~/.local/bin/claude, open a new session. The statusLine renders again.

Happy to add more logs or test a patched build. I have not confirmed whether this reproduces in a plain terminal versus only under a PTY host like cmux, so if that distinction is useful let me know and I will test it.