/fork command crashes with 'undefined is not an object (evaluating T.match)'

Bug Description

Running the /fork command causes Claude Code to crash with a runtime error.

Steps to Reproduce

1. Start a Claude Code session in a git repository
2. Have a conversation with some context
3. Run /fork
4. Claude Code crashes

Error

ERROR  undefined is not an object (evaluating 'T.match')

The crash occurs in the tool output rendering path. Based on the stack trace, a function that parses stderr for sandbox violation tags calls .match() on an undefined value — it appears stderr is undefined instead of an empty string when rendering the output of the fork operation.

Simplified stack trace (from minified source):

F_8 → d7T → Z$ → eH → WJ → Ev → GTT → Up → Np → oR

F_8 is the function that runs a regex match (/<sandbox_violations>...) against stderr, which is undefined at that point.

Environment

• Claude Code version: 2.1.38
• OS: macOS (Darwin 25.2.0)
• Platform: darwin arm64
• Shell: zsh

Expected Behavior

/fork should create a forked session without crashing.

Additional Context

The stderr parameter passed to the Bash output renderer appears to not be null-checked before .match() is called on it. A simple guard like stderr ?? "" before the regex match would prevent this crash.