[BUG] Stderr not in transript (Ctrl + O) when SessionStart hook fails with non-blocking status code

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?

I created a SessionStart hook which prints "TEST OUTPUT" to stderr and returns with exit code 1. For Claude Code v2.1.25 or prior, output to stderr can be observed when in transcript mode.

 ▐▛███▜▌   Claude Code v2.1.25
▝▜█████▛▘  Sonnet 4.5 · API Usage Billing
  ▘▘ ▝▝    /Users/katrinaliu
  ⎿ SessionStart:startup hook error: Failed with non-blocking status code: TEST OUTPUT


─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  Showing detailed transcript · ctrl+o to toggle · ctrl+e to show all

However, starting from v2.1.27, the output disappeared.

 ▐▛███▜▌   Claude Code v2.1.27
▝▜█████▛▘  Sonnet 4.5 · API Usage Billing
  ▘▘ ▝▝    /Users/katrinaliu
  ⎿  SessionStart:startup hook error

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  Showing detailed transcript · ctrl+o to toggle · ctrl+e to show all

What Should Happen?

According to the docs:

Any other exit code is a non-blocking error. stderr is shown in verbose mode (Ctrl+O) and execution continues.

if the exit code is not 0 or 2, anything outputted to stderr should be shown in verbose/ transcript mode, which means the behavior of Claude Code from v2.1.27 onwards is wrong. I have also tried it on the latest version, and the behavior remains.

Error Messages/Logs

Steps to Reproduce

1. Create a SessionStart hook that prints to stderr and returns with a non-blocking exit code
2. Run claude with the hook added
3. Toggle Ctrl + O
4. Observe that the output doesn't show up

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.25

Claude Code Version

2.1.27, 2.1.32

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_