[BUG] Shell profile echo statements pollute hook stdout, causing "JSON validation failed" errors

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 shell profile files (~/.zshrc, ~/.bashrc, etc.) contain unconditional echo statements, these statements execute when Claude Code spawns shell processes to run hooks. The echo output gets prepended to the hook's JSON response, causing "JSON validation failed" errors.

Example error displayed:

⏺ Ran 4 stop hooks
  ⎿  Stop hook error: JSON validation failed

The root cause is that a line like this in ~/.zshrc:

echo "✅ Shell ready on $(uname -m)"

Causes hook output to become:

✅ Shell ready on arm64
{}

Instead of just:

{}

This is invalid JSON and fails validation.

What Should Happen?

Either:

1. Documentation should warn about this common pitfall - that shell profiles should guard echo statements with interactive shell checks
2. Or Claude Code could spawn hook shells in a way that skips profile loading (e.g., bash --norc or zsh -f), though this may have other implications

Error Messages/Logs

⏺ Ran 4 stop hooks
  ⎿  Stop hook error: JSON validation failed

Also seen with SessionStart:

⎿  SessionStart:startup hook error

Steps to Reproduce

1. Add an unconditional echo to your shell profile:

``bash
# In ~/.zshrc or ~/.bashrc
echo "Shell ready"
``

1. Configure any command-type hook in .claude/settings.json:

``json
{
"hooks": {
"Stop": [{
"matcher": "*",
"hooks": [{
"type": "command",
"command": "echo '{}'"
}]
}]
}
}
``

1. Start a Claude Code session and let it complete (trigger Stop hook)

1. Observe "JSON validation failed" error

Workaround

Wrap any echo statements in shell profiles with an interactive shell check:

# Only print for interactive shells (not scripts/hooks)
[[ -o interactive ]] && echo "Shell ready"

Or for bash:

[[ $- == *i* ]] && echo "Shell ready"

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.25 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal (zsh)

Additional Information

This is a subtle issue because:

• The error message doesn't indicate the cause is stdout pollution
• Users don't typically think of their shell profile affecting JSON hook output
• The shell profile may have worked fine for years until hooks are introduced

Related issues:

• #18215 - Hook documentation contradicts stdout handling
• #19057 - Presence of ~/.bashrc triggers shell state capture issues