[BUG] PreToolUse Hook Causes TUI Rendering Corruption

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?

Using PreToolUse hook as guardrail for all tools (no matcher specified) to block operations based on keyword detection.

Symptoms:

• TUI rendering corruption occurs regardless of hook output format
• Text overlapping with previous output
• Layout corruption in TUI
• Inconsistent line breaks/spacing

Tested scenarios (all produce identical corruption):

• Simple string: echo "BLOCK: Test" >&2
• Official JSON format: echo '{"hookSpecificOutput": ...}' >&2
• Multi-line heredoc
• Empty output: exit 0 only (no echo)
• Different tools: Bash/Read/Write

Reproduction rate: 100%

Related to Issue #9661 (PreToolUse hook displayed instead of actual tool invocation) - still OPEN in v2.0.55

What Should Happen?

Hook output should display cleanly in TUI with proper formatting:

BLOCK: Test

Expected behavior:

• No text overlapping
• Proper cursor positioning
• Readable, well-formatted output

Error Messages/Logs

PreToolUse:Bash hook error: [bash .claude/hook_test.sh]: BLOCK: Test


*(Renders with visual corruption in TUI)*

Note: This occurs even with minimal hook script (exit 0 only, no output).

Steps to Reproduce

1. Create minimal hook script (.claude/hook_test.sh):

Simple string format:

#!/bin/bash
echo "BLOCK: Test" >&2
exit 2

Official JSON format (same corruption):

#!/bin/bash
echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "BLOCK: Test"}' >&2
exit 2

1. Enable in .claude/settings.local.json:

"hooks": {
  "PreToolUse": [{"hooks": [{"type": "command", "command": "bash .claude/hook_test.sh"}]}]
}

1. Execute any tool (Bash/Read/Write)
2. Observe TUI display corruption

Reproduction rate: 100%

Workaround: None found. Disabling hooks is the only way to avoid corruption.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Probably around 2.0.52

Claude Code Version

2.0.55

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

Environment:

• Claude Code Version: 2.0.55
• Platform: Anthropic API
• OS: Windows (NT 10.0-19044)
• Model: Sonnet

Video recordings attached showing:

• Hook enabled: corruption visible
• Hook disabled: normal display

Related issues:

• #9661: PreToolUse hook displayed instead of actual tool invocation (reported v2.0.15, claimed fixed v2.0.19, still occurs v2.0.55)
• #5557: Startup hook output re-displays on every turn with --verbose

Hook pattern used:

echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "reason"}' >&2
exit 2

(Based on official examples from anthropics/claude-code repository)

https://github.com/user-attachments/assets/dc10d45e-1582-4a75-ad6b-41608a866490
https://github.com/user-attachments/assets/7624baef-d4e5-459f-b351-269ef02060da