Hooks Development

Guide for developing Claude Code hooks with proper output visibility patterns.

When to Use This Skill

• Creating a new PostToolUse or PreToolUse hook
• Hook output is not visible to Claude (most common issue)
• User asks about decision: block pattern
• Debugging why hook messages don't appear
• User mentions "Claude Code hooks" or "hook visibility"

---

Quick Reference: Visibility Patterns

Critical insight: PostToolUse hook stdout is only visible to Claude when JSON contains "decision": "block".

Output Format	Claude Visibility
Plain text	Not visible
JSON without decision: block	Not visible
JSON with decision: block	Visible

Exit code behavior:

Exit Code	stdout Behavior	Claude Visibility
0	JSON parsed, shown in verbose mode only	Only if "decision": "block"
2	Ignored, uses stderr instead	stderr shown to Claude
Other	stderr shown in verbose mode	Not shown to Claude

---

Minimal Working Pattern

#!/usr/bin/env bash
set -euo pipefail

# Read hook payload from stdin
PAYLOAD=$(cat)
FILE_PATH=$(echo "$PAYLOAD" | jq -r '.tool_input.file_path // empty')

[[ -z "$FILE_PATH" ]] && exit 0

# Your condition here
if [[ condition_met ]]; then
    jq -n \
        --arg reason "[HOOK] Your message to Claude" \
        '{decision: "block", reason: $reason}'
fi

exit 0

Key points:

1. Use jq -n to generate valid JSON
2. Include "decision": "block" for visibility
3. Exit with code 0
4. The "blocking error" label is cosmetic - operation continues

---

TodoWrite Templates

Creating a PostToolUse Hook

1. [pending] Create hook script with shebang and set -euo pipefail
2. [pending] Parse PAYLOAD from stdin with jq
3. [pending] Add condition check for when to trigger
4. [pending] Output JSON with decision:block pattern
5. [pending] Register hook in hooks.json with matcher
6. [pending] Test by editing a matching file
7. [pending] Verify Claude sees the message in system-reminder

Debugging Invisible Hook Output

1. [pending] Verify hook executes (add debug log to /tmp)
2. [pending] Check JSON format is valid (pipe to jq .)
3. [pending] Confirm decision:block is present in output
4. [pending] Verify exit code is 0
5. [pending] Check hooks.json matcher pattern
6. [pending] Restart Claude Code session

---

Reference Documentation

• Lifecycle Reference - All 10 hook events, diagrams, use cases, configuration pitfalls
• Visibility Patterns - Full exit code and JSON schema details
• Hook Templates - Copy-paste templates for common patterns
• Debugging Guide - Troubleshooting invisible output

---

Post-Change Checklist (Self-Evolution)

When this skill is updated:

• Update evolution-log.md with discovery
• Verify code examples still work
• Check if ADR needs updating: PostToolUse Hook Visibility ADR

---

Related Resources

• ADR: PostToolUse Hook Visibility
• GitHub Issue #3983 - Original bug report
• Claude Code Hooks Reference - Official documentation