Hooks Development

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

Self-Evolving Skill: This skill improves through use. If instructions are wrong, parameters drifted, or a workaround was needed — fix this file immediately, don't defer. Only update for real, reproducible issues.

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"

decision: block

decision: block

Exit code behavior:

"decision": "block"

Minimal Working Pattern

/usr/bin/env bash << 'SKILL_SCRIPT_EOF'
#!/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
SKILL_SCRIPT_EOF

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

Troubleshooting

"decision": "block"

jq -n

brew install jq

Post-Execution Reflection

After this skill completes, check before closing:

1. Did the command succeed? — If not, fix the instruction or error table that caused the failure.
2. Did parameters or output change? — If the underlying tool's interface drifted, update Usage examples and Parameters table to match.
3. Was a workaround needed? — If you had to improvise (different flags, extra steps), update this SKILL.md so the next invocation doesn't need the same workaround.

Only update if the issue is real and reproducible — not speculative.