Writing Hooks

Scope: covers hooks.json authoring and hook script design. For plugin architecture, see [[writing-plugins]]. For rules (which are simpler but static), see [[writing-rules]].

1. Three Hook Types

Type	What it does	When to use	Complexity
command	Runs a shell script, reads JSON from stdin	Deterministic checks: file existence, JSON validation, regex matching	Medium
prompt	Injects text into Claude's context	Advisory: reminders, context injection, style guidance	Low
agent	Spawns a verification agent	Complex verification: code quality, semantic analysis, multi-file checks	High

Type Selection Flowchart

Is the check deterministic (regex, file exists, JSON schema)?
  YES --> command hook (shell script)
  NO  --> Does it need AI judgment?
    YES --> agent hook
    NO  --> prompt hook (context injection)

Command Hook Example

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/check-loc.sh",
            "timeout": 10000
          }
        ]
      }
    ]
  }
}

Hook script receives JSON on stdin with tool name and parameters. It outputs JSON to stdout.

Prompt Hook Example

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Remember: this project uses Result<T, E> for error handling. Never use try/catch directly."
          }
        ]
      }
    ]
  }
}

Agent Hook Example

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "agent",
            "agent": "Verify the written file follows project conventions. Check: import order, export style, naming conventions. Report any violations."
          }
        ]
      }
    ]
  }
}

2. Blocking vs Advisory

Blocking (PreToolUse with deny)

The hook prevents the tool from executing. Use for hard quality gates.

Script output for blocking:

{
  "hookSpecificOutput": {
    "permissionDecision": "deny",
    "permissionDecisionReason": "File exceeds 300 LOC limit (current: 342). Extract logic before writing."
  }
}

When to block:

• Tests must pass before committing
• File exceeds size limit
• Required field missing from config
• Dangerous operation detected (force push, drop table)

Advisory (PostToolUse with message)

The hook adds a message to Claude's context after the action completes. Use for suggestions and reminders.

Script output for advisory:

{
  "hookSpecificOutput": {
    "message": "The file you just edited has no tests. Consider adding tests in __tests__/."
  }
}

When to advise:

• Suggest related actions (run tests, update docs)
• Remind about conventions
• Surface contextual information
• Warn about potential issues without blocking

Decision Matrix

Situation	Block or Advise?	Rationale
Test failure on commit	Block	Broken tests should never be committed
File over LOC limit	Block	Enforce hard limit
Missing JSDoc on export	Advise	Nice to have, not a hard requirement
No tests for new file	Advise	Reminder, not a gate
Force push to main	Block	Destructive, irreversible
Large file creation (>500 lines)	Advise	Might be intentional (generated code)

Rule of thumb: block only what you would reject in a code review. Advise on everything else.

3. Event Selection Guide

Event	When it fires	Common use cases
PreToolUse	Before a tool executes	Block dangerous operations, validate inputs, check preconditions
PostToolUse	After a tool succeeds	Trigger follow-up actions, lint changed files, update state
PostToolUseFailure	After a tool fails	Error recovery, suggest alternatives, log failures
UserPromptSubmit	When user sends a message	Context injection, session setup, mode activation
Stop	When Claude stops responding	Cleanup, summary generation, state persistence
SessionStart	Session begins	Environment validation, context loading, config checks

Event Selection by Goal

Goal	Event	Hook type
Prevent bad writes	PreToolUse + matcher Write|Edit	command
Lint after edit	PostToolUse + matcher Write|Edit	command
Inject project context	UserPromptSubmit	prompt
Validate environment on start	SessionStart	command
Save session summary on exit	Stop	agent
Recover from failed bash commands	PostToolUseFailure + matcher Bash	prompt

4. Matcher Patterns

The matcher field uses regex to match tool names. It applies only to PreToolUse, PostToolUse, and PostToolUseFailure events.

Pattern	Matches	Use case
"Bash"	Bash tool only	Guard shell commands
"Write|Edit"	Write or Edit	Guard file modifications
"Write"	Write only	Guard new file creation
"Edit"	Edit only	Guard file edits (not creation)
"Read"	Read tool	Track what files Claude reads
"mcp__.*"	All MCP tool calls	Guard external integrations
"mcp__github__.*"	GitHub MCP tools	Guard GitHub operations
"Task"	Task tool (agent dispatch)	Monitor agent dispatching
".*"	Everything	Use carefully -- fires on every tool call

Matcher Testing

Before deploying, verify your matcher with test cases:

Matcher	Should match	Should NOT match
"Write|Edit"	Write, Edit	Bash, Read, WriteFile
"Bash"	Bash	BashScript, mcp__bash
"mcp__github__.*"	mcp__github__create_pr	mcp__slack__send

5. Portable Paths

Always use ${CLAUDE_PLUGIN_ROOT} for script paths in hooks.json. This variable resolves to the plugin's installation directory at runtime.

Correct

{
  "command": "${CLAUDE_PLUGIN_ROOT}/scripts/check-loc.sh"
}

Wrong (breaks on other machines)

{
  "command": "/Users/joker/.claude/plugins/cache/xiaolai/my-plugin/0.1.0/scripts/check-loc.sh"
}

Script Location Convention

my-plugin/
  hooks/
    hooks.json          # hook definitions
  scripts/
    check-loc.sh        # hook scripts
    validate-config.sh
    lint-output.sh

Script Requirements

Every hook script must have:

1. Shebang line: #!/bin/bash or #!/usr/bin/env node
2. Executable permission: chmod +x scripts/*.sh
3. JSON output: scripts must output valid JSON to stdout
4. Stderr for logging: debug output goes to stderr, not stdout (stdout is parsed as JSON)

#!/bin/bash
# Read input from stdin
input=$(cat)

# Debug logging goes to stderr
echo "Hook triggered: $(date)" >&2

# Business logic
file_path=$(echo "$input" | jq -r '.toolInput.file_path // empty')

if [ -z "$file_path" ]; then
  # Allow if we can't determine the file
  echo '{"hookSpecificOutput":{"decision":"allow"}}'
  exit 0
fi

loc=$(wc -l < "$file_path" 2>/dev/null || echo "0")

if [ "$loc" -gt 300 ]; then
  echo "{\"hookSpecificOutput\":{\"permissionDecision\":\"deny\",\"permissionDecisionReason\":\"File has $loc lines, exceeds 300 LOC limit\"}}"
else
  echo '{"hookSpecificOutput":{"decision":"allow"}}'
fi

6. Fail-Open vs Fail-Closed

What happens when your hook script crashes?

Fail-Open (recommended default)

If the script crashes, allow the action. Safer for advisory hooks and non-critical checks.

#!/bin/bash
# Fail-open wrapper
set +e  # Don't exit on error

result=$(your_check_logic 2>/dev/null)
exit_code=$?

if [ $exit_code -ne 0 ]; then
  # Script failed -- allow the action (fail-open)
  echo '{"hookSpecificOutput":{"decision":"allow"}}'
  exit 0
fi

# Normal processing...
echo "$result"

Fail-Closed (security-critical only)

If the script crashes, deny the action. Use only for critical security gates.

#!/bin/bash
# Fail-closed wrapper
set +e

result=$(your_check_logic 2>/dev/null)
exit_code=$?

if [ $exit_code -ne 0 ]; then
  # Script failed -- deny the action (fail-closed)
  echo '{"hookSpecificOutput":{"permissionDecision":"deny","permissionDecisionReason":"Safety check script failed -- blocking action as precaution"}}'
  exit 0
fi

# Normal processing...
echo "$result"

When to Use Each

Hook purpose	Fail mode	Rationale
LOC limit enforcement	Fail-open	Better to allow a large file than block all writes
Style reminder	Fail-open	Non-critical advisory
Prevent force push to main	Fail-closed	Destructive action, err on side of caution
Secret detection	Fail-closed	Security-critical, must not leak
Test runner	Fail-open	Test infra failures shouldn't block development

7. Common Mistakes

Mistake	Why it's wrong	Fix
Blocking on PostToolUse	Action already happened -- too late to block	Use PreToolUse for blocking
Wrong event case	pretooluse instead of PreToolUse -- case-sensitive	Use exact case: PreToolUse, PostToolUse, etc.
Script not executable	Hook fails silently	Run chmod +x scripts/*.sh
Missing shebang	Script may run with wrong interpreter	Add #!/bin/bash or #!/usr/bin/env node
Hardcoded paths	Breaks on other machines	Use ${CLAUDE_PLUGIN_ROOT}
stdout pollution	Debug output mixed into JSON response	Use stderr for logging: echo "debug" >&2
No timeout	Slow script blocks Claude indefinitely	Set "timeout": 10000 (10 seconds)
Matcher too broad (".*")	Fires on every tool call, performance impact	Narrow to specific tools
No fail-open wrapper	Script crash = broken hook = frustrated user	Wrap in fail-open try/catch

8. Quality Checklist

Before deploying hooks, verify:

• Each hook has the correct event type for its purpose
• Blocking hooks use PreToolUse, not PostToolUse
• Matchers are tested against expected and unexpected tool names
• All script paths use ${CLAUDE_PLUGIN_ROOT}
• All scripts have shebangs and executable permissions
• All scripts output valid JSON to stdout
• Debug logging goes to stderr, not stdout
• Fail-open or fail-closed is explicitly chosen for each hook
• Timeouts are set (default: 10 seconds)
• Hooks are tested with: normal input, edge case input, missing input