Hooks Reference Skill

This skill provides comprehensive guidance for implementing Claude Code hooks - event handlers that automate validation, context loading, and workflow enforcement.

Hook Events Overview

Event	When Triggered	Use Cases
PreToolUse	Before tool executes	Validate, block, modify
PostToolUse	After tool completes	Audit, react, verify
PermissionRequest	Permission dialog shown	Auto-allow, auto-deny
Stop	Claude finishes	Force continue, verify
SubagentStop	Subagent finishes	Verify task complete
SessionStart	Session begins	Load context, setup
SessionEnd	Session ends	Cleanup, save state
UserPromptSubmit	Prompt submitted	Validate, add context
PreCompact	Before compaction	Preserve info
Notification	Notification sent	Alert, log

hooks.json Structure

Basic structure:

{
  "description": "What these hooks do",
  "hooks": {
    "EventName": [
      {
        "matcher": "Pattern",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/handler.sh",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Matcher Patterns

For tool events (PreToolUse, PostToolUse, PermissionRequest):

• "Write" - Match exact tool name
• "Write|Edit" - Match multiple tools (regex)
• "Notebook.*" - Regex pattern
• "*" or "" - Match all tools

For SessionStart:

• "startup" - Initial startup
• "resume" - From --resume, --continue, /resume
• "clear" - From /clear
• "compact" - From auto/manual compact

For Notification:

• "permission_prompt" - Permission requests
• "idle_prompt" - Claude waiting for input

Hook Types

Command Hooks (type: "command")

Execute a bash script:

{
  "type": "command",
  "command": "${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh",
  "timeout": 30
}

Prompt Hooks (type: "prompt")

LLM-based evaluation (Stop, SubagentStop only):

{
  "type": "prompt",
  "prompt": "Evaluate if Claude should stop: $ARGUMENTS. Check if all tasks are complete.",
  "timeout": 30
}

Exit Codes

Exit Code	Meaning	Behavior
0	Success	Action proceeds, stdout to user (verbose)
2	Block	Action blocked, stderr shown to Claude
Other	Error	Non-blocking, stderr to user (verbose)

Hook Input (stdin JSON)

Common fields:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse"
}

PreToolUse specific:

{
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/path/to/file.txt",
    "content": "file content"
  },
  "tool_use_id": "toolu_01ABC..."
}

SessionStart specific:

{
  "source": "startup"
}

Stop specific:

{
  "stop_hook_active": false
}

Advanced JSON Output

Return structured decisions via stdout (exit code 0):

PreToolUse Decision Control

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "Auto-approved documentation file",
    "updatedInput": {
      "field_to_modify": "new value"
    }
  }
}

Decision values: "allow", "deny", "ask"

Stop Decision Control

{
  "decision": "block",
  "reason": "Not all tasks complete - still need to run tests"
}

UserPromptSubmit Context

{
  "hookSpecificOutput": {
    "hookEventName": "UserPromptSubmit",
    "additionalContext": "Current time: 2024-01-15 10:30:00"
  }
}

SessionStart Context

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "Project context loaded..."
  }
}

Example Hooks

1. Validate File Writes (PreToolUse)

hooks/hooks.json:

{
  "description": "Validate file write operations",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/validate-write.sh",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

scripts/validate-write.sh:

#!/usr/bin/env bash

INPUT=$(cat)
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // .tool_input.filePath // empty')

# Block writes to sensitive files
if [[ "$FILE_PATH" == *.env* ]] || [[ "$FILE_PATH" == *secret* ]]; then
  echo "Cannot write to sensitive files: $FILE_PATH" >&2
  exit 2
fi

# Block writes outside project
if [[ ! "$FILE_PATH" == "$CLAUDE_PROJECT_DIR"* ]]; then
  echo "Cannot write outside project directory" >&2
  exit 2
fi

exit 0

2. Block Dangerous Commands (PreToolUse)

hooks/hooks.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/validate-bash.sh"
          }
        ]
      }
    ]
  }
}

scripts/validate-bash.sh:

#!/usr/bin/env bash

INPUT=$(cat)
COMMAND=$(echo "$INPUT" | jq -r '.tool_input.command // empty')

# Block destructive commands
DANGEROUS_PATTERNS=(
  "rm -rf /"
  "rm -rf ~"
  ":(){ :|:& };:"
  "> /dev/sda"
)

for pattern in "${DANGEROUS_PATTERNS[@]}"; do
  if [[ "$COMMAND" == *"$pattern"* ]]; then
    echo "Blocked dangerous command pattern: $pattern" >&2
    exit 2
  fi
done

exit 0

3. Load Project Context (SessionStart)

hooks/hooks.json:

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "startup|resume",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh"
          }
        ]
      }
    ]
  }
}

scripts/load-context.sh:

#!/usr/bin/env bash

CONTEXT=""

# Load CLAUDE.md if exists
if [ -f "$CLAUDE_PROJECT_DIR/CLAUDE.md" ]; then
  CONTEXT+="Project instructions from CLAUDE.md have been loaded.\n"
fi

# Add git status
if [ -d "$CLAUDE_PROJECT_DIR/.git" ]; then
  BRANCH=$(git -C "$CLAUDE_PROJECT_DIR" branch --show-current 2>/dev/null)
  CONTEXT+="Current git branch: $BRANCH\n"
fi

# Output as JSON for structured context
cat << EOF
{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "$CONTEXT"
  }
}
EOF

exit 0

4. Verify Before Stop (Stop)

Using prompt-based hook:

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Evaluate if Claude should stop. Context: $ARGUMENTS\n\nCheck if:\n1. All requested tasks are complete\n2. No errors need addressing\n3. No tests need running\n\nRespond with: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
          }
        ]
      }
    ]
  }
}

5. Add Timestamp to Prompts (UserPromptSubmit)

hooks/hooks.json:

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/add-timestamp.sh"
          }
        ]
      }
    ]
  }
}

scripts/add-timestamp.sh:

#!/usr/bin/env bash

# Plain text stdout is added as context
echo "Current time: $(date '+%Y-%m-%d %H:%M:%S %Z')"

exit 0

Environment Variables

Available in hooks:

• ${CLAUDE_PLUGIN_ROOT} - Absolute path to plugin directory
• $CLAUDE_PROJECT_DIR - Project root directory
• $CLAUDE_ENV_FILE - (SessionStart only) File to persist env vars
• $CLAUDE_CODE_REMOTE - "true" if running in web environment

Persisting Environment (SessionStart)

#!/usr/bin/env bash

if [ -n "$CLAUDE_ENV_FILE" ]; then
  echo 'export NODE_ENV=development' >> "$CLAUDE_ENV_FILE"
  echo 'export API_URL=http://localhost:3000' >> "$CLAUDE_ENV_FILE"
fi

exit 0

Best Practices

1. Always quote variables: Use "$VAR" not $VAR
2. Validate input: Never trust stdin blindly
3. Use portable paths: ${CLAUDE_PLUGIN_ROOT} for plugin files
4. Set timeouts: Prevent hanging hooks
5. Handle errors: Check for missing fields with // empty
6. Keep hooks fast: Target <1 second execution
7. Use jq for JSON: Safer than string parsing

Debugging

Enable debug mode:

claude --debug

Test hook manually:

echo '{"tool_name":"Write","tool_input":{"file_path":"test.txt"}}' | \
  ./scripts/validate-write.sh
echo $?  # Check exit code

References

• Hooks documentation: https://code.claude.com/docs/en/hooks
• Hooks guide: https://code.claude.com/docs/en/hooks-guide
• Example hooks: https://github.com/anthropics/claude-code/tree/main/examples/hooks