Create Hooks

<objective> Hooks are event-driven automation for Claude Code that execute shell commands or LLM prompts in response to tool usage, session events, and user interactions. This skill teaches you how to create, configure, and debug hooks for validating commands, automating workflows, injecting context, and implementing custom completion criteria.

Hooks provide programmatic control over Claude's behavior without modifying core code, enabling project-specific automation, safety checks, and workflow customization. </objective>

<context> Hooks are shell commands or LLM-evaluated prompts that execute in response to Claude Code events. They operate within an event hierarchy: events (PreToolUse, PostToolUse, Stop, etc.) trigger matchers (tool patterns) which fire hooks (commands or prompts). Hooks can block actions, modify tool inputs, inject context, or simply observe and log Claude's operations. </context>

<quick_start> <workflow> 1. Create hooks config file:

• Project: .claude/hooks.json
• User: ~/.claude/hooks.json

2. Choose hook event (when it fires) 3. Choose hook type (command or prompt) 4. Configure matcher (which tools trigger it) 5. Test with claude --debug </workflow>

<example> Log all bash commands:

.claude/hooks.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"\\(.tool_input.command) - \\(.tool_input.description // \\\"No description\\\")\"' >> ~/.claude/bash-log.txt"
          }
        ]
      }
    ]
  }
}

This hook:

• Fires before (PreToolUse) every Bash tool use
• Executes a command (not an LLM prompt)
• Logs command + description to a file

</example> </quick_start>

<hook_types>

Blocking hooks can return "decision": "block" to prevent the action. See references/hook-types.md for detailed use cases. </hook_types>

<hook_anatomy> <hook_type name="command"> Type: Executes a shell command

Use when:

• Simple validation (check file exists)
• Logging (append to file)
• External tools (formatters, linters)
• Desktop notifications

Input: JSON via stdin Output: JSON via stdout (optional)

{
  "type": "command",
  "command": "/path/to/script.sh",
  "timeout": 30000
}

</hook_type>

<hook_type name="prompt"> Type: LLM evaluates a prompt

Use when:

• Complex decision logic
• Natural language validation
• Context-aware checks
• Reasoning required

Input: Prompt with $ARGUMENTS placeholder Output: JSON with decision and reason

{
  "type": "prompt",
  "prompt": "Evaluate if this command is safe: $ARGUMENTS\n\nReturn JSON: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
}

</hook_type> </hook_anatomy>

<matchers> Matchers filter which tools trigger the hook:

{
  "matcher": "Bash",           // Exact match
  "matcher": "Write|Edit",     // Multiple tools (regex OR)
  "matcher": "mcp__.*",        // All MCP tools
  "matcher": "mcp__memory__.*" // Specific MCP server
}

No matcher: Hook fires for all tools

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [...]  // No matcher - fires on every user prompt
      }
    ]
  }
}

</matchers>

<input_output> Hooks receive JSON via stdin with session info, current directory, and event-specific data. Blocking hooks can return JSON to approve/block actions or modify inputs.

Example output (blocking hooks):

{
  "decision": "approve" | "block",
  "reason": "Why this decision was made"
}

See references/input-output-schemas.md for complete schemas for each hook type. </input_output>

<environment_variables> Available in hook commands:

Example:

{
  "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/validate.sh"
}

</environment_variables>

<common_patterns> Desktop notification when input needed:

{
  "hooks": {
    "Notification": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "osascript -e 'display notification \"Claude needs input\" with title \"Claude Code\"'"
          }
        ]
      }
    ]
  }
}

Block destructive git commands:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Check if this command is destructive: $ARGUMENTS\n\nBlock if it contains: 'git push --force', 'rm -rf', 'git reset --hard'\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
          }
        ]
      }
    ]
  }
}

Auto-format code after edits:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write $CLAUDE_PROJECT_DIR",
            "timeout": 10000
          }
        ]
      }
    ]
  }
}

Add context at session start:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "echo '{\"hookSpecificOutput\": {\"hookEventName\": \"SessionStart\", \"additionalContext\": \"Current sprint: Sprint 23. Focus: User authentication\"}}'"
          }
        ]
      }
    ]
  }
}

</common_patterns>

<debugging> Always test hooks with the debug flag:

claude --debug

This shows which hooks matched, command execution, and output. See references/troubleshooting.md for common issues and solutions. </debugging>

<reference_guides> Hook types and events: references/hook-types.md

• Complete list of hook events
• When each event fires
• Input/output schemas for each
• Blocking vs non-blocking hooks

Command vs Prompt hooks: references/command-vs-prompt.md

• Decision tree: which type to use
• Command hook patterns and examples
• Prompt hook patterns and examples
• Performance considerations

Matchers and patterns: references/matchers.md

• Regex patterns for tool matching
• MCP tool matching patterns
• Multiple tool matching
• Debugging matcher issues

Input/Output schemas: references/input-output-schemas.md

• Complete schema for each hook type
• Field descriptions and types
• Hook-specific output fields
• Example JSON for each event

Working examples: references/examples.md

• Desktop notifications
• Command validation
• Auto-formatting workflows
• Logging and audit trails
• Stop logic patterns
• Session context injection

Troubleshooting: references/troubleshooting.md

• Hooks not triggering
• Command execution failures
• Prompt hook issues
• Permission problems
• Timeout handling
• Debug workflow

</reference_guides>

<security_checklist> Critical safety requirements:

• Infinite loop prevention: Check stop_hook_active flag in Stop hooks to prevent recursive triggering
• Timeout configuration: Set reasonable timeouts (default: 60s) to prevent hanging
• Permission validation: Ensure hook scripts have executable permissions (chmod +x)
• Path safety: Use absolute paths with $CLAUDE_PROJECT_DIR to avoid path injection
• JSON validation: Validate hook config with jq before use to catch syntax errors
• Selective blocking: Be conservative with blocking hooks to avoid workflow disruption

Testing protocol:

# Always test with debug flag first
claude --debug

# Validate JSON config
jq . .claude/hooks.json

</security_checklist>

<success_criteria> A working hook configuration has:

• Valid JSON in .claude/hooks.json (validated with jq)
• Appropriate hook event selected for the use case
• Correct matcher pattern that matches target tools
• Command or prompt that executes without errors
• Proper output schema (decision/reason for blocking hooks)
• Tested with --debug flag showing expected behavior
• No infinite loops in Stop hooks (checks stop_hook_active flag)
• Reasonable timeout set (especially for external commands)
• Executable permissions on script files if using file paths

</success_criteria>

Command vs Prompt Hooks

Decision guide for choosing between command-based and prompt-based hooks.

Decision Tree

Need to execute a hook?
│
├─ Simple yes/no validation?
│  └─ Use COMMAND (faster, cheaper)
│
├─ Need natural language understanding?
│  └─ Use PROMPT (LLM evaluation)
│
├─ External tool interaction?
│  └─ Use COMMAND (formatters, linters, git)
│
├─ Complex decision logic?
│  └─ Use PROMPT (reasoning required)
│
└─ Logging/notification only?
   └─ Use COMMAND (no decision needed)

---

Command Hooks

Characteristics

• Execution: Shell command
• Input: JSON via stdin
• Output: JSON via stdout (optional)
• Speed: Fast (no LLM call)
• Cost: Free (no API usage)
• Complexity: Limited to shell scripting logic

When to use

✅ Use command hooks for:

• File operations (read, write, check existence)
• Running tools (prettier, eslint, git)
• Simple pattern matching (grep, regex)
• Logging to files
• Desktop notifications
• Fast validation (file size, permissions)

❌ Don't use command hooks for:

• Natural language analysis
• Complex decision logic
• Context-aware validation
• Semantic understanding

Examples

1. Log bash commands

{
  "type": "command",
  "command": "jq -r '\"\\(.tool_input.command) - \\(.tool_input.description // \\\"No description\\\")\"' >> ~/.claude/bash-log.txt"
}

2. Block if file doesn't exist

#!/bin/bash
# check-file-exists.sh

input=$(cat)
file=$(echo "$input" | jq -r '.tool_input.file_path')

if [ ! -f "$file" ]; then
  echo '{"decision": "block", "reason": "File does not exist"}'
  exit 0
fi

echo '{"decision": "approve", "reason": "File exists"}'

3. Run prettier after edits

{
  "type": "command",
  "command": "prettier --write \"$(echo {} | jq -r '.tool_input.file_path')\"",
  "timeout": 10000
}

4. Desktop notification

{
  "type": "command",
  "command": "osascript -e 'display notification \"Claude needs input\" with title \"Claude Code\"'"
}

Parsing input in commands

Command hooks receive JSON via stdin. Use jq to parse:

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

# Extract fields
tool_name=$(echo "$input" | jq -r '.tool_name')
command=$(echo "$input" | jq -r '.tool_input.command')
session_id=$(echo "$input" | jq -r '.session_id')

# Your logic here
if [[ "$command" == *"rm -rf"* ]]; then
  echo '{"decision": "block", "reason": "Dangerous command"}'
else
  echo '{"decision": "approve", "reason": "Safe"}'
fi

---

Prompt Hooks

Characteristics

• Execution: LLM evaluates prompt
• Input: Prompt string with $ARGUMENTS placeholder
• Output: LLM generates JSON response
• Speed: Slower (~1-3s per evaluation)
• Cost: Uses API credits
• Complexity: Can reason, understand context, analyze semantics

When to use

✅ Use prompt hooks for:

• Natural language validation
• Semantic analysis (intent, safety, appropriateness)
• Complex decision trees
• Context-aware checks
• Reasoning about code quality
• Understanding user intent

❌ Don't use prompt hooks for:

• Simple pattern matching (use regex/grep)
• File operations (use command hooks)
• High-frequency events (too slow/expensive)
• Non-decision tasks (logging, notifications)

Examples

1. Validate commit messages

{
  "type": "prompt",
  "prompt": "Evaluate this git commit message: $ARGUMENTS\n\nCheck if it:\n1. Starts with conventional commit type (feat|fix|docs|refactor|test|chore)\n2. Is descriptive and clear\n3. Under 72 characters\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"specific feedback\"}"
}

2. Check if Stop is appropriate

{
  "type": "prompt",
  "prompt": "Review the conversation transcript: $ARGUMENTS\n\nDetermine if Claude should stop:\n1. All user tasks completed?\n2. Any errors that need fixing?\n3. Tests passing?\n4. Documentation updated?\n\nIf incomplete: {\"decision\": \"block\", \"reason\": \"what's missing\"}\nIf complete: {\"decision\": \"approve\", \"reason\": \"all done\"}"
}

3. Validate code changes for security

{
  "type": "prompt",
  "prompt": "Analyze this code change for security issues: $ARGUMENTS\n\nCheck for:\n- SQL injection vulnerabilities\n- XSS attack vectors\n- Authentication bypasses\n- Sensitive data exposure\n\nIf issues found: {\"decision\": \"block\", \"reason\": \"specific vulnerabilities\"}\nIf safe: {\"decision\": \"approve\", \"reason\": \"no issues found\"}"
}

4. Semantic prompt validation

{
  "type": "prompt",
  "prompt": "Evaluate user prompt: $ARGUMENTS\n\nIs this:\n1. Related to coding/development?\n2. Appropriate and professional?\n3. Clear and actionable?\n\nIf inappropriate: {\"decision\": \"block\", \"reason\": \"why\"}\nIf good: {\"decision\": \"approve\", \"reason\": \"ok\"}"
}

Writing effective prompts

Be specific about output format:

Return JSON: {"decision": "approve" or "block", "reason": "explanation"}

Provide clear criteria:

Block if:
1. Command contains 'rm -rf /'
2. Force push to main branch
3. Credentials in plain text

Otherwise approve.

Use $ARGUMENTS placeholder:

Analyze this input: $ARGUMENTS

Check for...

The $ARGUMENTS placeholder is replaced with the actual hook input JSON.

---

Performance Comparison

---

Combining Both

You can use multiple hooks for the same event:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "echo \"$input\" >> ~/bash-log.txt",
            "comment": "Log every command (fast)"
          },
          {
            "type": "prompt",
            "prompt": "Analyze this bash command for safety: $ARGUMENTS",
            "comment": "Validate with LLM (slower, smarter)"
          }
        ]
      }
    ]
  }
}

Hooks execute in order. If any hook blocks, execution stops.

---

Recommendations

High-frequency events (PreToolUse, PostToolUse):

• Prefer command hooks
• Use prompt hooks sparingly
• Cache LLM decisions when possible

Low-frequency events (Stop, UserPromptSubmit):

• Prompt hooks are fine
• Cost/latency less critical

Balance:

• Command hooks for simple checks
• Prompt hooks for complex validation
• Combine when appropriate

Working Examples

Real-world hook configurations ready to use.

Desktop Notifications

macOS notification when input needed

{
  "hooks": {
    "Notification": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "osascript -e 'display notification \"Claude needs your input\" with title \"Claude Code\" sound name \"Glass\"'"
          }
        ]
      }
    ]
  }
}

Linux notification (notify-send)

{
  "hooks": {
    "Notification": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "notify-send 'Claude Code' 'Awaiting your input' --urgency=normal"
          }
        ]
      }
    ]
  }
}

Play sound on notification

{
  "hooks": {
    "Notification": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "afplay /System/Library/Sounds/Glass.aiff"
          }
        ]
      }
    ]
  }
}

---

Logging

Log all bash commands

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"[\" + (.timestamp // now | todate) + \"] \" + .tool_input.command + \" - \" + (.tool_input.description // \"No description\")' >> ~/.claude/bash-log.txt"
          }
        ]
      }
    ]
  }
}

Log file operations

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"[\" + (now | todate) + \"] \" + .tool_name + \": \" + .tool_input.file_path' >> ~/.claude/file-operations.log"
          }
        ]
      }
    ]
  }
}

Audit trail for MCP operations

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "mcp__.*",
        "hooks": [
          {
            "type": "command",
            "command": "jq '. + {timestamp: now}' >> ~/.claude/mcp-audit.jsonl"
          }
        ]
      }
    ]
  }
}

---

Code Quality

Auto-format after edits

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write \"$(echo {} | jq -r '.tool_input.file_path')\" 2>/dev/null || true",
            "timeout": 10000
          }
        ]
      }
    ]
  }
}

Run linter after code changes

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "eslint \"$(echo {} | jq -r '.tool_input.file_path')\" --fix 2>/dev/null || true"
          }
        ]
      }
    ]
  }
}

Run tests before stopping

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/check-tests.sh"
          }
        ]
      }
    ]
  }
}

check-tests.sh:

#!/bin/bash
cd "$cwd" || exit 1

# Run tests
npm test > /dev/null 2>&1

if [ $? -eq 0 ]; then
  echo '{"decision": "approve", "reason": "All tests passing"}'
else
  echo '{"decision": "block", "reason": "Tests are failing. Please fix before stopping.", "systemMessage": "Run npm test to see failures"}'
fi

---

Safety and Validation

Block destructive commands

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/check-command-safety.sh"
          }
        ]
      }
    ]
  }
}

check-command-safety.sh:

#!/bin/bash
input=$(cat)
command=$(echo "$input" | jq -r '.tool_input.command')

# Check for dangerous patterns
if [[ "$command" == *"rm -rf /"* ]] || \
   [[ "$command" == *"mkfs"* ]] || \
   [[ "$command" == *"> /dev/sda"* ]]; then
  echo '{"decision": "block", "reason": "Destructive command detected", "systemMessage": "This command could cause data loss"}'
  exit 0
fi

# Check for force push to main
if [[ "$command" == *"git push"*"--force"* ]] && \
   [[ "$command" == *"main"* || "$command" == *"master"* ]]; then
  echo '{"decision": "block", "reason": "Force push to main branch blocked", "systemMessage": "Use a feature branch instead"}'
  exit 0
fi

echo '{"decision": "approve", "reason": "Command is safe"}'

Validate commit messages

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Check if this is a git commit command: $ARGUMENTS\n\nIf it's a git commit, validate the message follows conventional commits format (feat|fix|docs|refactor|test|chore): description\n\nIf invalid format: {\"decision\": \"block\", \"reason\": \"Commit message must follow conventional commits\"}\nIf valid or not a commit: {\"decision\": \"approve\", \"reason\": \"ok\"}"
          }
        ]
      }
    ]
  }
}

Block writes to critical files

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/check-protected-files.sh"
          }
        ]
      }
    ]
  }
}

check-protected-files.sh:

#!/bin/bash
input=$(cat)
file_path=$(echo "$input" | jq -r '.tool_input.file_path')

# Protected files
protected_files=(
  "package-lock.json"
  ".env.production"
  "credentials.json"
)

for protected in "${protected_files[@]}"; do
  if [[ "$file_path" == *"$protected"* ]]; then
    echo "{\"decision\": \"block\", \"reason\": \"Cannot modify $protected\", \"systemMessage\": \"This file is protected from automated changes\"}"
    exit 0
  fi
done

echo '{"decision": "approve", "reason": "File is not protected"}'

---

Context Injection

Load sprint context at session start

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/load-sprint-context.sh"
          }
        ]
      }
    ]
  }
}

load-sprint-context.sh:

#!/bin/bash

# Read sprint info from file
sprint_info=$(cat "$CLAUDE_PROJECT_DIR/.sprint-context.txt" 2>/dev/null || echo "No sprint context available")

# Return as SessionStart context
jq -n \
  --arg context "$sprint_info" \
  '{
    "hookSpecificOutput": {
      "hookEventName": "SessionStart",
      "additionalContext": $context
    }
  }'

Load git branch context

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "cd \"$cwd\" && git branch --show-current | jq -Rs '{\"hookSpecificOutput\": {\"hookEventName\": \"SessionStart\", \"additionalContext\": (\"Current branch: \" + .)}}'"
          }
        ]
      }
    ]
  }
}

Load environment info

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "echo '{\"hookSpecificOutput\": {\"hookEventName\": \"SessionStart\", \"additionalContext\": \"Environment: '$(hostname)'\\nNode version: '$(node --version 2>/dev/null || echo 'not installed')'\\nPython version: '$(python3 --version 2>/dev/null || echo 'not installed)'\"}}'"
          }
        ]
      }
    ]
  }
}

---

Workflow Automation

Auto-commit after major changes

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/auto-commit.sh"
          }
        ]
      }
    ]
  }
}

auto-commit.sh:

#!/bin/bash
cd "$cwd" || exit 1

# Check if there are changes
if ! git diff --quiet; then
  git add -A
  git commit -m "chore: auto-commit from claude session" --no-verify
  echo '{"systemMessage": "Changes auto-committed"}'
fi

Update documentation after code changes

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/update-docs.sh",
            "timeout": 30000
          }
        ]
      }
    ]
  }
}

Run pre-commit hooks

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/check-pre-commit.sh"
          }
        ]
      }
    ]
  }
}

check-pre-commit.sh:

#!/bin/bash
input=$(cat)
command=$(echo "$input" | jq -r '.tool_input.command')

# If git commit, run pre-commit hooks first
if [[ "$command" == *"git commit"* ]]; then
  pre-commit run --all-files > /dev/null 2>&1

  if [ $? -ne 0 ]; then
    echo '{"decision": "block", "reason": "Pre-commit hooks failed", "systemMessage": "Fix formatting/linting issues first"}'
    exit 0
  fi
fi

echo '{"decision": "approve", "reason": "ok"}'

---

Session Management

Archive transcript on session end

{
  "hooks": {
    "SessionEnd": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/archive-session.sh"
          }
        ]
      }
    ]
  }
}

archive-session.sh:

#!/bin/bash
input=$(cat)
transcript_path=$(echo "$input" | jq -r '.transcript_path')
session_id=$(echo "$input" | jq -r '.session_id')

# Create archive directory
archive_dir="$HOME/.claude/archives"
mkdir -p "$archive_dir"

# Copy transcript with timestamp
timestamp=$(date +%Y%m%d-%H%M%S)
cp "$transcript_path" "$archive_dir/${timestamp}-${session_id}.jsonl"

echo "Session archived to $archive_dir"

Save session stats

{
  "hooks": {
    "SessionEnd": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "jq '. + {ended_at: now}' >> ~/.claude/session-stats.jsonl"
          }
        ]
      }
    ]
  }
}

---

Advanced Patterns

Intelligent stop logic

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Review the conversation: $ARGUMENTS\n\nCheck if:\n1. All user-requested tasks are complete\n2. Tests are passing (if code changes made)\n3. No errors that need fixing\n4. Documentation updated (if applicable)\n\nIf incomplete: {\"decision\": \"block\", \"reason\": \"specific issue\", \"systemMessage\": \"what needs to be done\"}\n\nIf complete: {\"decision\": \"approve\", \"reason\": \"all tasks done\"}\n\nIMPORTANT: If stop_hook_active is true, return {\"decision\": undefined} to avoid infinite loop",
            "timeout": 30000
          }
        ]
      }
    ]
  }
}

Chain multiple hooks

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'First hook' >> /tmp/hook-chain.log"
          },
          {
            "type": "command",
            "command": "echo 'Second hook' >> /tmp/hook-chain.log"
          },
          {
            "type": "prompt",
            "prompt": "Final validation: $ARGUMENTS"
          }
        ]
      }
    ]
  }
}

Hooks execute in order. First block stops the chain.

Conditional execution based on file type

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/format-by-type.sh"
          }
        ]
      }
    ]
  }
}

format-by-type.sh:

#!/bin/bash
input=$(cat)
file_path=$(echo "$input" | jq -r '.tool_input.file_path')

case "$file_path" in
  *.js|*.jsx|*.ts|*.tsx)
    prettier --write "$file_path"
    ;;
  *.py)
    black "$file_path"
    ;;
  *.go)
    gofmt -w "$file_path"
    ;;
esac

---

Project-Specific Hooks

Use $CLAUDE_PROJECT_DIR for project-specific hooks:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/init-session.sh"
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/validate-changes.sh"
          }
        ]
      }
    ]
  }
}

This keeps hook scripts versioned with the project.

Hook Types and Events

Complete reference for all Claude Code hook events.

PreToolUse

When it fires: Before any tool is executed

Can block: Yes

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Bash",
  "tool_input": {
    "command": "npm install",
    "description": "Install dependencies"
  }
}

Output schema (to control execution):

{
  "decision": "approve" | "block",
  "reason": "Explanation",
  "permissionDecision": "allow" | "deny" | "ask",
  "permissionDecisionReason": "Why",
  "updatedInput": {
    "command": "npm install --save-exact"
  }
}

Use cases:

• Validate commands before execution
• Block dangerous operations
• Modify tool inputs
• Log command attempts
• Ask user for confirmation

Example: Block force pushes to main

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Check if this git command is safe: $ARGUMENTS\n\nBlock if: force push to main/master\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
          }
        ]
      }
    ]
  }
}

---

PostToolUse

When it fires: After a tool completes execution

Can block: No (tool already executed)

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PostToolUse",
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/path/to/file.js",
    "content": "..."
  },
  "tool_output": "File created successfully"
}

Output schema:

{
  "systemMessage": "Optional message to display",
  "suppressOutput": false
}

Use cases:

• Auto-format code after Write/Edit
• Run tests after code changes
• Update documentation
• Trigger CI builds
• Send notifications

Example: Auto-format after edits

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write $CLAUDE_PROJECT_DIR",
            "timeout": 10000
          }
        ]
      }
    ]
  }
}

---

UserPromptSubmit

When it fires: User submits a prompt to Claude

Can block: Yes

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "UserPromptSubmit",
  "prompt": "Write a function to calculate factorial"
}

Output schema:

{
  "decision": "approve" | "block",
  "reason": "Explanation",
  "systemMessage": "Message to user"
}

Use cases:

• Validate prompt format
• Block inappropriate requests
• Preprocess user input
• Add context to prompts
• Enforce prompt templates

Example: Require issue numbers in prompts

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Check if prompt mentions an issue number (e.g., #123 or PROJ-456): $ARGUMENTS\n\nIf no issue number: {\"decision\": \"block\", \"reason\": \"Please include issue number\"}\nOtherwise: {\"decision\": \"approve\", \"reason\": \"ok\"}"
          }
        ]
      }
    ]
  }
}

---

Stop

When it fires: Claude attempts to stop working

Can block: Yes

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "Stop",
  "stop_hook_active": false
}

Output schema:

{
  "decision": "block" | undefined,
  "reason": "Why Claude should continue",
  "continue": true,
  "systemMessage": "Additional instructions"
}

Use cases:

• Verify all tasks completed
• Check for errors that need fixing
• Ensure tests pass before stopping
• Validate deliverables
• Custom completion criteria

Example: Verify tests pass before stopping

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "npm test && echo '{\"decision\": \"approve\"}' || echo '{\"decision\": \"block\", \"reason\": \"Tests failing\"}'"
          }
        ]
      }
    ]
  }
}

Important: Check stop_hook_active to avoid infinite loops. If true, don't block again.

---

SubagentStop

When it fires: A subagent attempts to stop

Can block: Yes

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "SubagentStop",
  "stop_hook_active": false
}

Output schema: Same as Stop

Use cases:

• Verify subagent completed its task
• Check for errors in subagent output
• Validate subagent deliverables
• Ensure quality before accepting results

Example: Check if code-reviewer provided feedback

{
  "hooks": {
    "SubagentStop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Review the subagent transcript: $ARGUMENTS\n\nDid the code-reviewer provide:\n1. Specific issues found\n2. Severity ratings\n3. Remediation steps\n\nIf missing: {\"decision\": \"block\", \"reason\": \"Incomplete review\"}\nOtherwise: {\"decision\": \"approve\", \"reason\": \"Complete\"}"
          }
        ]
      }
    ]
  }
}

---

SessionStart

When it fires: At the beginning of a Claude session

Can block: No

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "SessionStart",
  "source": "startup"
}

Output schema:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "Context to inject into session"
  }
}

Use cases:

• Load project context
• Inject sprint information
• Set environment variables
• Initialize state
• Display welcome messages

Example: Load current sprint context

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "cat $CLAUDE_PROJECT_DIR/.sprint-context.txt | jq -Rs '{\"hookSpecificOutput\": {\"hookEventName\": \"SessionStart\", \"additionalContext\": .}}'"
          }
        ]
      }
    ]
  }
}

---

SessionEnd

When it fires: When a Claude session ends

Can block: No (cannot prevent session end)

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "SessionEnd",
  "reason": "exit" | "error" | "timeout"
}

Output schema: None (hook output ignored)

Use cases:

• Save session state
• Cleanup temporary files
• Update logs
• Send analytics
• Archive transcripts

Example: Archive session transcript

{
  "hooks": {
    "SessionEnd": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "cp $transcript_path $CLAUDE_PROJECT_DIR/.claude/archives/$(date +%Y%m%d-%H%M%S).jsonl"
          }
        ]
      }
    ]
  }
}

---

PreCompact

When it fires: Before context window compaction

Can block: Yes

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PreCompact",
  "trigger": "manual" | "auto",
  "custom_instructions": "User's compaction instructions"
}

Output schema:

{
  "decision": "approve" | "block",
  "reason": "Explanation"
}

Use cases:

• Validate state before compaction
• Save important context
• Custom compaction logic
• Prevent compaction at critical moments

---

Notification

When it fires: Claude needs user input (awaiting response)

Can block: No

Input schema:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "Notification"
}

Output schema: None

Use cases:

• Desktop notifications
• Sound alerts
• Status bar updates
• External notifications (Slack, etc.)

Example: macOS notification

{
  "hooks": {
    "Notification": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "osascript -e 'display notification \"Claude needs input\" with title \"Claude Code\"'"
          }
        ]
      }
    ]
  }
}

Input/Output Schemas

Complete JSON schemas for all hook types.

Common Input Fields

All hooks receive these fields:

{
  session_id: string           // Unique session identifier
  transcript_path: string      // Path to session transcript (.jsonl file)
  cwd: string                  // Current working directory
  permission_mode: string      // "default" | "plan" | "acceptEdits" | "bypassPermissions"
  hook_event_name: string      // Name of the hook event
}

---

PreToolUse

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Bash",
  "tool_input": {
    "command": "npm install",
    "description": "Install dependencies"
  }
}

Output (optional, for control):

{
  "decision": "approve" | "block",
  "reason": "Explanation for the decision",
  "permissionDecision": "allow" | "deny" | "ask",
  "permissionDecisionReason": "Why this permission decision",
  "updatedInput": {
    "command": "npm install --save-exact"
  },
  "systemMessage": "Message displayed to user",
  "suppressOutput": false,
  "continue": true
}

Fields:

• decision: Whether to allow the tool call
• reason: Explanation (required if blocking)
• permissionDecision: Override permission system
• updatedInput: Modified tool input (partial update)
• systemMessage: Message shown to user
• suppressOutput: Hide hook output from user
• continue: If false, stop execution

---

PostToolUse

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "PostToolUse",
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/path/to/file.js",
    "content": "const x = 1;"
  },
  "tool_output": "File created successfully at: /path/to/file.js"
}

Output (optional):

{
  "systemMessage": "Code formatted successfully",
  "suppressOutput": false
}

Fields:

• systemMessage: Additional message to display
• suppressOutput: Hide tool output from user

---

UserPromptSubmit

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "UserPromptSubmit",
  "prompt": "Write a function to calculate factorial"
}

Output:

{
  "decision": "approve" | "block",
  "reason": "Prompt is clear and actionable",
  "systemMessage": "Optional message to user"
}

Fields:

• decision: Whether to allow the prompt
• reason: Explanation (required if blocking)
• systemMessage: Message shown to user

---

Stop

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "Stop",
  "stop_hook_active": false
}

Output:

{
  "decision": "block" | undefined,
  "reason": "Tests are still failing - please fix before stopping",
  "continue": true,
  "stopReason": "Cannot stop yet",
  "systemMessage": "Additional context"
}

Fields:

• decision: "block" to prevent stopping, undefined to allow
• reason: Why Claude should continue (required if blocking)
• continue: If true and blocking, Claude continues working
• stopReason: Message shown when stopping is blocked
• systemMessage: Additional context for Claude
• stop_hook_active: If true, don't block again (prevents infinite loops)

Important: Always check stop_hook_active to avoid infinite loops:

if (input.stop_hook_active) {
  return { decision: undefined }; // Don't block again
}

---

SubagentStop

Input: Same as Stop

Output: Same as Stop

Usage: Same as Stop, but for subagent completion

---

SessionStart

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "SessionStart",
  "source": "startup" | "continue" | "checkpoint"
}

Output:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "Current sprint: Sprint 23\nFocus: User authentication\nDeadline: Friday"
  }
}

Fields:

• additionalContext: Text injected into session context
• Multiple SessionStart hooks' contexts are concatenated

---

SessionEnd

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "SessionEnd",
  "reason": "exit" | "error" | "timeout" | "compact"
}

Output: None (ignored)

Usage: Cleanup tasks only. Cannot prevent session end.

---

PreCompact

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "PreCompact",
  "trigger": "manual" | "auto",
  "custom_instructions": "Preserve all git commit messages"
}

Output:

{
  "decision": "approve" | "block",
  "reason": "Safe to compact" | "Wait until task completes"
}

Fields:

• trigger: How compaction was initiated
• custom_instructions: User's compaction preferences (if manual)
• decision: Whether to proceed with compaction
• reason: Explanation

---

Notification

Input:

{
  "session_id": "abc123",
  "transcript_path": "~/.claude/projects/.../session.jsonl",
  "cwd": "/Users/username/project",
  "permission_mode": "default",
  "hook_event_name": "Notification"
}

Output: None (hook just performs notification action)

Usage: Trigger external notifications (desktop, sound, status bar)

---

Common Output Fields

These fields can be returned by any hook:

{
  "continue": true | false,
  "stopReason": "Reason shown when stopping",
  "suppressOutput": true | false,
  "systemMessage": "Additional context or message"
}

Fields:

• continue: If false, stop Claude's execution immediately
• stopReason: Message displayed when execution stops
• suppressOutput: If true, hide hook's stdout/stderr from user
• systemMessage: Context added to Claude's next message

---

LLM Prompt Hook Response

When using type: "prompt", the LLM must return JSON:

{
  "decision": "approve" | "block",
  "reason": "Detailed explanation",
  "systemMessage": "Optional message",
  "continue": true | false,
  "stopReason": "Optional stop message"
}

Example prompt:

Evaluate this command: $ARGUMENTS

Check if it's safe to execute.

Return JSON:
{
  "decision": "approve" or "block",
  "reason": "your explanation"
}

The $ARGUMENTS placeholder is replaced with the hook's input JSON.

---

Tool-Specific Input Fields

Different tools provide different tool_input fields:

Bash

{
  "tool_input": {
    "command": "npm install",
    "description": "Install dependencies",
    "timeout": 120000,
    "run_in_background": false
  }
}

Write

{
  "tool_input": {
    "file_path": "/path/to/file.js",
    "content": "const x = 1;"
  }
}

Edit

{
  "tool_input": {
    "file_path": "/path/to/file.js",
    "old_string": "const x = 1;",
    "new_string": "const x = 2;",
    "replace_all": false
  }
}

Read

{
  "tool_input": {
    "file_path": "/path/to/file.js",
    "offset": 0,
    "limit": 100
  }
}

Grep

{
  "tool_input": {
    "pattern": "function.*",
    "path": "/path/to/search",
    "output_mode": "content"
  }
}

MCP tools

{
  "tool_input": {
    // MCP tool-specific parameters
  }
}

Access these in hooks:

command=$(echo "$input" | jq -r '.tool_input.command')
file_path=$(echo "$input" | jq -r '.tool_input.file_path')

---

Modifying Tool Input

PreToolUse hooks can modify tool_input before execution:

Original input:

{
  "tool_input": {
    "command": "npm install lodash"
  }
}

Hook output:

{
  "decision": "approve",
  "reason": "Adding --save-exact flag",
  "updatedInput": {
    "command": "npm install --save-exact lodash"
  }
}

Result: Tool executes with modified input.

Partial updates: Only specify fields you want to change:

{
  "updatedInput": {
    "timeout": 300000  // Only update timeout, keep other fields
  }
}

---

Error Handling

Command hooks: Return non-zero exit code to indicate error

if [ error ]; then
  echo '{"decision": "block", "reason": "Error occurred"}' >&2
  exit 1
fi

Prompt hooks: LLM should return valid JSON. If malformed, hook fails gracefully.

Timeout: Set timeout (ms) to prevent hanging:

{
  "type": "command",
  "command": "/path/to/slow-script.sh",
  "timeout": 30000
}

Default: 60000ms (60s)

Matchers and Pattern Matching

Complete guide to matching tools with hook matchers.

What are matchers?

Matchers are regex patterns that filter which tools trigger a hook. They allow you to:

• Target specific tools (e.g., only Bash)
• Match multiple tools (e.g., Write|Edit)
• Match tool categories (e.g., all MCP tools)
• Match everything (omit matcher)

---

Syntax

Matchers use JavaScript regex syntax:

{
  "matcher": "pattern"
}

The pattern is tested against the tool name using new RegExp(pattern).test(toolName).

---

Common Patterns

Exact match

{
  "matcher": "Bash"
}

Matches: Bash Doesn't match: bash, BashOutput

Multiple tools (OR)

{
  "matcher": "Write|Edit"
}

Matches: Write, Edit Doesn't match: Read, Bash

Starts with

{
  "matcher": "^Bash"
}

Matches: Bash, BashOutput Doesn't match: Read

Ends with

{
  "matcher": "Output$"
}

Matches: BashOutput Doesn't match: Bash, Read

Contains

{
  "matcher": ".*write.*"
}

Matches: Write, NotebookWrite, TodoWrite Doesn't match: Read, Edit

Case-sensitive! write won't match Write.

Any tool (no matcher)

{
  "hooks": {
    "PreToolUse": [
      {
        "hooks": [...]  // No matcher = matches all tools
      }
    ]
  }
}

---

Tool Categories

All file operations

{
  "matcher": "Read|Write|Edit|Glob|Grep"
}

All bash tools

{
  "matcher": "Bash.*"
}

Matches: Bash, BashOutput, BashKill

All MCP tools

{
  "matcher": "mcp__.*"
}

Matches: mcp__memory__store, mcp__filesystem__read, etc.

Specific MCP server

{
  "matcher": "mcp__memory__.*"
}

Matches: mcp__memory__store, mcp__memory__retrieve Doesn't match: mcp__filesystem__read

Specific MCP tool

{
  "matcher": "mcp__.*__write.*"
}

Matches: mcp__filesystem__write, mcp__memory__write Doesn't match: mcp__filesystem__read

---

MCP Tool Naming

MCP tools follow the pattern: mcp__{server}__{tool}

Examples:

• mcp__memory__store
• mcp__filesystem__read
• mcp__github__create_issue

Match all tools from a server:

{
  "matcher": "mcp__github__.*"
}

Match specific tool across all servers:

{
  "matcher": "mcp__.*__read.*"
}

---

Real-World Examples

Log all bash commands

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.command' >> ~/bash-log.txt"
          }
        ]
      }
    ]
  }
}

Format code after any file write

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit|NotebookEdit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write $CLAUDE_PROJECT_DIR"
          }
        ]
      }
    ]
  }
}

Validate all MCP memory writes

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "mcp__memory__.*",
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Validate this memory operation: $ARGUMENTS\n\nCheck if data is appropriate to store.\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"why\"}"
          }
        ]
      }
    ]
  }
}

Block destructive git commands

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/check-git-safety.sh"
          }
        ]
      }
    ]
  }
}

check-git-safety.sh:

#!/bin/bash
input=$(cat)
command=$(echo "$input" | jq -r '.tool_input.command')

if [[ "$command" == *"git push --force"* ]] || \
   [[ "$command" == *"rm -rf /"* ]] || \
   [[ "$command" == *"git reset --hard"* ]]; then
  echo '{"decision": "block", "reason": "Destructive command detected"}'
else
  echo '{"decision": "approve", "reason": "Safe"}'
fi

---

Multiple Matchers

You can have multiple matcher blocks for the same event:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/bash-validator.sh"
          }
        ]
      },
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/file-validator.sh"
          }
        ]
      },
      {
        "matcher": "mcp__.*",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/mcp-logger.sh"
          }
        ]
      }
    ]
  }
}

Each matcher is evaluated independently. A tool can match multiple matchers.

---

Debugging Matchers

Enable debug mode

claude --debug

Debug output shows:

[DEBUG] Getting matching hook commands for PreToolUse with query: Bash
[DEBUG] Found 3 hook matchers in settings
[DEBUG] Matched 1 hooks for query "Bash"

Test your matcher

Use JavaScript regex to test patterns:

const toolName = "mcp__memory__store";
const pattern = "mcp__memory__.*";
const regex = new RegExp(pattern);
console.log(regex.test(toolName)); // true

Or in Node.js:

node -e "console.log(/mcp__memory__.*/.test('mcp__memory__store'))"

Common mistakes

❌ Case sensitivity

{
  "matcher": "bash"  // Won't match "Bash"
}

✅ Correct

{
  "matcher": "Bash"
}

---

❌ Missing escape

{
  "matcher": "mcp__memory__*"  // * is literal, not wildcard
}

✅ Correct

{
  "matcher": "mcp__memory__.*"  // .* is regex for "any characters"
}

---

❌ Unintended partial match

{
  "matcher": "Write"  // Matches "Write", "TodoWrite", "NotebookWrite"
}

✅ Exact match only

{
  "matcher": "^Write$"
}

---

Advanced Patterns

Negative lookahead (exclude tools)

{
  "matcher": "^(?!Read).*"
}

Matches: Everything except Read

Match any file operation except Grep

{
  "matcher": "^(Read|Write|Edit|Glob)$"
}

Case-insensitive match

{
  "matcher": "(?i)bash"
}

Matches: Bash, bash, BASH

(Note: Claude Code tools are PascalCase by convention, so this is rarely needed)

---

Performance Considerations

Broad matchers (e.g., .*) run on every tool use:

• Simple command hooks: negligible impact
• Prompt hooks: can slow down significantly

Recommendation: Be as specific as possible with matchers to minimize unnecessary hook executions.

Example: Instead of matching all tools and checking inside the hook:

{
  "matcher": ".*",  // Runs on EVERY tool
  "hooks": [
    {
      "type": "command",
      "command": "if [[ $(jq -r '.tool_name') == 'Bash' ]]; then ...; fi"
    }
  ]
}

Do this:

{
  "matcher": "Bash",  // Only runs on Bash
  "hooks": [
    {
      "type": "command",
      "command": "..."
    }
  ]
}

---

Tool Name Reference

Common Claude Code tool names:

• Bash
• BashOutput
• KillShell
• Read
• Write
• Edit
• Glob
• Grep
• TodoWrite
• NotebookEdit
• WebFetch
• WebSearch
• Task
• Skill
• SlashCommand
• AskUserQuestion
• ExitPlanMode

MCP tools: mcp__{server}__{tool} (varies by installed servers)

Run claude --debug and watch tool calls to discover available tool names.

Troubleshooting

Common issues and solutions when working with hooks.

Hook Not Triggering

Symptom

Hook never executes, even when expected event occurs.

Diagnostic steps

1. Enable debug mode

claude --debug

Look for:

[DEBUG] Getting matching hook commands for PreToolUse with query: Bash
[DEBUG] Found 0 hooks

2. Check hook file location

Hooks must be in:

• Project: .claude/hooks.json
• User: ~/.claude/hooks.json
• Plugin: {plugin}/hooks.json

Verify:

cat .claude/hooks.json
# or
cat ~/.claude/hooks.json

3. Validate JSON syntax

Invalid JSON is silently ignored:

jq . .claude/hooks.json

If error: fix JSON syntax.

4. Check matcher pattern

Common mistakes:

❌ Case sensitivity

{
  "matcher": "bash"  // Won't match "Bash"
}

✅ Fix

{
  "matcher": "Bash"
}

---

❌ Missing escape for regex

{
  "matcher": "mcp__memory__*"  // Literal *, not wildcard
}

✅ Fix

{
  "matcher": "mcp__memory__.*"  // Regex wildcard
}

5. Test matcher in isolation

node -e "console.log(/Bash/.test('Bash'))"  # true
node -e "console.log(/bash/.test('Bash'))"  # false

Solutions

Missing hook file: Create .claude/hooks.json or ~/.claude/hooks.json

Invalid JSON: Use jq to validate and format:

jq . .claude/hooks.json > temp.json && mv temp.json .claude/hooks.json

Wrong matcher: Check tool names with --debug and update matcher

No matcher specified: If you want to match all tools, omit the matcher field entirely:

{
  "hooks": {
    "PreToolUse": [
      {
        "hooks": [...]  // No matcher = all tools
      }
    ]
  }
}

---

Command Hook Failing

Symptom

Hook executes but fails with error.

Diagnostic steps

1. Check debug output

[DEBUG] Hook command completed with status 1: <error message>

Status 1 = command failed.

2. Test command directly

Copy the command and run in terminal:

echo '{"session_id":"test","tool_name":"Bash"}' | /path/to/your/hook.sh

3. Check permissions

ls -l /path/to/hook.sh
chmod +x /path/to/hook.sh  # If not executable

4. Verify dependencies

Does the command require tools?

which jq  # Check if jq is installed
which osascript  # macOS only

Common issues

Missing executable permission

chmod +x /path/to/hook.sh

Missing dependencies

Install required tools:

# macOS
brew install jq

# Linux
apt-get install jq

Bad path

Use absolute paths:

{
  "command": "/Users/username/.claude/hooks/script.sh"
}

Or use environment variables:

{
  "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/script.sh"
}

Timeout

If command takes too long:

{
  "command": "/path/to/slow-script.sh",
  "timeout": 120000  // 2 minutes
}

---

Prompt Hook Not Working

Symptom

Prompt hook blocks everything or doesn't block when expected.

Diagnostic steps

1. Check LLM response format

Debug output shows:

[DEBUG] Hook command completed with status 0: {"decision": "approve", "reason": "ok"}

Verify JSON is valid.

2. Check prompt structure

Ensure prompt is clear:

{
  "prompt": "Evaluate: $ARGUMENTS\n\nReturn JSON: {\"decision\": \"approve\" or \"block\", \"reason\": \"why\"}"
}

3. Test prompt manually

Submit similar prompt to Claude directly to see response format.

Common issues

Ambiguous instructions

❌ Vague

{
  "prompt": "Is this ok? $ARGUMENTS"
}

✅ Clear

{
  "prompt": "Check if this command is safe: $ARGUMENTS\n\nBlock if: contains 'rm -rf', 'mkfs', or force push to main\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
}

Missing $ARGUMENTS

❌ No placeholder

{
  "prompt": "Validate this command"
}

✅ With placeholder

{
  "prompt": "Validate this command: $ARGUMENTS"
}

Invalid JSON response

The LLM must return valid JSON. If it returns plain text, the hook fails.

Add explicit formatting instructions:

IMPORTANT: Return ONLY valid JSON, no other text:
{
  "decision": "approve" or "block",
  "reason": "your explanation"
}

---

Hook Blocks Everything

Symptom

Hook blocks all operations, even safe ones.

Diagnostic steps

1. Check hook logic

Review the script/prompt logic. Is the condition too broad?

2. Test with known-safe input

echo '{"tool_name":"Read","tool_input":{"file_path":"test.txt"}}' | /path/to/hook.sh

Expected: {"decision": "approve"}

3. Check for errors in script

Add error output:

#!/bin/bash
set -e  # Exit on error
input=$(cat)
# ... rest of script

Solutions

Logic error

Review conditions:

# Before (blocks everything)
if [[ "$command" != "safe_command" ]]; then
  block
fi

# After (blocks dangerous commands)
if [[ "$command" == *"dangerous"* ]]; then
  block
fi

Default to approve

If logic is complex, default to approve on unclear cases:

# Default
decision="approve"
reason="ok"

# Only change if dangerous
if [[ "$command" == *"rm -rf"* ]]; then
  decision="block"
  reason="Dangerous command"
fi

echo "{\"decision\": \"$decision\", \"reason\": \"$reason\"}"

---

Infinite Loop in Stop Hook

Symptom

Stop hook runs repeatedly, Claude never stops.

Cause

Hook blocks stop without checking stop_hook_active flag.

Solution

Always check the flag:

#!/bin/bash
input=$(cat)
stop_hook_active=$(echo "$input" | jq -r '.stop_hook_active')

# If hook already active, don't block again
if [[ "$stop_hook_active" == "true" ]]; then
  echo '{"decision": undefined}'
  exit 0
fi

# Your logic here
if [ tests_passing ]; then
  echo '{"decision": "approve", "reason": "Tests pass"}'
else
  echo '{"decision": "block", "reason": "Tests failing"}'
fi

Or in prompt hooks:

{
  "prompt": "Evaluate stopping: $ARGUMENTS\n\nIMPORTANT: If stop_hook_active is true, return {\"decision\": undefined}\n\nOtherwise check if tasks complete..."
}

---

Hook Output Not Visible

Symptom

Hook runs but output not shown to user.

Cause

suppressOutput: true or output goes to stderr.

Solutions

Don't suppress output:

{
  "decision": "approve",
  "reason": "ok",
  "suppressOutput": false
}

Use systemMessage:

{
  "decision": "approve",
  "reason": "ok",
  "systemMessage": "This message will be shown to user"
}

Write to stdout, not stderr:

echo "This is shown" >&1
echo "This is hidden" >&2

---

Permission Errors

Symptom

Hook script can't read files or execute commands.

Solutions

Make script executable:

chmod +x /path/to/hook.sh

Check file ownership:

ls -l /path/to/hook.sh
chown $USER /path/to/hook.sh

Use absolute paths:

# Instead of
command="./script.sh"

# Use
command="$CLAUDE_PROJECT_DIR/.claude/hooks/script.sh"

---

Hook Timeouts

Symptom

[DEBUG] Hook command timed out after 60000ms

Solutions

Increase timeout:

{
  "type": "command",
  "command": "/path/to/slow-script.sh",
  "timeout": 300000  // 5 minutes
}

Optimize script:

• Reduce unnecessary operations
• Cache results when possible
• Run expensive operations in background

Run in background:

#!/bin/bash
# Start long operation in background
/path/to/long-operation.sh &

# Return immediately
echo '{"decision": "approve", "reason": "ok"}'

---

Matcher Conflicts

Symptom

Multiple hooks triggering when only one expected.

Cause

Tool name matches multiple matchers.

Diagnostic

[DEBUG] Matched 3 hooks for query "Bash"

Solutions

Be more specific:

// Instead of
{"matcher": ".*"}  // Matches everything

// Use
{"matcher": "Bash"}  // Exact match

Check overlapping patterns:

{
  "hooks": {
    "PreToolUse": [
      {"matcher": "Bash", ...},        // Matches Bash
      {"matcher": "Bash.*", ...},      // Also matches Bash!
      {"matcher": ".*", ...}           // Also matches everything!
    ]
  }
}

Remove overlaps or make them mutually exclusive.

---

Environment Variables Not Working

Symptom

$CLAUDE_PROJECT_DIR or other variables are empty.

Solutions

Check variable spelling:

• $CLAUDE_PROJECT_DIR (correct)
• $CLAUDE_PROJECT_ROOT (wrong)

Use double quotes:

{
  "command": "$CLAUDE_PROJECT_DIR/hooks/script.sh"
}

In shell scripts, use from input:

#!/bin/bash
input=$(cat)
cwd=$(echo "$input" | jq -r '.cwd')
cd "$cwd" || exit 1

---

Debugging Workflow

Step 1: Enable debug mode

claude --debug

Step 2: Look for hook execution logs

[DEBUG] Executing hooks for PreToolUse:Bash
[DEBUG] Found 1 hook matchers
[DEBUG] Executing hook command: /path/to/script.sh
[DEBUG] Hook command completed with status 0

Step 3: Test hook in isolation

echo '{"test":"data"}' | /path/to/hook.sh

Step 4: Check script with set -x

#!/bin/bash
set -x  # Print each command before executing
# ... your script

Step 5: Add logging

#!/bin/bash
echo "Hook started" >> /tmp/hook-debug.log
input=$(cat)
echo "Input: $input" >> /tmp/hook-debug.log
# ... your logic
echo "Decision: $decision" >> /tmp/hook-debug.log

Step 6: Verify JSON output

echo '{"decision":"approve","reason":"test"}' | jq .

If jq fails, JSON is invalid.