customaize-agent:create-hook

Analyze the project, suggest practical hooks, and create them with proper testing.

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Install skill "customaize-agent:create-hook" with this command: npx skills add neolabhq/context-engineering-kit/neolabhq-context-engineering-kit-customaize-agent-create-hook

Create Hook Command

Analyze the project, suggest practical hooks, and create them with proper testing.

Your Task (/create-hook)

  • Analyze environment - Detect tooling and existing hooks

  • Suggest hooks - Based on your project configuration

  • Configure hook - Ask targeted questions and create the script

  • Test & validate - Ensure the hook works correctly

Your Workflow

  1. Environment Analysis & Suggestions

Automatically detect the project tooling and suggest relevant hooks:

When TypeScript is detected (tsconfig.json ):

  • PostToolUse hook: "Type-check files after editing"

  • PreToolUse hook: "Block edits with type errors"

When Prettier is detected (.prettierrc , prettier.config.js ):

  • PostToolUse hook: "Auto-format files after editing"

  • PreToolUse hook: "Require formatted code"

When ESLint is detected (.eslintrc.* ):

  • PostToolUse hook: "Lint and auto-fix after editing"

  • PreToolUse hook: "Block commits with linting errors"

When package.json has scripts:

  • test script → "Run tests before commits"

  • build script → "Validate build before commits"

When a git repository is detected:

  • PreToolUse/Bash hook: "Prevent commits with secrets"

  • PostToolUse hook: "Security scan on file changes"

Decision Tree:

Project has TypeScript? → Suggest type checking hooks Project has formatter? → Suggest formatting hooks Project has tests? → Suggest test validation hooks Security sensitive? → Suggest security hooks

  • Scan for additional patterns and suggest custom hooks based on:
    • Custom scripts in package.json
    • Unique file patterns or extensions
    • Development workflow indicators
    • Project-specific tooling configurations
  1. Hook Configuration

Start by asking: "What should this hook do?" and offer relevant suggestions from your analysis.

Then understand the context from the user's description and only ask about details you're unsure about:

Trigger timing: When should it run?

  • PreToolUse : Before file operations (can block)

  • PostToolUse : After file operations (feedback/fixes)

  • UserPromptSubmit : Before processing requests

  • Other event types as needed

Tool matcher: Which tools should trigger it? (Write , Edit , Bash , * etc)

Scope: global , project , or project-local

Response approach:

  • Exit codes only: Simple (exit 0 = success, exit 2 = block in PreToolUse)

  • JSON response: Advanced control (blocking, context, decisions)

  • Guide based on complexity: simple pass/fail → exit codes, rich feedback → JSON

Blocking behavior (if relevant): "Should this stop operations when issues are found?"

  • PreToolUse: Can block operations (security, validation)

  • PostToolUse: Usually provide feedback only

Claude integration (CRITICAL): "Should Claude Code automatically see and fix issues this hook detects?"

  • If YES: Use additionalContext for error communication

  • If NO: Use suppressOutput: true for silent operation

Context pollution: "Should successful operations be silent to avoid noise?"

  • Recommend YES for formatting, routine checks

  • Recommend NO for security alerts, critical errors

File filtering: "What file types should this hook process?"

  1. Hook Creation

You should:

  • Create hooks directory: ~/.claude/hooks/ or .claude/hooks/ based on scope

  • Generate script: Create hook script with:

  • Proper shebang and executable permissions

  • Project-specific commands (use detected config paths)

  • Comments explaining the hook's purpose

  • Update settings: Add hook configuration to appropriate settings.json

  • Use absolute paths: Avoid relative paths to scripts and executables. Use $CLAUDE_PROJECT_DIR to reference project root

  • Offer validation: Ask if the user wants you to test the hook

Key Implementation Standards:

  • Read JSON from stdin (never use argv)

  • Use top-level additionalContext /systemMessage for Claude communication

  • Include suppressOutput: true for successful operations

  • Provide specific error counts and actionable feedback

  • Focus on changed files rather than entire codebase

  • Support common development workflows

⚠️ CRITICAL: Input/Output Format

This is where most hook implementations fail. Pay extra attention to:

  • Input: Reading JSON from stdin correctly (not argv)

  • Output: Using correct top-level JSON structure for Claude communication

  • Documentation: Consulting official docs for exact schemas when in doubt

  1. Testing & Validation

CRITICAL: Test both happy and sad paths:

Happy Path Testing:

  • Test expected success scenario - Create conditions where hook should pass

  • Examples: TypeScript (valid code), Linting (formatted code), Security (safe commands)

Sad Path Testing: 2. Test expected failure scenario - Create conditions where hook should fail/warn

  • Examples: TypeScript (type errors), Linting (unformatted code), Security (dangerous operations)

Verification Steps: 3. Verify expected behavior: Check if it blocks/warns/provides context as intended

Example Testing Process:

  • For a hook preventing file deletion: Create a test file, attempt the protected action, and verify the hook prevents it

If Issues Occur, you should:

  • Check hook registration in settings

  • Verify script permissions (chmod +x )

  • Test with simplified version first

  • Debug with detailed hook execution analysis

Hook Templates

Type Checking (PostToolUse)

#!/usr/bin/env node // Read stdin JSON, check .ts/.tsx files only // Run: npx tsc --noEmit --pretty // Output: JSON with additionalContext for errors

Auto-formatting (PostToolUse)

#!/usr/bin/env node // Read stdin JSON, check supported file types // Run: npx prettier --write [file] // Output: JSON with suppressOutput: true

Security Scanning (PreToolUse)

#!/bin/bash

Read stdin JSON, check for secrets/keys

Block if dangerous patterns found

Exit 2 to block, 0 to continue

Complete templates available at: https://docs.claude.com/en/docs/claude-code/hooks#examples

Quick Reference

📖 Official Docs: https://docs.claude.com/en/docs/claude-code/hooks.md

Common Patterns:

  • stdin input: JSON.parse(process.stdin.read())

  • File filtering: Check extensions before processing

  • Success response: {continue: true, suppressOutput: true}

  • Error response: {continue: true, additionalContext: "error details"}

  • Block operation: exit(2) in PreToolUse hooks

Hook Types by Use Case:

  • Code Quality: PostToolUse for feedback and fixes

  • Security: PreToolUse to block dangerous operations

  • CI/CD: PreToolUse to validate before commits

  • Development: PostToolUse for automated improvements

Hook Execution Best Practices:

  • Hooks run in parallel according to official documentation

  • Design for independence since execution order isn't guaranteed

  • Plan hook interactions carefully when multiple hooks affect the same files

Success Criteria

✅ Hook created successfully when:

  • Script has executable permissions

  • Registered in correct settings.json

  • Responds correctly to test scenarios

  • Integrates properly with Claude for automated fixes

  • Follows project conventions and detected tooling

Result: The user gets a working hook that enhances their development workflow with intelligent automation and quality checks.

Documentation Index

Fetch the complete documentation index at: https://code.claude.com/docs/llms.txt Use this file to discover all available pages before exploring further.

Automate workflows with hooks

Run shell commands automatically when Claude Code edits files, finishes tasks, or needs input. Format code, send notifications, validate commands, and enforce project rules.

Hooks are user-defined shell commands that execute at specific points in Claude Code's lifecycle. They provide deterministic control over Claude Code's behavior, ensuring certain actions always happen rather than relying on the LLM to choose to run them. Use hooks to enforce project rules, automate repetitive tasks, and integrate Claude Code with your existing tools.

For decisions that require judgment rather than deterministic rules, you can also use prompt-based hooks or agent-based hooks that use a Claude model to evaluate conditions.

For other ways to extend Claude Code, see skills for giving Claude additional instructions and executable commands, subagents for running tasks in isolated contexts, and plugins for packaging extensions to share across projects.

Set up your first hook

The fastest way to create a hook is through the /hooks interactive menu in Claude Code. This walkthrough creates a desktop notification hook, so you get alerted whenever Claude is waiting for your input instead of watching the terminal.

<Tabs> <Tab title="macOS"> Uses osascript to trigger a native macOS notification through AppleScript:

```
osascript -e 'display notification "Claude Code needs your attention" with title "Claude Code"'
```

</Tab>

<Tab title="Linux"> Uses notify-send, which is pre-installed on most Linux desktops with a notification daemon:

```
notify-send 'Claude Code' 'Claude Code needs your attention'
```

</Tab>

<Tab title="Windows (PowerShell)"> Uses PowerShell to show a native message box through .NET's Windows Forms:

```
powershell.exe -Command "[System.Reflection.Assembly]::LoadWithPartialName('System.Windows.Forms'); [System.Windows.Forms.MessageBox]::Show('Claude Code needs your attention', 'Claude Code')"
```

</Tab> </Tabs>

What you can automate

Hooks let you run code at key points in Claude Code's lifecycle: format files after edits, block commands before they execute, send notifications when Claude needs input, inject context at session start, and more. For the full list of hook events, see the Hooks reference.

Each example includes a ready-to-use configuration block that you add to a settings file. The most common patterns:

  • Get notified when Claude needs input

  • Auto-format code after edits

  • Block edits to protected files

  • Re-inject context after compaction

Get notified when Claude needs input

Get a desktop notification whenever Claude finishes working and needs your input, so you can switch to other tasks without checking the terminal.

This hook uses the Notification event, which fires when Claude is waiting for input or permission. Each tab below uses the platform's native notification command. Add this to ~/.claude/settings.json , or use the interactive walkthrough above to configure it with /hooks :

Auto-format code after edits

Automatically run Prettier on every file Claude edits, so formatting stays consistent without manual intervention.

This hook uses the PostToolUse event with an Edit|Write matcher, so it runs only after file-editing tools. The command extracts the edited file path with jq and passes it to Prettier. Add this to .claude/settings.json in your project root:

{ "hooks": { "PostToolUse": [ { "matcher": "Edit|Write", "hooks": [ { "type": "command", "command": "jq -r '.tool_input.file_path' | xargs npx prettier --write" } ] } ] } }

Block edits to protected files

Prevent Claude from modifying sensitive files like .env , package-lock.json , or anything in .git/ . Claude receives feedback explaining why the edit was blocked, so it can adjust its approach.

This example uses a separate script file that the hook calls. The script checks the target file path against a list of protected patterns and exits with code 2 to block the edit.

#!/bin/bash
# protect-files.sh

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

PROTECTED_PATTERNS=(".env" "package-lock.json" ".git/")

for pattern in "${PROTECTED_PATTERNS[@]}"; do
  if [[ "$FILE_PATH" == *"$pattern"* ]]; then
    echo "Blocked: $FILE_PATH matches protected pattern '$pattern'" >&#x26;2
    exit 2
  fi
done

exit 0

chmod +x .claude/hooks/protect-files.sh

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/protect-files.sh"
          }
        ]
      }
    ]
  }
}

Re-inject context after compaction

When Claude's context window fills up, compaction summarizes the conversation to free space. This can lose important details. Use a SessionStart hook with a compact matcher to re-inject critical context after every compaction.

Any text your command writes to stdout is added to Claude's context. This example reminds Claude of project conventions and recent work. Add this to .claude/settings.json in your project root:

{ "hooks": { "SessionStart": [ { "matcher": "compact", "hooks": [ { "type": "command", "command": "echo 'Reminder: use Bun, not npm. Run bun test before committing. Current sprint: auth refactor.'" } ] } ] } }

You can replace the echo with any command that produces dynamic output, like git log --oneline -5 to show recent commits. For injecting context on every session start, consider using CLAUDE.md instead. For environment variables, see CLAUDE_ENV_FILE in the reference.

How hooks work

Hook events fire at specific lifecycle points in Claude Code. When an event fires, all matching hooks run in parallel, and identical hook commands are automatically deduplicated. The table below shows each event and when it triggers:

Event When it fires

SessionStart

When a session begins or resumes

UserPromptSubmit

When you submit a prompt, before Claude processes it

PreToolUse

Before a tool call executes. Can block it

PermissionRequest

When a permission dialog appears

PostToolUse

After a tool call succeeds

PostToolUseFailure

After a tool call fails

Notification

When Claude Code sends a notification

SubagentStart

When a subagent is spawned

SubagentStop

When a subagent finishes

Stop

When Claude finishes responding

PreCompact

Before context compaction

SessionEnd

When a session terminates

Each hook has a type that determines how it runs. Most hooks use "type": "command" , which runs a shell command. Two other options use a Claude model to make decisions: "type": "prompt" for single-turn evaluation and "type": "agent" for multi-turn verification with tool access. See Prompt-based hooks and Agent-based hooks for details.

Read input and return output

Hooks communicate with Claude Code through stdin, stdout, stderr, and exit codes. When an event fires, Claude Code passes event-specific data as JSON to your script's stdin. Your script reads that data, does its work, and tells Claude Code what to do next via the exit code.

Hook input

Every event includes common fields like session_id and cwd , but each event type adds different data. For example, when Claude runs a Bash command, a PreToolUse hook receives something like this on stdin:

{ "session_id": "abc123", // unique ID for this session "cwd": "/Users/sarah/myproject", // working directory when the event fired "hook_event_name": "PreToolUse", // which event triggered this hook "tool_name": "Bash", // the tool Claude is about to use "tool_input": { // the arguments Claude passed to the tool "command": "npm test" // for Bash, this is the shell command } }

Your script can parse that JSON and act on any of those fields. UserPromptSubmit hooks get the prompt text instead, SessionStart hooks get the source (startup, resume, compact), and so on. See Common input fields in the reference for shared fields, and each event's section for event-specific schemas.

Hook output

Your script tells Claude Code what to do next by writing to stdout or stderr and exiting with a specific code. For example, a PreToolUse hook that wants to block a command:

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

if echo "$COMMAND" | grep -q "drop table"; then echo "Blocked: dropping tables is not allowed" >&2 # stderr becomes Claude's feedback exit 2 # exit 2 = block the action fi

exit 0 # exit 0 = let it proceed

The exit code determines what happens next:

  • Exit 0: the action proceeds. For UserPromptSubmit and SessionStart hooks, anything you write to stdout is added to Claude's context.

  • Exit 2: the action is blocked. Write a reason to stderr, and Claude receives it as feedback so it can adjust.

  • Any other exit code: the action proceeds. Stderr is logged but not shown to Claude. Toggle verbose mode with Ctrl+O to see these messages in the transcript.

Structured JSON output

Exit codes give you two options: allow or block. For more control, exit 0 and print a JSON object to stdout instead.

For example, a PreToolUse hook can deny a tool call and tell Claude why, or escalate it to the user for approval:

{ "hookSpecificOutput": { "hookEventName": "PreToolUse", "permissionDecision": "deny", "permissionDecisionReason": "Use rg instead of grep for better performance" } }

Claude Code reads permissionDecision and cancels the tool call, then feeds permissionDecisionReason back to Claude as feedback. These three options are specific to PreToolUse :

  • "allow" : proceed without showing a permission prompt

  • "deny" : cancel the tool call and send the reason to Claude

  • "ask" : show the permission prompt to the user as normal

Other events use different decision patterns. For example, PostToolUse and Stop hooks use a top-level decision: "block" field, while PermissionRequest uses hookSpecificOutput.decision.behavior . See the summary table in the reference for a full breakdown by event.

For UserPromptSubmit hooks, use additionalContext instead to inject text into Claude's context. Prompt-based hooks (type: "prompt" ) handle output differently: see Prompt-based hooks.

Filter hooks with matchers

Without a matcher, a hook fires on every occurrence of its event. Matchers let you narrow that down. For example, if you want to run a formatter only after file edits (not after every tool call), add a matcher to your PostToolUse hook:

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

The "Edit|Write" matcher is a regex pattern that matches the tool name. The hook only fires when Claude uses the Edit or Write tool, not when it uses Bash , Read , or any other tool.

Each event type matches on a specific field. Matchers support exact strings and regex patterns:

Event What the matcher filters Example matcher values

PreToolUse , PostToolUse , PostToolUseFailure , PermissionRequest

tool name Bash , Edit|Write , mcp__.*

SessionStart

how the session started startup , resume , clear , compact

SessionEnd

why the session ended clear , logout , prompt_input_exit , other

Notification

notification type permission_prompt , idle_prompt , auth_success , elicitation_dialog

SubagentStart

agent type Bash , Explore , Plan , or custom agent names

PreCompact

what triggered compaction manual , auto

UserPromptSubmit , Stop

no matcher support always fires on every occurrence

SubagentStop

agent type same values as SubagentStart

A few more examples showing matchers on different event types:

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

The command below extracts the tool name from the hook's JSON input with jq and writes it to stderr, where it shows up in verbose mode (Ctrl+O):

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "mcp__github__.*",
        "hooks": [
          {
            "type": "command",
            "command": "echo \"GitHub tool called: $(jq -r '.tool_name')\" >&#x26;2"
          }
        ]
      }
    ]
  }
}

{
  "hooks": {
    "SessionEnd": [
      {
        "matcher": "clear",
        "hooks": [
          {
            "type": "command",
            "command": "rm -f /tmp/claude-scratch-*.txt"
          }
        ]
      }
    ]
  }
}

For full matcher syntax, see the Hooks reference.

Configure hook location

Where you add a hook determines its scope:

Location Scope Shareable

~/.claude/settings.json

All your projects No, local to your machine

.claude/settings.json

Single project Yes, can be committed to the repo

.claude/settings.local.json

Single project No, gitignored

Managed policy settings Organization-wide Yes, admin-controlled

Plugin hooks/hooks.json

When plugin is enabled Yes, bundled with the plugin

Skill or agent frontmatter While the skill or agent is active Yes, defined in the component file

You can also use the /hooks menu in Claude Code to add, delete, and view hooks interactively. To disable all hooks at once, use the toggle at the bottom of the /hooks menu or set "disableAllHooks": true in your settings file.

Hooks added through the /hooks menu take effect immediately. If you edit settings files directly while Claude Code is running, the changes won't take effect until you review them in the /hooks menu or restart your session.

Prompt-based hooks

For decisions that require judgment rather than deterministic rules, use type: "prompt" hooks. Instead of running a shell command, Claude Code sends your prompt and the hook's input data to a Claude model (Haiku by default) to make the decision. You can specify a different model with the model field if you need more capability.

The model's only job is to return a yes/no decision as JSON:

  • "ok": true : the action proceeds

  • "ok": false : the action is blocked. The model's "reason" is fed back to Claude so it can adjust.

This example uses a Stop hook to ask the model whether all requested tasks are complete. If the model returns "ok": false , Claude keeps working and uses the reason as its next instruction:

{ "hooks": { "Stop": [ { "hooks": [ { "type": "prompt", "prompt": "Check if all tasks are complete. If not, respond with {"ok": false, "reason": "what remains to be done"}." } ] } ] } }

For full configuration options, see Prompt-based hooks in the reference.

Agent-based hooks

When verification requires inspecting files or running commands, use type: "agent" hooks. Unlike prompt hooks which make a single LLM call, agent hooks spawn a subagent that can read files, search code, and use other tools to verify conditions before returning a decision.

Agent hooks use the same "ok" / "reason" response format as prompt hooks, but with a longer default timeout of 60 seconds and up to 50 tool-use turns.

This example verifies that tests pass before allowing Claude to stop:

{ "hooks": { "Stop": [ { "hooks": [ { "type": "agent", "prompt": "Verify that all unit tests pass. Run the test suite and check the results. $ARGUMENTS", "timeout": 120 } ] } ] } }

Use prompt hooks when the hook input data alone is enough to make a decision. Use agent hooks when you need to verify something against the actual state of the codebase.

For full configuration options, see Agent-based hooks in the reference.

Limitations and troubleshooting

Limitations

  • Hooks communicate through stdout, stderr, and exit codes only. They cannot trigger slash commands or tool calls directly.

  • Hook timeout is 10 minutes by default, configurable per hook with the timeout field (in seconds).

  • PostToolUse hooks cannot undo actions since the tool has already executed.

  • PermissionRequest hooks do not fire in non-interactive mode (-p ). Use PreToolUse hooks for automated permission decisions.

  • Stop hooks fire whenever Claude finishes responding, not only at task completion. They do not fire on user interrupts.

Hook not firing

The hook is configured but never executes.

  • Run /hooks and confirm the hook appears under the correct event

  • Check that the matcher pattern matches the tool name exactly (matchers are case-sensitive)

  • Verify you're triggering the right event type (e.g., PreToolUse fires before tool execution, PostToolUse fires after)

  • If using PermissionRequest hooks in non-interactive mode (-p ), switch to PreToolUse instead

Hook error in output

You see a message like "PreToolUse hook error: ..." in the transcript.

Your script exited with a non-zero code unexpectedly. Test it manually by piping sample JSON:

echo '{"tool_name":"Bash","tool_input":{"command":"ls"}}' | ./my-hook.sh echo $? # Check the exit code

  • If you see "command not found", use absolute paths or $CLAUDE_PROJECT_DIR to reference scripts

  • If you see "jq: command not found", install jq or use Python/Node.js for JSON parsing

  • If the script isn't running at all, make it executable: chmod +x ./my-hook.sh

/hooks shows no hooks configured

You edited a settings file but the hooks don't appear in the menu.

  • Restart your session or open /hooks to reload. Hooks added through the /hooks menu take effect immediately, but manual file edits require a reload.

  • Verify your JSON is valid (trailing commas and comments are not allowed)

  • Confirm the settings file is in the correct location: .claude/settings.json for project hooks, ~/.claude/settings.json for global hooks

Stop hook runs forever

Claude keeps working in an infinite loop instead of stopping.

Your Stop hook script needs to check whether it already triggered a continuation. Parse the stop_hook_active field from the JSON input and exit early if it's true :

#!/bin/bash INPUT=$(cat) if [ "$(echo "$INPUT" | jq -r '.stop_hook_active')" = "true" ]; then exit 0 # Allow Claude to stop fi

... rest of your hook logic

JSON validation failed

Claude Code shows a JSON parsing error even though your hook script outputs valid JSON.

When Claude Code runs a hook, it spawns a shell that sources your profile (~/.zshrc or ~/.bashrc ). If your profile contains unconditional echo statements, that output gets prepended to your hook's JSON:

Shell ready on arm64 {"decision": "block", "reason": "Not allowed"}

Claude Code tries to parse this as JSON and fails. To fix this, wrap echo statements in your shell profile so they only run in interactive shells:

In ~/.zshrc or ~/.bashrc

if [[ $- == i ]]; then echo "Shell ready" fi

The $- variable contains shell flags, and i means interactive. Hooks run in non-interactive shells, so the echo is skipped.

Debug techniques

Toggle verbose mode with Ctrl+O to see hook output in the transcript, or run claude --debug for full execution details including which hooks matched and their exit codes.

Learn more

  • Hooks reference: full event schemas, JSON output format, async hooks, and MCP tool hooks

  • Security considerations: review before deploying hooks in shared or production environments

  • Bash command validator example: complete reference implementation

Documentation Index

Fetch the complete documentation index at: https://code.claude.com/docs/llms.txt Use this file to discover all available pages before exploring further.

Hooks reference

Reference for Claude Code hook events, configuration schema, JSON input/output formats, exit codes, async hooks, prompt hooks, and MCP tool hooks.

Hooks are user-defined shell commands or LLM prompts that execute automatically at specific points in Claude Code's lifecycle. Use this reference to look up event schemas, configuration options, JSON input/output formats, and advanced features like async hooks and MCP tool hooks. If you're setting up hooks for the first time, start with the guide instead.

Hook lifecycle

Hooks fire at specific points during a Claude Code session. When an event fires and a matcher matches, Claude Code passes JSON context about the event to your hook handler. For command hooks, this arrives on stdin. Your handler can then inspect the input, take action, and optionally return a decision. Some events fire once per session, while others fire repeatedly inside the agentic loop:

The table below summarizes when each event fires. The Hook events section documents the full input schema and decision control options for each one.

Event When it fires

SessionStart

When a session begins or resumes

UserPromptSubmit

When you submit a prompt, before Claude processes it

PreToolUse

Before a tool call executes. Can block it

PermissionRequest

When a permission dialog appears

PostToolUse

After a tool call succeeds

PostToolUseFailure

After a tool call fails

Notification

When Claude Code sends a notification

SubagentStart

When a subagent is spawned

SubagentStop

When a subagent finishes

Stop

When Claude finishes responding

PreCompact

Before context compaction

SessionEnd

When a session terminates

How a hook resolves

To see how these pieces fit together, consider this PreToolUse hook that blocks destructive shell commands. The hook runs block-rm.sh before every Bash tool call:

{ "hooks": { "PreToolUse": [ { "matcher": "Bash", "hooks": [ { "type": "command", "command": ".claude/hooks/block-rm.sh" } ] } ] } }

The script reads the JSON input from stdin, extracts the command, and returns a permissionDecision of "deny" if it contains rm -rf :

#!/bin/bash

.claude/hooks/block-rm.sh

COMMAND=$(jq -r '.tool_input.command')

if echo "$COMMAND" | grep -q 'rm -rf'; then jq -n '{ hookSpecificOutput: { hookEventName: "PreToolUse", permissionDecision: "deny", permissionDecisionReason: "Destructive command blocked by hook" } }' else exit 0 # allow the command fi

Now suppose Claude Code decides to run Bash "rm -rf /tmp/build" . Here's what happens:

{ "tool_name": "Bash", "tool_input": { "command": "rm -rf /tmp/build" }, ... }

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "deny",
    "permissionDecisionReason": "Destructive command blocked by hook"
  }
}

If the command had been safe (like npm test), the script would hit exit 0 instead, which tells Claude Code to allow the tool call with no further action.

The Configuration section below documents the full schema, and each hook event section documents what input your command receives and what output it can return.

Configuration

Hooks are defined in JSON settings files. The configuration has three levels of nesting:

  • Choose a hook event to respond to, like PreToolUse or Stop

  • Add a matcher group to filter when it fires, like "only for the Bash tool"

  • Define one or more hook handlers to run when matched

See How a hook resolves above for a complete walkthrough with an annotated example.

Hook locations

Where you define a hook determines its scope:

Location Scope Shareable

~/.claude/settings.json

All your projects No, local to your machine

.claude/settings.json

Single project Yes, can be committed to the repo

.claude/settings.local.json

Single project No, gitignored

Managed policy settings Organization-wide Yes, admin-controlled

Plugin hooks/hooks.json

When plugin is enabled Yes, bundled with the plugin

Skill or agent frontmatter While the component is active Yes, defined in the component file

For details on settings file resolution, see settings. Enterprise administrators can use allowManagedHooksOnly to block user, project, and plugin hooks. See Hook configuration.

Matcher patterns

The matcher field is a regex string that filters when hooks fire. Use "*" , "" , or omit matcher entirely to match all occurrences. Each event type matches on a different field:

Event What the matcher filters Example matcher values

PreToolUse , PostToolUse , PostToolUseFailure , PermissionRequest

tool name Bash , Edit|Write , mcp__.*

SessionStart

how the session started startup , resume , clear , compact

SessionEnd

why the session ended clear , logout , prompt_input_exit , bypass_permissions_disabled , other

Notification

notification type permission_prompt , idle_prompt , auth_success , elicitation_dialog

SubagentStart

agent type Bash , Explore , Plan , or custom agent names

PreCompact

what triggered compaction manual , auto

SubagentStop

agent type same values as SubagentStart

UserPromptSubmit , Stop

no matcher support always fires on every occurrence

The matcher is a regex, so Edit|Write matches either tool and Notebook.* matches any tool starting with Notebook. The matcher runs against a field from the JSON input that Claude Code sends to your hook on stdin. For tool events, that field is tool_name . Each hook event section lists the full set of matcher values and the input schema for that event.

This example runs a linting script only when Claude writes or edits a file:

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

UserPromptSubmit and Stop don't support matchers and always fire on every occurrence. If you add a matcher field to these events, it is silently ignored.

Match MCP tools

MCP server tools appear as regular tools in tool events (PreToolUse , PostToolUse , PostToolUseFailure , PermissionRequest ), so you can match them the same way you match any other tool name.

MCP tools follow the naming pattern mcp__<server>__<tool> , for example:

  • mcp__memory__create_entities : Memory server's create entities tool

  • mcp__filesystem__read_file : Filesystem server's read file tool

  • mcp__github__search_repositories : GitHub server's search tool

Use regex patterns to target specific MCP tools or groups of tools:

  • mcp__memory__.* matches all tools from the memory server

  • mcp__.__write. matches any tool containing "write" from any server

This example logs all memory server operations and validates write operations from any MCP server:

{ "hooks": { "PreToolUse": [ { "matcher": "mcp__memory__.", "hooks": [ { "type": "command", "command": "echo 'Memory operation initiated' >> ~/mcp-operations.log" } ] }, { "matcher": "mcp__.__write.*", "hooks": [ { "type": "command", "command": "/home/user/scripts/validate-mcp-write.py" } ] } ] } }

Hook handler fields

Each object in the inner hooks array is a hook handler: the shell command, LLM prompt, or agent that runs when the matcher matches. There are three types:

  • Command hooks (type: "command" ): run a shell command. Your script receives the event's JSON input on stdin and communicates results back through exit codes and stdout.

  • Prompt hooks (type: "prompt" ): send a prompt to a Claude model for single-turn evaluation. The model returns a yes/no decision as JSON. See Prompt-based hooks.

  • Agent hooks (type: "agent" ): spawn a subagent that can use tools like Read, Grep, and Glob to verify conditions before returning a decision. See Agent-based hooks.

Common fields

These fields apply to all hook types:

Field Required Description

type

yes "command" , "prompt" , or "agent"

timeout

no Seconds before canceling. Defaults: 600 for command, 30 for prompt, 60 for agent

statusMessage

no Custom spinner message displayed while the hook runs

once

no If true , runs only once per session then is removed. Skills only, not agents. See Hooks in skills and agents

Command hook fields

In addition to the common fields, command hooks accept these fields:

Field Required Description

command

yes Shell command to execute

async

no If true , runs in the background without blocking. See Run hooks in the background

Prompt and agent hook fields

In addition to the common fields, prompt and agent hooks accept these fields:

Field Required Description

prompt

yes Prompt text to send to the model. Use $ARGUMENTS as a placeholder for the hook input JSON

model

no Model to use for evaluation. Defaults to a fast model

All matching hooks run in parallel, and identical handlers are deduplicated automatically. Handlers run in the current directory with Claude Code's environment. The $CLAUDE_CODE_REMOTE environment variable is set to "true" in remote web environments and not set in the local CLI.

Reference scripts by path

Use environment variables to reference hook scripts relative to the project or plugin root, regardless of the working directory when the hook runs:

  • $CLAUDE_PROJECT_DIR : the project root. Wrap in quotes to handle paths with spaces.

  • ${CLAUDE_PLUGIN_ROOT} : the plugin's root directory, for scripts bundled with a plugin.

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/check-style.sh"
          }
        ]
      }
    ]
  }
}

This example runs a formatting script bundled with the plugin:

{
  "description": "Automatic code formatting",
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/format.sh",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

See the plugin components reference for details on creating plugin hooks.

Hooks in skills and agents

In addition to settings files and plugins, hooks can be defined directly in skills and subagents using frontmatter. These hooks are scoped to the component's lifecycle and only run when that component is active.

All hook events are supported. For subagents, Stop hooks are automatically converted to SubagentStop since that is the event that fires when a subagent completes.

Hooks use the same configuration format as settings-based hooks but are scoped to the component's lifetime and cleaned up when it finishes.

This skill defines a PreToolUse hook that runs a security validation script before each Bash command:

name: secure-operations description: Perform operations with security checks hooks: PreToolUse: - matcher: "Bash" hooks: - type: command command: "./scripts/security-check.sh"

Agents use the same format in their YAML frontmatter.

The /hooks menu

Type /hooks in Claude Code to open the interactive hooks manager, where you can view, add, and delete hooks without editing settings files directly. For a step-by-step walkthrough, see Set up your first hook in the guide.

Each hook in the menu is labeled with a bracket prefix indicating its source:

  • [User] : from ~/.claude/settings.json

  • [Project] : from .claude/settings.json

  • [Local] : from .claude/settings.local.json

  • [Plugin] : from a plugin's hooks/hooks.json , read-only

Disable or remove hooks

To remove a hook, delete its entry from the settings JSON file, or use the /hooks menu and select the hook to delete it.

To temporarily disable all hooks without removing them, set "disableAllHooks": true in your settings file or use the toggle in the /hooks menu. There is no way to disable an individual hook while keeping it in the configuration.

Direct edits to hooks in settings files don't take effect immediately. Claude Code captures a snapshot of hooks at startup and uses it throughout the session. This prevents malicious or accidental hook modifications from taking effect mid-session without your review. If hooks are modified externally, Claude Code warns you and requires review in the /hooks menu before changes apply.

Hook input and output

Hooks receive JSON data via stdin and communicate results through exit codes, stdout, and stderr. This section covers fields and behavior common to all events. Each event's section under Hook events includes its specific input schema and decision control options.

Common input fields

All hook events receive these fields via stdin as JSON, in addition to event-specific fields documented in each hook event section:

Field Description

session_id

Current session identifier

transcript_path

Path to conversation JSON

cwd

Current working directory when the hook is invoked

permission_mode

Current permission mode: "default" , "plan" , "acceptEdits" , "dontAsk" , or "bypassPermissions"

hook_event_name

Name of the event that fired

For example, a PreToolUse hook for a Bash command receives this on stdin:

{ "session_id": "abc123", "transcript_path": "/home/user/.claude/projects/.../transcript.jsonl", "cwd": "/home/user/my-project", "permission_mode": "default", "hook_event_name": "PreToolUse", "tool_name": "Bash", "tool_input": { "command": "npm test" } }

The tool_name and tool_input fields are event-specific. Each hook event section documents the additional fields for that event.

Exit code output

The exit code from your hook command tells Claude Code whether the action should proceed, be blocked, or be ignored.

Exit 0 means success. Claude Code parses stdout for JSON output fields. JSON output is only processed on exit 0. For most events, stdout is only shown in verbose mode (Ctrl+O ). The exceptions are UserPromptSubmit and SessionStart , where stdout is added as context that Claude can see and act on.

Exit 2 means a blocking error. Claude Code ignores stdout and any JSON in it. Instead, stderr text is fed back to Claude as an error message. The effect depends on the event: PreToolUse blocks the tool call, UserPromptSubmit rejects the prompt, and so on. See exit code 2 behavior for the full list.

Any other exit code is a non-blocking error. stderr is shown in verbose mode (Ctrl+O ) and execution continues.

For example, a hook command script that blocks dangerous Bash commands:

#!/bin/bash

Reads JSON input from stdin, checks the command

command=$(jq -r '.tool_input.command' < /dev/stdin)

if [[ "$command" == rm* ]]; then echo "Blocked: rm commands are not allowed" >&2 exit 2 # Blocking error: tool call is prevented fi

exit 0 # Success: tool call proceeds

Exit code 2 behavior per event

Exit code 2 is the way a hook signals "stop, don't do this." The effect depends on the event, because some events represent actions that can be blocked (like a tool call that hasn't happened yet) and others represent things that already happened or can't be prevented.

Hook event Can block? What happens on exit 2

PreToolUse

Yes Blocks the tool call

PermissionRequest

Yes Denies the permission

UserPromptSubmit

Yes Blocks prompt processing and erases the prompt

Stop

Yes Prevents Claude from stopping, continues the conversation

SubagentStop

Yes Prevents the subagent from stopping

PostToolUse

No Shows stderr to Claude (tool already ran)

PostToolUseFailure

No Shows stderr to Claude (tool already failed)

Notification

No Shows stderr to user only

SubagentStart

No Shows stderr to user only

SessionStart

No Shows stderr to user only

SessionEnd

No Shows stderr to user only

PreCompact

No Shows stderr to user only

JSON output

Exit codes let you allow or block, but JSON output gives you finer-grained control. Instead of exiting with code 2 to block, exit 0 and print a JSON object to stdout. Claude Code reads specific fields from that JSON to control behavior, including decision control for blocking, allowing, or escalating to the user.

Your hook's stdout must contain only the JSON object. If your shell profile prints text on startup, it can interfere with JSON parsing. See JSON validation failed in the troubleshooting guide.

The JSON object supports three kinds of fields:

  • Universal fields like continue work across all events. These are listed in the table below.

  • Top-level decision and reason are used by some events to block or provide feedback.

  • hookSpecificOutput is a nested object for events that need richer control. It requires a hookEventName field set to the event name.

Field Default Description

continue

true

If false , Claude stops processing entirely after the hook runs. Takes precedence over any event-specific decision fields

stopReason

none Message shown to the user when continue is false . Not shown to Claude

suppressOutput

false

If true , hides stdout from verbose mode output

systemMessage

none Warning message shown to the user

To stop Claude entirely regardless of event type:

{ "continue": false, "stopReason": "Build failed, fix errors before continuing" }

Decision control

Not every event supports blocking or controlling behavior through JSON. The events that do each use a different set of fields to express that decision. Use this table as a quick reference before writing a hook:

Events Decision pattern Key fields

UserPromptSubmit, PostToolUse, PostToolUseFailure, Stop, SubagentStop Top-level decision

decision: "block" , reason

PreToolUse hookSpecificOutput

permissionDecision (allow/deny/ask), permissionDecisionReason

PermissionRequest hookSpecificOutput

decision.behavior (allow/deny)

Here are examples of each pattern in action:

{
  "decision": "block",
  "reason": "Test suite must pass before proceeding"
}

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "deny",
    "permissionDecisionReason": "Database writes are not allowed"
  }
}

{
  "hookSpecificOutput": {
    "hookEventName": "PermissionRequest",
    "decision": {
      "behavior": "allow",
      "updatedInput": {
        "command": "npm run lint"
      }
    }
  }
}

For extended examples including Bash command validation, prompt filtering, and auto-approval scripts, see What you can automate in the guide and the Bash command validator reference implementation.

Hook events

Each event corresponds to a point in Claude Code's lifecycle where hooks can run. The sections below are ordered to match the lifecycle: from session setup through the agentic loop to session end. Each section describes when the event fires, what matchers it supports, the JSON input it receives, and how to control behavior through output.

SessionStart

Runs when Claude Code starts a new session or resumes an existing session. Useful for loading development context like existing issues or recent changes to your codebase, or setting up environment variables. For static context that does not require a script, use CLAUDE.md instead.

SessionStart runs on every session, so keep these hooks fast.

The matcher value corresponds to how the session was initiated:

Matcher When it fires

startup

New session

resume

--resume , --continue , or /resume

clear

/clear

compact

Auto or manual compaction

SessionStart input

In addition to the common input fields, SessionStart hooks receive source , model , and optionally agent_type . The source field indicates how the session started: "startup" for new sessions, "resume" for resumed sessions, "clear" after /clear , or "compact" after compaction. The model field contains the model identifier. If you start Claude Code with claude --agent <name> , an agent_type field contains the agent name.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "SessionStart", "source": "startup", "model": "claude-sonnet-4-5-20250929" }

SessionStart decision control

Any text your hook script prints to stdout is added as context for Claude. In addition to the JSON output fields available to all hooks, you can return these event-specific fields:

Field Description

additionalContext

String added to Claude's context. Multiple hooks' values are concatenated

{ "hookSpecificOutput": { "hookEventName": "SessionStart", "additionalContext": "My additional context here" } }

Persist environment variables

SessionStart hooks have access to the CLAUDE_ENV_FILE environment variable, which provides a file path where you can persist environment variables for subsequent Bash commands.

To set individual environment variables, write export statements to CLAUDE_ENV_FILE . Use append (>> ) to preserve variables set by other hooks:

#!/bin/bash

if [ -n "$CLAUDE_ENV_FILE" ]; then echo 'export NODE_ENV=production' >> "$CLAUDE_ENV_FILE" echo 'export DEBUG_LOG=true' >> "$CLAUDE_ENV_FILE" echo 'export PATH="$PATH:./node_modules/.bin"' >> "$CLAUDE_ENV_FILE" fi

exit 0

To capture all environment changes from setup commands, compare the exported variables before and after:

#!/bin/bash

ENV_BEFORE=$(export -p | sort)

Run your setup commands that modify the environment

source ~/.nvm/nvm.sh nvm use 20

if [ -n "$CLAUDE_ENV_FILE" ]; then ENV_AFTER=$(export -p | sort) comm -13 <(echo "$ENV_BEFORE") <(echo "$ENV_AFTER") >> "$CLAUDE_ENV_FILE" fi

exit 0

Any variables written to this file will be available in all subsequent Bash commands that Claude Code executes during the session.

UserPromptSubmit

Runs when the user submits a prompt, before Claude processes it. This allows you to add additional context based on the prompt/conversation, validate prompts, or block certain types of prompts.

UserPromptSubmit input

In addition to the common input fields, UserPromptSubmit hooks receive the prompt field containing the text the user submitted.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "UserPromptSubmit", "prompt": "Write a function to calculate the factorial of a number" }

UserPromptSubmit decision control

UserPromptSubmit hooks can control whether a user prompt is processed and add context. All JSON output fields are available.

There are two ways to add context to the conversation on exit code 0:

  • Plain text stdout: any non-JSON text written to stdout is added as context

  • JSON with additionalContext : use the JSON format below for more control. The additionalContext field is added as context

Plain stdout is shown as hook output in the transcript. The additionalContext field is added more discretely.

To block a prompt, return a JSON object with decision set to "block" :

Field Description

decision

"block" prevents the prompt from being processed and erases it from context. Omit to allow the prompt to proceed

reason

Shown to the user when decision is "block" . Not added to context

additionalContext

String added to Claude's context

{ "decision": "block", "reason": "Explanation for decision", "hookSpecificOutput": { "hookEventName": "UserPromptSubmit", "additionalContext": "My additional context here" } }

PreToolUse

Runs after Claude creates tool parameters and before processing the tool call. Matches on tool name: Bash , Edit , Write , Read , Glob , Grep , Task , WebFetch , WebSearch , and any MCP tool names.

Use PreToolUse decision control to allow, deny, or ask for permission to use the tool.

PreToolUse input

In addition to the common input fields, PreToolUse hooks receive tool_name , tool_input , and tool_use_id . The tool_input fields depend on the tool:

Bash

Executes shell commands.

Field Type Example Description

command

string "npm test"

The shell command to execute

description

string "Run test suite"

Optional description of what the command does

timeout

number 120000

Optional timeout in milliseconds

run_in_background

boolean false

Whether to run the command in background

Write

Creates or overwrites a file.

Field Type Example Description

file_path

string "/path/to/file.txt"

Absolute path to the file to write

content

string "file content"

Content to write to the file

Edit

Replaces a string in an existing file.

Field Type Example Description

file_path

string "/path/to/file.txt"

Absolute path to the file to edit

old_string

string "original text"

Text to find and replace

new_string

string "replacement text"

Replacement text

replace_all

boolean false

Whether to replace all occurrences

Read

Reads file contents.

Field Type Example Description

file_path

string "/path/to/file.txt"

Absolute path to the file to read

offset

number 10

Optional line number to start reading from

limit

number 50

Optional number of lines to read

Glob

Finds files matching a glob pattern.

Field Type Example Description

pattern

string "**/*.ts"

Glob pattern to match files against

path

string "/path/to/dir"

Optional directory to search in. Defaults to current working directory

Grep

Searches file contents with regular expressions.

Field Type Example Description

pattern

string "TODO.*fix"

Regular expression pattern to search for

path

string "/path/to/dir"

Optional file or directory to search in

glob

string "*.ts"

Optional glob pattern to filter files

output_mode

string "content"

"content" , "files_with_matches" , or "count" . Defaults to "files_with_matches"

-i

boolean true

Case insensitive search

multiline

boolean false

Enable multiline matching

WebFetch

Fetches and processes web content.

Field Type Example Description

url

string "https://example.com/api"

URL to fetch content from

prompt

string "Extract the API endpoints"

Prompt to run on the fetched content

WebSearch

Searches the web.

Field Type Example Description

query

string "react hooks best practices"

Search query

allowed_domains

array ["docs.example.com"]

Optional: only include results from these domains

blocked_domains

array ["spam.example.com"]

Optional: exclude results from these domains

Task

Spawns a subagent.

Field Type Example Description

prompt

string "Find all API endpoints"

The task for the agent to perform

description

string "Find API endpoints"

Short description of the task

subagent_type

string "Explore"

Type of specialized agent to use

model

string "sonnet"

Optional model alias to override the default

PreToolUse decision control

PreToolUse hooks can control whether a tool call proceeds. Unlike other hooks that use a top-level decision field, PreToolUse returns its decision inside a hookSpecificOutput object. This gives it richer control: three outcomes (allow, deny, or ask) plus the ability to modify tool input before execution.

Field Description

permissionDecision

"allow" bypasses the permission system, "deny" prevents the tool call, "ask" prompts the user to confirm

permissionDecisionReason

For "allow" and "ask" , shown to the user but not Claude. For "deny" , shown to Claude

updatedInput

Modifies the tool's input parameters before execution. Combine with "allow" to auto-approve, or "ask" to show the modified input to the user

additionalContext

String added to Claude's context before the tool executes

{ "hookSpecificOutput": { "hookEventName": "PreToolUse", "permissionDecision": "allow", "permissionDecisionReason": "My reason here", "updatedInput": { "field_to_modify": "new value" }, "additionalContext": "Current environment: production. Proceed with caution." } }

PermissionRequest

Runs when the user is shown a permission dialog. Use PermissionRequest decision control to allow or deny on behalf of the user.

Matches on tool name, same values as PreToolUse.

PermissionRequest input

PermissionRequest hooks receive tool_name and tool_input fields like PreToolUse hooks, but without tool_use_id . An optional permission_suggestions array contains the "always allow" options the user would normally see in the permission dialog. The difference is when the hook fires: PermissionRequest hooks run when a permission dialog is about to be shown to the user, while PreToolUse hooks run before tool execution regardless of permission status.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "PermissionRequest", "tool_name": "Bash", "tool_input": { "command": "rm -rf node_modules", "description": "Remove node_modules directory" }, "permission_suggestions": [ { "type": "toolAlwaysAllow", "tool": "Bash" } ] }

PermissionRequest decision control

PermissionRequest hooks can allow or deny permission requests. In addition to the JSON output fields available to all hooks, your hook script can return a decision object with these event-specific fields:

Field Description

behavior

"allow" grants the permission, "deny" denies it

updatedInput

For "allow" only: modifies the tool's input parameters before execution

updatedPermissions

For "allow" only: applies permission rule updates, equivalent to the user selecting an "always allow" option

message

For "deny" only: tells Claude why the permission was denied

interrupt

For "deny" only: if true , stops Claude

{ "hookSpecificOutput": { "hookEventName": "PermissionRequest", "decision": { "behavior": "allow", "updatedInput": { "command": "npm run lint" } } } }

PostToolUse

Runs immediately after a tool completes successfully.

Matches on tool name, same values as PreToolUse.

PostToolUse input

PostToolUse hooks fire after a tool has already executed successfully. The input includes both tool_input , the arguments sent to the tool, and tool_response , the result it returned. The exact schema for both depends on the tool.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "PostToolUse", "tool_name": "Write", "tool_input": { "file_path": "/path/to/file.txt", "content": "file content" }, "tool_response": { "filePath": "/path/to/file.txt", "success": true }, "tool_use_id": "toolu_01ABC123..." }

PostToolUse decision control

PostToolUse hooks can provide feedback to Claude after tool execution. In addition to the JSON output fields available to all hooks, your hook script can return these event-specific fields:

Field Description

decision

"block" prompts Claude with the reason . Omit to allow the action to proceed

reason

Explanation shown to Claude when decision is "block"

additionalContext

Additional context for Claude to consider

updatedMCPToolOutput

For MCP tools only: replaces the tool's output with the provided value

{ "decision": "block", "reason": "Explanation for decision", "hookSpecificOutput": { "hookEventName": "PostToolUse", "additionalContext": "Additional information for Claude" } }

PostToolUseFailure

Runs when a tool execution fails. This event fires for tool calls that throw errors or return failure results. Use this to log failures, send alerts, or provide corrective feedback to Claude.

Matches on tool name, same values as PreToolUse.

PostToolUseFailure input

PostToolUseFailure hooks receive the same tool_name and tool_input fields as PostToolUse, along with error information as top-level fields:

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "PostToolUseFailure", "tool_name": "Bash", "tool_input": { "command": "npm test", "description": "Run test suite" }, "tool_use_id": "toolu_01ABC123...", "error": "Command exited with non-zero status code 1", "is_interrupt": false }

Field Description

error

String describing what went wrong

is_interrupt

Optional boolean indicating whether the failure was caused by user interruption

PostToolUseFailure decision control

PostToolUseFailure hooks can provide context to Claude after a tool failure. In addition to the JSON output fields available to all hooks, your hook script can return these event-specific fields:

Field Description

additionalContext

Additional context for Claude to consider alongside the error

{ "hookSpecificOutput": { "hookEventName": "PostToolUseFailure", "additionalContext": "Additional information about the failure for Claude" } }

Notification

Runs when Claude Code sends notifications. Matches on notification type: permission_prompt , idle_prompt , auth_success , elicitation_dialog . Omit the matcher to run hooks for all notification types.

Use separate matchers to run different handlers depending on the notification type. This configuration triggers a permission-specific alert script when Claude needs permission approval and a different notification when Claude has been idle:

{ "hooks": { "Notification": [ { "matcher": "permission_prompt", "hooks": [ { "type": "command", "command": "/path/to/permission-alert.sh" } ] }, { "matcher": "idle_prompt", "hooks": [ { "type": "command", "command": "/path/to/idle-notification.sh" } ] } ] } }

Notification input

In addition to the common input fields, Notification hooks receive message with the notification text, an optional title , and notification_type indicating which type fired.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "Notification", "message": "Claude needs your permission to use Bash", "title": "Permission needed", "notification_type": "permission_prompt" }

Notification hooks cannot block or modify notifications. In addition to the JSON output fields available to all hooks, you can return additionalContext to add context to the conversation:

Field Description

additionalContext

String added to Claude's context

SubagentStart

Runs when a Claude Code subagent is spawned via the Task tool. Supports matchers to filter by agent type name (built-in agents like Bash , Explore , Plan , or custom agent names from .claude/agents/ ).

SubagentStart input

In addition to the common input fields, SubagentStart hooks receive agent_id with the unique identifier for the subagent and agent_type with the agent name (built-in agents like "Bash" , "Explore" , "Plan" , or custom agent names).

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "SubagentStart", "agent_id": "agent-abc123", "agent_type": "Explore" }

SubagentStart hooks cannot block subagent creation, but they can inject context into the subagent. In addition to the JSON output fields available to all hooks, you can return:

Field Description

additionalContext

String added to the subagent's context

{ "hookSpecificOutput": { "hookEventName": "SubagentStart", "additionalContext": "Follow security guidelines for this task" } }

SubagentStop

Runs when a Claude Code subagent has finished responding. Matches on agent type, same values as SubagentStart.

SubagentStop input

In addition to the common input fields, SubagentStop hooks receive stop_hook_active , agent_id , agent_type , and agent_transcript_path . The agent_type field is the value used for matcher filtering. The transcript_path is the main session's transcript, while agent_transcript_path is the subagent's own transcript stored in a nested subagents/ folder.

{ "session_id": "abc123", "transcript_path": "/.claude/projects/.../abc123.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "SubagentStop", "stop_hook_active": false, "agent_id": "def456", "agent_type": "Explore", "agent_transcript_path": "/.claude/projects/.../abc123/subagents/agent-def456.jsonl" }

SubagentStop hooks use the same decision control format as Stop hooks.

Stop

Runs when the main Claude Code agent has finished responding. Does not run if the stoppage occurred due to a user interrupt.

Stop input

In addition to the common input fields, Stop hooks receive stop_hook_active . This field is true when Claude Code is already continuing as a result of a stop hook. Check this value or process the transcript to prevent Claude Code from running indefinitely.

{ "session_id": "abc123", "transcript_path": "~/.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "Stop", "stop_hook_active": true }

Stop decision control

Stop and SubagentStop hooks can control whether Claude continues. In addition to the JSON output fields available to all hooks, your hook script can return these event-specific fields:

Field Description

decision

"block" prevents Claude from stopping. Omit to allow Claude to stop

reason

Required when decision is "block" . Tells Claude why it should continue

{ "decision": "block", "reason": "Must be provided when Claude is blocked from stopping" }

PreCompact

Runs before Claude Code is about to run a compact operation.

The matcher value indicates whether compaction was triggered manually or automatically:

Matcher When it fires

manual

/compact

auto

Auto-compact when the context window is full

PreCompact input

In addition to the common input fields, PreCompact hooks receive trigger and custom_instructions . For manual , custom_instructions contains what the user passes into /compact . For auto , custom_instructions is empty.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "PreCompact", "trigger": "manual", "custom_instructions": "" }

SessionEnd

Runs when a Claude Code session ends. Useful for cleanup tasks, logging session statistics, or saving session state. Supports matchers to filter by exit reason.

The reason field in the hook input indicates why the session ended:

Reason Description

clear

Session cleared with /clear command

logout

User logged out

prompt_input_exit

User exited while prompt input was visible

bypass_permissions_disabled

Bypass permissions mode was disabled

other

Other exit reasons

SessionEnd input

In addition to the common input fields, SessionEnd hooks receive a reason field indicating why the session ended. See the reason table above for all values.

{ "session_id": "abc123", "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl", "cwd": "/Users/...", "permission_mode": "default", "hook_event_name": "SessionEnd", "reason": "other" }

SessionEnd hooks have no decision control. They cannot block session termination but can perform cleanup tasks.

Prompt-based hooks

In addition to Bash command hooks (type: "command" ), Claude Code supports prompt-based hooks (type: "prompt" ) that use an LLM to evaluate whether to allow or block an action. Prompt-based hooks work with the following events: PreToolUse , PostToolUse , PostToolUseFailure , PermissionRequest , UserPromptSubmit , Stop , and SubagentStop .

How prompt-based hooks work

Instead of executing a Bash command, prompt-based hooks:

  • Send the hook input and your prompt to a Claude model, Haiku by default

  • The LLM responds with structured JSON containing a decision

  • Claude Code processes the decision automatically

Prompt hook configuration

Set type to "prompt" and provide a prompt string instead of a command . Use the $ARGUMENTS placeholder to inject the hook's JSON input data into your prompt text. Claude Code sends the combined prompt and input to a fast Claude model, which returns a JSON decision.

This Stop hook asks the LLM to evaluate whether all tasks are complete before allowing Claude to finish:

{ "hooks": { "Stop": [ { "hooks": [ { "type": "prompt", "prompt": "Evaluate if Claude should stop: $ARGUMENTS. Check if all tasks are complete." } ] } ] } }

Field Required Description

type

yes Must be "prompt"

prompt

yes The prompt text to send to the LLM. Use $ARGUMENTS as a placeholder for the hook input JSON. If $ARGUMENTS is not present, input JSON is appended to the prompt

model

no Model to use for evaluation. Defaults to a fast model

timeout

no Timeout in seconds. Default: 30

Response schema

The LLM must respond with JSON containing:

{ "ok": true | false, "reason": "Explanation for the decision" }

Field Description

ok

true allows the action, false prevents it

reason

Required when ok is false . Explanation shown to Claude

Example: Multi-criteria Stop hook

This Stop hook uses a detailed prompt to check three conditions before allowing Claude to stop. If "ok" is false , Claude continues working with the provided reason as its next instruction. SubagentStop hooks use the same format to evaluate whether a subagent should stop:

{ "hooks": { "Stop": [ { "hooks": [ { "type": "prompt", "prompt": "You are evaluating whether Claude should stop working. Context: $ARGUMENTS\n\nAnalyze the conversation and determine if:\n1. All user-requested tasks are complete\n2. Any errors need to be addressed\n3. Follow-up work is needed\n\nRespond with JSON: {"ok": true} to allow stopping, or {"ok": false, "reason": "your explanation"} to continue working.", "timeout": 30 } ] } ] } }

Agent-based hooks

Agent-based hooks (type: "agent" ) are like prompt-based hooks but with multi-turn tool access. Instead of a single LLM call, an agent hook spawns a subagent that can read files, search code, and inspect the codebase to verify conditions. Agent hooks support the same events as prompt-based hooks.

How agent hooks work

When an agent hook fires:

  • Claude Code spawns a subagent with your prompt and the hook's JSON input

  • The subagent can use tools like Read, Grep, and Glob to investigate

  • After up to 50 turns, the subagent returns a structured { "ok": true/false } decision

  • Claude Code processes the decision the same way as a prompt hook

Agent hooks are useful when verification requires inspecting actual files or test output, not just evaluating the hook input data alone.

Agent hook configuration

Set type to "agent" and provide a prompt string. The configuration fields are the same as prompt hooks, with a longer default timeout:

Field Required Description

type

yes Must be "agent"

prompt

yes Prompt describing what to verify. Use $ARGUMENTS as a placeholder for the hook input JSON

model

no Model to use. Defaults to a fast model

timeout

no Timeout in seconds. Default: 60

The response schema is the same as prompt hooks: { "ok": true } to allow or { "ok": false, "reason": "..." } to block.

This Stop hook verifies that all unit tests pass before allowing Claude to finish:

{ "hooks": { "Stop": [ { "hooks": [ { "type": "agent", "prompt": "Verify that all unit tests pass. Run the test suite and check the results. $ARGUMENTS", "timeout": 120 } ] } ] } }

Run hooks in the background

By default, hooks block Claude's execution until they complete. For long-running tasks like deployments, test suites, or external API calls, set "async": true to run the hook in the background while Claude continues working. Async hooks cannot block or control Claude's behavior: response fields like decision , permissionDecision , and continue have no effect, because the action they would have controlled has already completed.

Configure an async hook

Add "async": true to a command hook's configuration to run it in the background without blocking Claude. This field is only available on type: "command" hooks.

This hook runs a test script after every Write tool call. Claude continues working immediately while run-tests.sh executes for up to 120 seconds. When the script finishes, its output is delivered on the next conversation turn:

{ "hooks": { "PostToolUse": [ { "matcher": "Write", "hooks": [ { "type": "command", "command": "/path/to/run-tests.sh", "async": true, "timeout": 120 } ] } ] } }

The timeout field sets the maximum time in seconds for the background process. If not specified, async hooks use the same 10-minute default as sync hooks.

How async hooks execute

When an async hook fires, Claude Code starts the hook process and immediately continues without waiting for it to finish. The hook receives the same JSON input via stdin as a synchronous hook.

After the background process exits, if the hook produced a JSON response with a systemMessage or additionalContext field, that content is delivered to Claude as context on the next conversation turn.

Example: run tests after file changes

This hook starts a test suite in the background whenever Claude writes a file, then reports the results back to Claude when the tests finish. Save this script to .claude/hooks/run-tests-async.sh in your project and make it executable with chmod +x :

#!/bin/bash

run-tests-async.sh

Read hook input from stdin

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

Only run tests for source files

if [[ "$FILE_PATH" != *.ts && "$FILE_PATH" != *.js ]]; then exit 0 fi

Run tests and report results via systemMessage

RESULT=$(npm test 2>&1) EXIT_CODE=$?

if [ $EXIT_CODE -eq 0 ]; then echo "{"systemMessage": "Tests passed after editing $FILE_PATH"}" else echo "{"systemMessage": "Tests failed after editing $FILE_PATH: $RESULT"}" fi

Then add this configuration to .claude/settings.json in your project root. The async: true flag lets Claude keep working while tests run:

{ "hooks": { "PostToolUse": [ { "matcher": "Write|Edit", "hooks": [ { "type": "command", "command": ""$CLAUDE_PROJECT_DIR"/.claude/hooks/run-tests-async.sh", "async": true, "timeout": 300 } ] } ] } }

Limitations

Async hooks have several constraints compared to synchronous hooks:

  • Only type: "command" hooks support async . Prompt-based hooks cannot run asynchronously.

  • Async hooks cannot block tool calls or return decisions. By the time the hook completes, the triggering action has already proceeded.

  • Hook output is delivered on the next conversation turn. If the session is idle, the response waits until the next user interaction.

  • Each execution creates a separate background process. There is no deduplication across multiple firings of the same async hook.

Security considerations

Disclaimer

Hooks run with your system user's full permissions.

Security best practices

Keep these practices in mind when writing hooks:

  • Validate and sanitize inputs: never trust input data blindly

  • Always quote shell variables: use "$VAR" not $VAR

  • Block path traversal: check for .. in file paths

  • Use absolute paths: specify full paths for scripts, using "$CLAUDE_PROJECT_DIR" for the project root

  • Skip sensitive files: avoid .env , .git/ , keys, etc.

Debug hooks

Run claude --debug to see hook execution details, including which hooks matched, their exit codes, and output. Toggle verbose mode with Ctrl+O to see hook progress in the transcript.

[DEBUG] Executing hooks for PostToolUse:Write [DEBUG] Getting matching hook commands for PostToolUse with query: Write [DEBUG] Found 1 hook matchers in settings [DEBUG] Matched 1 hooks for query "Write" [DEBUG] Found 1 hook commands to execute [DEBUG] Executing hook command: <Your command> with timeout 600000ms [DEBUG] Hook command completed with status 0: <Your stdout>

For troubleshooting common issues like hooks not firing, infinite Stop hook loops, or configuration errors, see Limitations and troubleshooting in the guide.

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