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Intercept and control agent behavior with hooks

Intercept and customize agent behavior at key execution points with hooks

Hooks are callback functions that run your code in response to agent events, like a tool being called, a session starting, or execution stopping. With hooks, you can:

• Block dangerous operations before they execute, like destructive shell commands or unauthorized file access
• Log and audit every tool call for compliance, debugging, or analytics
• Transform inputs and outputs to sanitize data, inject credentials, or redirect file paths
• Require human approval for sensitive actions like database writes or API calls
• Track session lifecycle to manage state, clean up resources, or send notifications

This guide covers how hooks work, how to configure them, and provides examples for common patterns like blocking tools, modifying inputs, and forwarding notifications.

How hooks work

Something happens during agent execution and the SDK fires an event: a tool is about to be called (`PreToolUse`), a tool returned a result (`PostToolUse`), a subagent started or stopped, the agent is idle, or execution finished. See the [full list of events](#available-hooks).



The SDK checks for hooks registered for that event type. This includes callback hooks you pass in `options.hooks` and shell command hooks from settings files when the corresponding [`settingSources`](/en/agent-sdk/typescript#setting-source) or [`setting_sources`](/en/agent-sdk/python#setting-source) entry is enabled, which it is for default `query()` options.



If a hook has a [`matcher`](#matchers) pattern (like `"Write|Edit"`), the SDK tests it against the event's target (for example, the tool name). Hooks without a matcher run for every event of that type.



Each matching hook's [callback function](#callback-functions) receives input about what's happening: the tool name, its arguments, the session ID, and other event-specific details.



After performing any operations (logging, API calls, validation), your callback returns an [output object](#outputs) that tells the agent what to do: allow the operation, block it, modify the input, or inject context into the conversation.

The following example puts these steps together. It registers a PreToolUse hook (step 1) with a "Write|Edit" matcher (step 3) so the callback only fires for file-writing tools. When triggered, the callback receives the tool's input (step 4), checks if the file path targets a .env file, and returns permissionDecision: "deny" to block the operation (step 5):

from claude_agent_sdk import (
    AssistantMessage,
    ClaudeSDKClient,
    ClaudeAgentOptions,
    HookMatcher,
    ResultMessage,
)

# Define a hook callback that receives tool call details
async def protect_env_files(input_data, tool_use_id, context):
    # Extract the file path from the tool's input arguments
    file_path = input_data["tool_input"].get("file_path", "")
    file_name = file_path.split("/")[-1]

    # Block the operation if targeting a .env file
    if file_name == ".env":
        return {
            "hookSpecificOutput": {
                "hookEventName": input_data["hook_event_name"],
                "permissionDecision": "deny",
                "permissionDecisionReason": "Cannot modify .env files",
            }
        }

    # Return empty object to allow the operation
    return {}

async def main():
    options = ClaudeAgentOptions(
        hooks={
            # Register the hook for PreToolUse events
            # The matcher filters to only Write and Edit tool calls
            "PreToolUse": [HookMatcher(matcher="Write|Edit", hooks=[protect_env_files])]
        }
    )

    async with ClaudeSDKClient(options=options) as client:
        await client.query("Update the database configuration")
        async for message in client.receive_response():
            # Filter for assistant and result messages
            if isinstance(message, (AssistantMessage, ResultMessage)):
                print(message)

asyncio.run(main())

// Define a hook callback with the HookCallback type
const protectEnvFiles: HookCallback = async (input, toolUseID, { signal }) => {
  // Cast input to the specific hook type for type safety
  const preInput = input as PreToolUseHookInput;

  // Cast tool_input to access its properties (typed as unknown in the SDK)
  const toolInput = preInput.tool_input as Record<string, unknown>;
  const filePath = toolInput?.file_path as string;
  const fileName = filePath?.split("/").pop();

  // Block the operation if targeting a .env file
  if (fileName === ".env") {
    return {
      hookSpecificOutput: {
        hookEventName: preInput.hook_event_name,
        permissionDecision: "deny",
        permissionDecisionReason: "Cannot modify .env files"
      }
    };
  }

  // Return empty object to allow the operation
  return {};
};

for await (const message of query({
  prompt: "Update the database configuration",
  options: {
    hooks: {
      // Register the hook for PreToolUse events
      // The matcher filters to only Write and Edit tool calls
      PreToolUse: [{ matcher: "Write|Edit", hooks: [protectEnvFiles] }]
    }
  }
})) {
  // Filter for assistant and result messages
  if (message.type === "assistant" || message.type === "result") {
    console.log(message);
  }
}

Available hooks

The SDK provides hooks for different stages of agent execution. Some hooks are available in both SDKs, while others are TypeScript-only.

Configure hooks

To configure a hook, pass it in the hooks field of your agent options (ClaudeAgentOptions in Python, the options object in TypeScript):

options = ClaudeAgentOptions(
    hooks={"PreToolUse": [HookMatcher(matcher="Bash", hooks=[my_callback])]}
)

async with ClaudeSDKClient(options=options) as client:
    await client.query("Your prompt")
    async for message in client.receive_response():
        print(message)

for await (const message of query({
  prompt: "Your prompt",
  options: {
    hooks: {
      PreToolUse: [{ matcher: "Bash", hooks: [myCallback] }]
    }
  }
})) {
  console.log(message);
}

The hooks option is a dictionary (Python) or object (TypeScript) where:

• Keys are hook event names (e.g., 'PreToolUse', 'PostToolUse', 'Stop')
• Values are arrays of matchers, each containing an optional filter pattern and your callback functions

Matchers

Use matchers to filter when your callbacks fire. The matcher field is a regex string that matches against a different value depending on the hook event type. For example, tool-based hooks match against the tool name, while Notification hooks match against the notification type. See the Claude Code hooks reference for the full list of matcher values for each event type.

Use the matcher pattern to target specific tools whenever possible. A matcher with 'Bash' only runs for Bash commands, while omitting the pattern runs your callbacks for every occurrence of the event. Note that for tool-based hooks, matchers only filter by tool name, not by file paths or other arguments. To filter by file path, check tool_input.file_path inside your callback.

Discovering tool names: See Tool Input Types for the full list of built-in tool names, or add a hook without a matcher to log all tool calls your session makes.

MCP tool naming: MCP tools always start with mcp__ followed by the server name and action: mcp__<server>__<action>. For example, if you configure a server named playwright, its tools will be named mcp__playwright__browser_screenshot, mcp__playwright__browser_click, etc. The server name comes from the key you use in the mcpServers configuration.

Callback functions

Inputs

Every hook callback receives three arguments:

• Input data: a typed object containing event details. Each hook type has its own input shape (for example, PreToolUseHookInput includes tool_name and tool_input, while `NotificationH