# Hooks reference

This document provides the technical specification for Gemini CLI hooks,
including JSON schemas and API details.

## Global hook mechanics

- **Communication**: `stdin` for Input (JSON), `stdout` for Output (JSON), and
  `stderr` for logs and feedback.
- **Exit codes**:
  - `0`: Success. `stdout` is parsed as JSON. **Preferred for all logic.**
  - `2`: System Block. The action is blocked; `stderr` is used as the rejection
    reason.
  - `Other`: Warning. A non-fatal failure occurred; the CLI continues with a
    warning.
- **Silence is Mandatory**: Your script **must not** print any plain text to
  `stdout` other than the final JSON.

---

## Configuration schema

Hooks are defined in `settings.json` within the `hooks` object. Each event (for
example, `BeforeTool`) contains an array of **hook definitions**.

### Hook definition

| Field        | Type      | Required | Description                                                                             |
| :----------- | :-------- | :------- | :-------------------------------------------------------------------------------------- |
| `matcher`    | `string`  | No       | A regex (for tools) or exact string (for lifecycle) to filter when the hook runs.       |
| `sequential` | `boolean` | No       | If `true`, hooks in this group run one after another. If `false`, they run in parallel. |
| `hooks`      | `array`   | **Yes**  | An array of **hook configurations**.                                                    |

### Hook configuration

| Field         | Type     | Required  | Description                                                          |
| :------------ | :------- | :-------- | :------------------------------------------------------------------- |
| `type`        | `string` | **Yes**   | The execution engine. Currently only `"command"` is supported.       |
| `command`     | `string` | **Yes\*** | The shell command to execute. (Required when `type` is `"command"`). |
| `name`        | `string` | No        | A friendly name for identifying the hook in logs and CLI commands.   |
| `timeout`     | `number` | No        | Execution timeout in milliseconds (default: 60000).                  |
| `description` | `string` | No        | A brief explanation of the hook's purpose.                           |

---

## Base input schema

All hooks receive these common fields via `stdin`:

```typescript
{
  "session_id": string,      // Unique ID for the current session
  "transcript_path": string, // Absolute path to session transcript JSON
  "cwd": string,             // Current working directory
  "hook_event_name": string, // The firing event (for example "BeforeTool")
  "timestamp": string        // ISO 8601 execution time
}
```

---

## Common output fields

Most hooks support these fields in their `stdout` JSON:

| Field            | Type      | Description                                                                    |
| :--------------- | :-------- | :----------------------------------------------------------------------------- |
| `systemMessage`  | `string`  | Displayed immediately to the user in the terminal.                             |
| `suppressOutput` | `boolean` | If `true`, hides internal hook metadata from logs/telemetry.                   |
| `continue`       | `boolean` | If `false`, stops the entire agent loop immediately.                           |
| `stopReason`     | `string`  | Displayed to the user when `continue` is `false`.                              |
| `decision`       | `string`  | `"allow"` or `"deny"` (alias `"block"`). Specific impact depends on the event. |
| `reason`         | `string`  | The feedback/error message provided when a `decision` is `"deny"`.             |

---

## Tool hooks

### Matchers and tool names

For `BeforeTool` and `AfterTool` events, the `matcher` field in your settings is
compared against the name of the tool being executed.

- **Built-in Tools**: You can match any built-in tool (for example, `read_file`,
  `run_shell_command`). See the [Tools Reference](../reference/tools) for a full
  list of available tool names.
- **MCP Tools**: Tools from MCP servers follow the naming pattern
  `mcp_<server_name>_<tool_name>`.
- **Regex Support**: Matchers support regular expressions (for example,
  `matcher: "read_.*"` matches all file reading tools).

### `BeforeTool`

Fires before a tool is invoked. Used for argument validation, security checks,
and parameter rewriting.

- **Input Fields**:
  - `tool_name`: (`string`) The name of the tool being called.
  - `tool_input`: (`object`) The raw arguments generated by the model.
  - `mcp_context`: (`object`) Optional metadata for MCP-based tools.
  - `original_request_name`: (`string`) The original name of the tool being
    called, if this is a tail tool call.
- **Relevant Output Fields**:
  - `decision`: Set to `"deny"` (or `"block"`) to prevent the tool from
    executing.
  - `reason`: Required if denied. This text is sent **to the agent** as a tool
    error, allowing it to respond or retry.
  - `hookSpecificOutput.tool_input`: An object that **merges with and
    overrides** the model's arguments before execution.
  - `continue`: Set to `false` to **kill the entire agent loop** immediately.
- **Exit Code 2 (Block Tool)**: Prevents execution. Uses `stderr` as the
  `reason` sent to the agent. **The turn continues.**

### `AfterTool`

Fires after a tool executes. Used for result auditing, context injection, or
hiding sensitive output from the agent.

- **Input Fields**:
  - `tool_name`: (`string`)
  - `tool_input`: (`object`) The original arguments.
  - `tool_response`: (`object`) The result containing `llmContent`,
    `returnDisplay`, and optional `error`.
  - `mcp_context`: (`object`)
  - `original_request_name`: (`string`) The original name of the tool being
    called, if this is a tail tool call.
- **Relevant Output Fields**:
  - `decision`: Set to `"deny"` to hide the real tool output from the agent.
  - `reason`: Required if denied. This text **replaces** the tool result sent
    back to the model.
  - `hookSpecificOutput.additionalContext`: Text that is **appended** to the
    tool result for the agent.
  - `hookSpecificOutput.tailToolCallRequest`: (`{ name: string, args: object }`)
    A request to execute another tool immediately after this one. The result of
    this "tail call" will replace the original tool's response. Ideal for
    programmatic tool routing.
  - `continue`: Set to `false` to **kill the entire agent loop** immediately.
- **Exit Code 2 (Block Result)**: Hides the tool result. Uses `stderr` as the
  replacement content sent to the agent. **The turn continues.**

---

## Agent hooks

### `BeforeAgent`

Fires after a user submits a prompt, but before the agent begins planning. Used
for prompt validation or injecting dynamic context.

- **Input Fields**:
  - `prompt`: (`string`) The original text submitted by the user.
- **Relevant Output Fields**:
  - `hookSpecificOutput.additionalContext`: Text that is **appended** to the
    prompt for this turn only.
  - `decision`: Set to `"deny"` to block the turn and **discard the user's
    message** (it will not appear in history).
  - `continue`: Set to `false` to block the turn but **save the message to
    history**.
  - `reason`: Required if denied or stopped.
- **Exit Code 2 (Block Turn)**: Aborts the turn and erases the prompt from
  context. Same as `decision: "deny"`.

### `AfterAgent`

Fires once per turn after the model generates its final response. Primary use
case is response validation and automatic retries.

- **Input Fields**:
  - `prompt`: (`string`) The user's original request.
  - `prompt_response`: (`string`) The final text generated by the agent.
  - `stop_hook_active`: (`boolean`) Indicates if this hook is already running as
    part of a retry sequence.
- **Relevant Output Fields**:
  - `decision`: Set to `"deny"` to **reject the response** and force a retry.
  - `reason`: Required if denied. This text is sent **to the agent as a new
    prompt** to request a correction.
  - `continue`: Set to `false` to **stop the session** without retrying.
  - `hookSpecificOutput.clearContext`: If `true`, clears conversation history
    (LLM memory) while preserving UI display.
- **Exit Code 2 (Retry)**: Rejects the response and triggers an automatic retry
  turn using `stderr` as the feedback prompt.

---

## Model hooks

### `BeforeModel`

Fires before sending a request to the LLM. Operates on a stable, SDK-agnostic
request format.

- **Input Fields**:
  - `llm_request`: (`object`) Contains `model`, `messages`, and `config`
    (generation params).
- **Relevant Output Fields**:
  - `hookSpecificOutput.llm_request`: An object that **overrides** parts of the
    outgoing request (for example, changing models or temperature).
  - `hookSpecificOutput.llm_response`: A **Synthetic Response** object. If
    provided, the CLI skips the LLM call entirely and uses this as the response.
  - `decision`: Set to `"deny"` to block the request and abort the turn.
- **Exit Code 2 (Block Turn)**: Aborts the turn and skips the LLM call. Uses
  `stderr` as the error message.

### `BeforeToolSelection`

Fires before the LLM decides which tools to call. Used to filter the available
toolset or force specific tool modes.

- **Input Fields**:
  - `llm_request`: (`object`) Same format as `BeforeModel`.
- **Relevant Output Fields**:
  - `hookSpecificOutput.toolConfig.mode`: (`"AUTO" | "ANY" | "NONE"`)
    - `"NONE"`: Disables all tools (Wins over other hooks).
    - `"ANY"`: Forces at least one tool call.
  - `hookSpecificOutput.toolConfig.allowedFunctionNames`: (`string[]`) Whitelist
    of tool names.
- **Union Strategy**: Multiple hooks' whitelists are **combined**.
- **Limitations**: Does **not** support `decision`, `continue`, or
  `systemMessage`.

### `AfterModel`

Fires immediately after an LLM response chunk is received. Used for real-time
redaction or PII filtering.

- **Input Fields**:
  - `llm_request`: (`object`) The original request.
  - `llm_response`: (`object`) The model's response (or a single chunk during
    streaming).
- **Relevant Output Fields**:
  - `hookSpecificOutput.llm_response`: An object that **replaces** the model's
    response chunk.
  - `decision`: Set to `"deny"` to discard the response chunk and block the
    turn.
  - `continue`: Set to `false` to **kill the entire agent loop** immediately.
- **Note on Streaming**: Fired for **every chunk** generated by the model.
  Modifying the response only affects the current chunk.
- **Exit Code 2 (Block Response)**: Aborts the turn and discards the model's
  output. Uses `stderr` as the error message.

---

## Lifecycle & system hooks

### `SessionStart`

Fires on application startup, resuming a session, or after a `/clear` command.
Used for loading initial context.

- **Input fields**:
  - `source`: (`"startup" | "resume" | "clear"`)
- **Relevant output fields**:
  - `hookSpecificOutput.additionalContext`: (`string`)
    - **Interactive**: Injected as the first turn in history.
    - **Non-interactive**: Prepended to the user's prompt.
  - `systemMessage`: Shown at the start of the session.
- **Advisory only**: `continue` and `decision` fields are **ignored**. Startup
  is never blocked.

### `SessionEnd`

Fires when the CLI exits or a session is cleared. Used for cleanup or final
telemetry.

- **Input Fields**:
  - `reason`: (`"exit" | "clear" | "logout" | "prompt_input_exit" | "other"`)
- **Relevant Output Fields**:
  - `systemMessage`: Displayed to the user during shutdown.
- **Best Effort**: The CLI **will not wait** for this hook to complete and
  ignores all flow-control fields (`continue`, `decision`).

### `Notification`

Fires when the CLI emits a system alert (for example, Tool Permissions). Used
for external logging or cross-platform alerts.

- **Input Fields**:
  - `notification_type`: (`"ToolPermission"`)
  - `message`: Summary of the alert.
  - `details`: JSON object with alert-specific metadata (for example, tool name,
    file path).
- **Relevant Output Fields**:
  - `systemMessage`: Displayed alongside the system alert.
- **Observability Only**: This hook **cannot** block alerts or grant permissions
  automatically. Flow-control fields are ignored.

### `PreCompress`

Fires before the CLI summarizes history to save tokens. Used for logging or
state saving.

- **Input Fields**:
  - `trigger`: (`"auto" | "manual"`)
- **Relevant Output Fields**:
  - `systemMessage`: Displayed to the user before compression.
- **Advisory Only**: Fired asynchronously. It **cannot** block or modify the
  compression process. Flow-control fields are ignored.

---

## Stable Model API

Gemini CLI uses these structures to ensure hooks don't break across SDK updates.

**LLMRequest**:

```typescript
{
  "model": string,
  "messages": Array<{
    "role": "user" | "model" | "system",
    "content": string // Non-text parts are filtered out for hooks
  }>,
  "config": { "temperature": number, ... },
  "toolConfig": { "mode": string, "allowedFunctionNames": string[] }
}

```

**LLMResponse**:

```typescript
{
  "candidates": Array<{
    "content": { "role": "model", "parts": string[] },
    "finishReason": string
  }>,
  "usageMetadata": { "totalTokenCount": number }
}
```
