(docs): Add reference section to hooks documentation (#15159)

docs/hooks/index.md

@@ -7,6 +7,9 @@ the CLI's source code.
 See [writing hooks guide](writing-hooks.md) for a tutorial on creating your
 first hook and a comprehensive example.
 
+See [hooks reference](reference.md) for the technical specification of the I/O
+schemas.
+
 See [best practices](best-practices.md) for guidelines on security, performance,
 and debugging.
 

docs/hooks/reference.md

@@ -0,0 +1,168 @@
+# Hooks Reference
+
+This document provides the technical specification for Gemini CLI hooks,
+including the JSON schemas for input and output, exit code behaviors, and the
+stable model API.
+
+## Communication Protocol
+
+Hooks communicate with Gemini CLI via standard streams and exit codes:
+
+- **Input**: Gemini CLI sends a JSON object to the hook's `stdin`.
+- **Output**: The hook sends a JSON object (or plain text) to `stdout`.
+- **Exit Codes**: Used to signal success or blocking errors.
+
+### Exit Code Behavior
+
+| Exit Code | Meaning            | Behavior                                                                                        |
+| :-------- | :----------------- | :---------------------------------------------------------------------------------------------- |
+| `0`       | **Success**        | `stdout` is parsed as JSON. If parsing fails, it's treated as a `systemMessage`.                |
+| `2`       | **Blocking Error** | Interrupts the current operation. `stderr` is shown to the agent (for tool events) or the user. |
+| Other     | **Warning**        | Execution continues. `stderr` is logged as a non-blocking warning.                              |
+
+---
+
+## Input Schema (`stdin`)
+
+Every hook receives a base JSON object. Extra fields are added depending on the
+specific event.
+
+### Base Fields (All Events)
+
+| Field             | Type     | Description                                           |
+| :---------------- | :------- | :---------------------------------------------------- |
+| `session_id`      | `string` | Unique identifier for the current CLI session.        |
+| `transcript_path` | `string` | Path to the session's JSON transcript (if available). |
+| `cwd`             | `string` | The current working directory.                        |
+| `hook_event_name` | `string` | The name of the firing event (e.g., `BeforeTool`).    |
+| `timestamp`       | `string` | ISO 8601 timestamp of the event.                      |
+
+### Event-Specific Fields
+
+#### Tool Events (`BeforeTool`, `AfterTool`)
+
+- `tool_name`: (`string`) The internal name of the tool (e.g., `write_file`,
+  `run_shell_command`).
+- `tool_input`: (`object`) The arguments passed to the tool.
+- `tool_response`: (`object`, **AfterTool only**) The raw output from the tool
+  execution.
+
+#### Agent Events (`BeforeAgent`, `AfterAgent`)
+
+- `prompt`: (`string`) The user's submitted prompt.
+- `prompt_response`: (`string`, **AfterAgent only**) The final response text
+  from the model.
+- `stop_hook_active`: (`boolean`, **AfterAgent only**) Indicates if a stop hook
+  is already handling a continuation.
+
+#### Model Events (`BeforeModel`, `AfterModel`, `BeforeToolSelection`)
+
+- `llm_request`: (`LLMRequest`) A stable representation of the outgoing request.
+  See [Stable Model API](#stable-model-api).
+- `llm_response`: (`LLMResponse`, **AfterModel only**) A stable representation
+  of the incoming response.
+
+#### Session & Notification Events
+
+- `source`: (`startup` | `resume` | `clear`, **SessionStart only**) The trigger
+  source.
+- `reason`: (`exit` | `clear` | `logout` | `prompt_input_exit` | `other`,
+  **SessionEnd only**) The reason for session end.
+- `trigger`: (`manual` | `auto`, **PreCompress only**) What triggered the
+  compression event.
+- `notification_type`: (`ToolPermission`, **Notification only**) The type of
+  notification being fired.
+- `message`: (`string`, **Notification only**) The notification message.
+- `details`: (`object`, **Notification only**) Payload-specific details for the
+  notification.
+
+---
+
+## Output Schema (`stdout`)
+
+If the hook exits with `0`, the CLI attempts to parse `stdout` as JSON.
+
+### Common Output Fields
+
+| Field                | Type      | Description                                                              |
+| :------------------- | :-------- | :----------------------------------------------------------------------- |
+| `decision`           | `string`  | One of: `allow`, `deny`, `block`, `ask`, `approve`.                      |
+| `reason`             | `string`  | Explanation shown to the **agent** when a decision is `deny` or `block`. |
+| `systemMessage`      | `string`  | Message displayed to the **user** in the CLI terminal.                   |
+| `continue`           | `boolean` | If `false`, immediately terminates the agent loop for this turn.         |
+| `stopReason`         | `string`  | Message shown to the user when `continue` is `false`.                    |
+| `suppressOutput`     | `boolean` | If `true`, the hook execution is hidden from the CLI transcript.         |
+| `hookSpecificOutput` | `object`  | Container for event-specific data (see below).                           |
+
+### `hookSpecificOutput` Reference
+
+| Field               | Supported Events                           | Description                                                                       |
+| :------------------ | :----------------------------------------- | :-------------------------------------------------------------------------------- |
+| `additionalContext` | `SessionStart`, `BeforeAgent`, `AfterTool` | Appends text directly to the agent's context.                                     |
+| `llm_request`       | `BeforeModel`                              | A `Partial<LLMRequest>` to override parameters of the outgoing call.              |
+| `llm_response`      | `BeforeModel`                              | A **full** `LLMResponse` to bypass the model and provide a synthetic result.      |
+| `llm_response`      | `AfterModel`                               | A `Partial<LLMResponse>` to modify the model's response before the agent sees it. |
+| `toolConfig`        | `BeforeToolSelection`                      | Object containing `mode` (`AUTO`/`ANY`/`NONE`) and `allowedFunctionNames`.        |
+
+---
+
+## Stable Model API
+
+Gemini CLI uses a decoupled format for model interactions to ensure hooks remain
+stable even if the underlying Gemini SDK changes.
+
+### `LLMRequest` Object
+
+Used in `BeforeModel` and `BeforeToolSelection`.
+
+> 💡 **Note**: In v1, model hooks are primarily text-focused. Non-text parts
+> (like images or function calls) provided in the `content` array will be
+> simplified to their string representation by the translator.
+
+```typescript
+{
+  "model": string,
+  "messages": Array<{
+    "role": "user" | "model" | "system",
+    "content": string | Array<{ "type": string, [key: string]: any }>
+  }>,
+  "config"?: {
+    "temperature"?: number,
+    "maxOutputTokens"?: number,
+    "topP"?: number,
+    "topK"?: number
+  },
+  "toolConfig"?: {
+    "mode"?: "AUTO" | "ANY" | "NONE",
+    "allowedFunctionNames"?: string[]
+  }
+}
+```
+
+### `LLMResponse` Object
+
+Used in `AfterModel` and as a synthetic response in `BeforeModel`.
+
+```typescript
+{
+  "text"?: string,
+  "candidates": Array<{
+    "content": {
+      "role": "model",
+      "parts": string[]
+    },
+    "finishReason"?: "STOP" | "MAX_TOKENS" | "SAFETY" | "RECITATION" | "OTHER",
+    "index"?: number,
+    "safetyRatings"?: Array<{
+      "category": string,
+      "probability": string,
+      "blocked"?: boolean
+    }>
+  }>,
+  "usageMetadata"?: {
+    "promptTokenCount"?: number,
+    "candidatesTokenCount"?: number,
+    "totalTokenCount"?: number
+  }
+}
+```

docs/sidebar.json

@@ -204,6 +204,10 @@
         "label": "Writing hooks",
         "slug": "docs/hooks/writing-hooks"
       },
+      {
+        "label": "Hooks reference",
+        "slug": "docs/hooks/reference"
+      },
       {
         "label": "Best practices",
         "slug": "docs/hooks/best-practices"