Hooks implementation & automatic Claude Code hooks migration

docs/cli/hooks.md

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+# Hooks
+
+Hooks allow you to execute custom shell scripts at specific points in the Gemini
+CLI lifecycle. This enables powerful customizations like audit logging, context
+injection, tool filtering, and integration with external systems.
+
+## Quick Start
+
+### 1. Enable Hooks
+
+In `~/.gemini/settings.json`:
+
+```json
+{
+  "tools": {
+    "enableHooks": true
+  }
+}
+```
+
+### 2. Configure a Hook
+
+```json
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "~/.gemini/hooks/my-hook.sh",
+            "timeout": 10000
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+### 3. Create the Hook Script
+
+```bash
+#!/bin/bash
+# ~/.gemini/hooks/my-hook.sh
+
+# Read JSON input from stdin
+INPUT=$(cat)
+
+# Parse event name
+EVENT=$(echo "$INPUT" | jq -r '.hook_event_name')
+
+# Log to file
+echo "[$(date)] Event: $EVENT" >> /tmp/gemini_hooks.log
+
+# Output valid JSON
+echo '{"continue": true}'
+```
+
+Make it executable:
+
+```bash
+chmod +x ~/.gemini/hooks/my-hook.sh
+```
+
+## Hook Events
+
+| Event                 | When It Fires                     | Key Input Fields                           |
+| --------------------- | --------------------------------- | ------------------------------------------ |
+| `SessionStart`        | CLI starts up, resumes, or clears | `source`                                   |
+| `SessionEnd`          | CLI exits                         | `reason`                                   |
+| `BeforeAgent`         | Before processing user prompt     | `prompt`                                   |
+| `AfterAgent`          | After agent responds              | `prompt`, `prompt_response`                |
+| `BeforeTool`          | Before a tool executes            | `tool_name`, `tool_input`                  |
+| `AfterTool`           | After a tool completes            | `tool_name`, `tool_input`, `tool_response` |
+| `PreCompress`         | Before context compression        | `trigger`                                  |
+| `BeforeModel`         | Before LLM API call               | `llm_request`                              |
+| `AfterModel`          | After LLM API response            | `llm_request`, `llm_response`              |
+| `BeforeToolSelection` | Before tool selection             | `llm_request`                              |
+
+## Hook Input Format
+
+All hooks receive JSON via stdin with these common fields:
+
+```json
+{
+  "session_id": "abc123...",
+  "transcript_path": "/path/to/session.json",
+  "cwd": "/current/working/directory",
+  "hook_event_name": "BeforeAgent",
+  "timestamp": "2025-01-15T10:30:00.000Z"
+}
+```
+
+Plus event-specific fields (see table above).
+
+## Hook Output Format
+
+Hooks should output JSON to stdout:
+
+```json
+{
+  "continue": true,
+  "decision": "allow",
+  "reason": "Optional reason text",
+  "systemMessage": "Message to display to user"
+}
+```
+
+### Exit Codes
+
+| Code | Meaning                    |
+| ---- | -------------------------- |
+| `0`  | Success                    |
+| `1`  | Warning (non-blocking)     |
+| `2`  | Blocking error (deny/stop) |
+
+## Configuration Structure
+
+```json
+{
+  "hooks": {
+    "[EventName]": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "path/to/script.sh",
+            "timeout": 60000,
+            "enabled": true
+          }
+        ],
+        "matcher": "pattern",
+        "sequential": false
+      }
+    ]
+  }
+}
+```
+
+### Fields
+
+| Field        | Type    | Description                              |
+| ------------ | ------- | ---------------------------------------- |
+| `type`       | string  | Always `"command"`                       |
+| `command`    | string  | Path to script or shell command          |
+| `timeout`    | number  | Timeout in milliseconds (default: 60000) |
+| `enabled`    | boolean | Enable/disable this hook (default: true) |
+| `matcher`    | string  | Pattern for tool-related hooks (regex)   |
+| `sequential` | boolean | Run hooks sequentially vs parallel       |
+
+## Tool Name Reference
+
+For `BeforeTool` and `AfterTool` hooks, use these tool names in matchers:
+
+| Tool              | Name in Gemini CLI    |
+| ----------------- | --------------------- |
+| File editing      | `replace`             |
+| File writing      | `write_file`          |
+| File reading      | `read_file`           |
+| Shell commands    | `run_shell_command`   |
+| Todo lists        | `write_todos`         |
+| File globbing     | `glob`                |
+| Content search    | `search_file_content` |
+| Directory listing | `list_directory`      |
+| Web fetch         | `web_fetch`           |
+| Web search        | `google_web_search`   |
+
+### Matcher Examples
+
+```json
+{
+  "AfterTool": [
+    {
+      "hooks": [
+        { "type": "command", "command": "~/.gemini/hooks/log-edits.sh" }
+      ],
+      "matcher": "replace|write_file"
+    },
+    {
+      "hooks": [
+        { "type": "command", "command": "~/.gemini/hooks/log-shell.sh" }
+      ],
+      "matcher": "run_shell_command"
+    }
+  ]
+}
+```
+
+## Migrating from Claude Code
+
+### Automatic Migration
+
+```bash
+gemini hooks migrate
+```
+
+This command:
+
+1. Reads your `~/.claude/settings.json`
+2. Transforms event names and tool matchers
+3. Converts timeouts (seconds → milliseconds)
+4. Updates script paths (`.claude/hooks` → `.gemini/hooks`)
+5. Saves to `~/.gemini/settings.json`
+
+### Event Name Mapping
+
+| Claude Code        | Gemini CLI     |
+| ------------------ | -------------- |
+| `UserPromptSubmit` | `BeforeAgent`  |
+| `PostToolUse`      | `AfterTool`    |
+| `Stop`             | `AfterAgent`   |
+| `PreCompact`       | `PreCompress`  |
+| `SessionStart`     | `SessionStart` |
+| `SessionEnd`       | `SessionEnd`   |
+
+### Tool Name Mapping
+
+| Claude Code | Gemini CLI            |
+| ----------- | --------------------- |
+| `Edit`      | `replace`             |
+| `MultiEdit` | `replace`             |
+| `Write`     | `write_file`          |
+| `Read`      | `read_file`           |
+| `Bash`      | `run_shell_command`   |
+| `TodoWrite` | `write_todos`         |
+| `Glob`      | `glob`                |
+| `Grep`      | `search_file_content` |
+
+### Field Name Differences
+
+| Concept    | Claude Code | Gemini CLI            |
+| ---------- | ----------- | --------------------- |
+| User input | `.message`  | `.prompt`             |
+| Event name | N/A         | `.hook_event_name`    |
+| Tool name  | Same        | Different (see table) |
+
+### Compatibility Shim
+
+For existing Claude Code scripts, use the compatibility shim:
+
+```bash
+#!/bin/bash
+source ~/.gemini/hooks/utils/claude-compat-shim.sh
+
+# Now use normalized variables:
+echo "User message: $USER_MESSAGE"
+echo "Session ID: $SESSION_ID"
+echo "Event: $HOOK_EVENT_NAME"
+echo "Tool (normalized): $TOOL_NAME_NORMALIZED"
+```
+
+The shim provides:
+
+- `$USER_MESSAGE` - Works with both `.message` (Claude) and `.prompt` (Gemini)
+- `$TOOL_NAME_NORMALIZED` - Maps Gemini tool names to Claude-style names
+- Helper functions: `hook_success()`, `hook_block()`, `hook_warn()`
+
+## Examples
+
+### Audit Logger
+
+Log all tool executions:
+
+```bash
+#!/bin/bash
+# ~/.gemini/hooks/audit-logger.sh
+
+INPUT=$(cat)
+EVENT=$(echo "$INPUT" | jq -r '.hook_event_name')
+TOOL=$(echo "$INPUT" | jq -r '.tool_name // "N/A"')
+
+echo "[$(date)] $EVENT - Tool: $TOOL" >> /tmp/gemini_audit.log
+echo '{"continue": true}'
+```
+
+### Context Injector
+
+Add project context to prompts:
+
+```bash
+#!/bin/bash
+# ~/.gemini/hooks/inject-context.sh
+
+INPUT=$(cat)
+
+# Read project context
+if [ -f ".gemini/context.md" ]; then
+  CONTEXT=$(cat .gemini/context.md)
+  echo "{\"continue\": true, \"hookSpecificOutput\": {\"additionalContext\": \"$CONTEXT\"}}"
+else
+  echo '{"continue": true}'
+fi
+```
+
+### Tool Guardian
+
+Block dangerous commands:
+
+```bash
+#!/bin/bash
+# ~/.gemini/hooks/tool-guard.sh
+
+INPUT=$(cat)
+TOOL_INPUT=$(echo "$INPUT" | jq -r '.tool_input.command // ""')
+
+# Block rm -rf commands
+if echo "$TOOL_INPUT" | grep -q 'rm.*-rf'; then
+  echo '{"decision": "block", "reason": "Dangerous rm -rf command blocked"}'
+  exit 2
+fi
+
+echo '{"continue": true}'
+```
+
+## Hook Commands
+
+Manage hooks with these CLI commands:
+
+```bash
+# List all configured hooks
+gemini hooks list
+
+# Disable a hook
+gemini hooks disable "~/.gemini/hooks/my-hook.sh"
+
+# Enable a disabled hook
+gemini hooks enable "~/.gemini/hooks/my-hook.sh"
+
+# Migrate from Claude Code
+gemini hooks migrate
+```
+
+## Debugging
+
+### Test Your Hook
+
+```bash
+# Manually test with sample input
+echo '{"hook_event_name": "BeforeAgent", "prompt": "test", "session_id": "123", "cwd": "/tmp", "timestamp": "2025-01-01T00:00:00Z"}' | ~/.gemini/hooks/my-hook.sh
+```
+
+### View Hook Execution
+
+Check the debug log for hook execution details:
+
+```bash
+# Enable debug mode
+export DEBUG=gemini:*
+
+# Or check hook output in your script
+tail -f /tmp/gemini_hooks.log
+```
+
+### Common Issues
+
+1. **Hook not firing**: Ensure `tools.enableHooks: true` in settings
+2. **"UnknownEvent" logged**: Read event name from JSON stdin, not command args
+3. **Permission denied**: Make script executable with `chmod +x`
+4. **Timeout errors**: Increase `timeout` value or optimize script
+5. **Matcher not working**: Use Gemini tool names (e.g., `replace` not `Edit`)
+
+## Environment Variables
+
+Hooks have access to these environment variables:
+
+| Variable                                  | Description               |
+| ----------------------------------------- | ------------------------- |
+| `GEMINI_PROJECT_DIR`                      | Current working directory |
+| `CLAUDE_PROJECT_DIR`                      | Same (for compatibility)  |
+| Plus all existing `process.env` variables |