chore: import upstream snapshot with attribution

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+# Hooks Best Practices
+
+This guide covers security considerations, performance optimization, debugging
+techniques, and privacy considerations for developing and deploying hooks in
+Gemini CLI.
+
+## Performance
+
+### Keep hooks fast
+
+Hooks run synchronously—slow hooks delay the agent loop. Optimize for speed by
+using parallel operations:
+
+```javascript
+// Sequential operations are slower
+const data1 = await fetch(url1).then((r) => r.json());
+const data2 = await fetch(url2).then((r) => r.json());
+
+// Prefer parallel operations for better performance
+// Start requests concurrently
+const p1 = fetch(url1).then((r) => r.json());
+const p2 = fetch(url2).then((r) => r.json());
+
+// Wait for all results
+const [data1, data2] = await Promise.all([p1, p2]);
+```
+
+### Cache expensive operations
+
+Store results between invocations to avoid repeated computation, especially for
+hooks that run frequently (like `BeforeTool` or `AfterModel`).
+
+```javascript
+const fs = require('fs');
+const path = require('path');
+
+const CACHE_FILE = '.gemini/hook-cache.json';
+
+function readCache() {
+  try {
+    return JSON.parse(fs.readFileSync(CACHE_FILE, 'utf8'));
+  } catch {
+    return {};
+  }
+}
+
+function writeCache(data) {
+  fs.writeFileSync(CACHE_FILE, JSON.stringify(data, null, 2));
+}
+
+async function main() {
+  const cache = readCache();
+  const cacheKey = `tool-list-${(Date.now() / 3600000) | 0}`; // Hourly cache
+
+  if (cache[cacheKey]) {
+    // Write JSON to stdout
+    console.log(JSON.stringify(cache[cacheKey]));
+    return;
+  }
+
+  // Expensive operation
+  const result = await computeExpensiveResult();
+  cache[cacheKey] = result;
+  writeCache(cache);
+
+  console.log(JSON.stringify(result));
+}
+```
+
+### Use appropriate events
+
+Choose hook events that match your use case to avoid unnecessary execution.
+
+- **`AfterAgent`**: Fires **once** per turn after the model finishes its final
+  response. Use this for quality validation (Retries) or final logging.
+- **`AfterModel`**: Fires after **every chunk** of LLM output. Use this for
+  real-time redaction, PII filtering, or monitoring output as it streams.
+
+If you only need to check the final completion, use `AfterAgent` to save
+performance.
+
+### Filter with matchers
+
+Use specific matchers to avoid unnecessary hook execution. Instead of matching
+all tools with `*`, specify only the tools you need. This saves the overhead of
+spawning a process for irrelevant events.
+
+```json
+{
+  "matcher": "write_file|replace",
+  "hooks": [
+    {
+      "name": "validate-writes",
+      "type": "command",
+      "command": "./validate.sh"
+    }
+  ]
+}
+```
+
+### Optimize JSON parsing
+
+For large inputs (like `AfterModel` receiving a large context), standard JSON
+parsing can be slow. If you only need one field, consider streaming parsers or
+lightweight extraction logic, though for most shell scripts `jq` is sufficient.
+
+## Debugging
+
+### The "Strict JSON" rule
+
+The most common cause of hook failure is "polluting" the standard output.
+
+- **stdout** is for **JSON only**.
+- **stderr** is for **logs and text**.
+
+**Good:**
+
+```bash
+#!/bin/bash
+echo "Starting check..." >&2  # <--- Redirect to stderr
+echo '{"decision": "allow"}'
+
+```
+
+### Log to files
+
+Since hooks run in the background, writing to a dedicated log file is often the
+easiest way to debug complex logic.
+
+```bash
+#!/usr/bin/env bash
+LOG_FILE=".gemini/hooks/debug.log"
+
+# Log with timestamp
+log() {
+  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" >> "$LOG_FILE"
+}
+
+input=$(cat)
+log "Received input: ${input:0:100}..."
+
+# Hook logic here
+
+log "Hook completed successfully"
+# Always output valid JSON to stdout at the end, even if just empty
+echo "{}"
+
+```
+
+### Use stderr for errors
+
+Error messages on stderr are surfaced appropriately based on exit codes:
+
+```javascript
+try {
+  const result = dangerousOperation();
+  console.log(JSON.stringify({ result }));
+} catch (error) {
+  // Write the error description to stderr so the user/agent sees it
+  console.error(`Hook error: ${error.message}`);
+  process.exit(2); // Blocking error
+}
+```
+
+### Test hooks independently
+
+Run hook scripts manually with sample JSON input to verify they behave as
+expected before hooking them up to the CLI.
+
+**macOS/Linux**
+
+```bash
+# Create test input
+cat > test-input.json << 'EOF'
+{
+  "session_id": "test-123",
+  "cwd": "/tmp/test",
+  "hook_event_name": "BeforeTool",
+  "tool_name": "write_file",
+  "tool_input": {
+    "file_path": "test.txt",
+    "content": "Test content"
+  }
+}
+EOF
+
+# Test the hook
+cat test-input.json | .gemini/hooks/my-hook.sh
+
+# Check exit code
+echo "Exit code: $?"
+```
+
+**Windows (PowerShell)**
+
+```powershell
+# Create test input
+@"
+{
+  "session_id": "test-123",
+  "cwd": "C:\\temp\\test",
+  "hook_event_name": "BeforeTool",
+  "tool_name": "write_file",
+  "tool_input": {
+    "file_path": "test.txt",
+    "content": "Test content"
+  }
+}
+"@ | Out-File -FilePath test-input.json -Encoding utf8
+
+# Test the hook
+Get-Content test-input.json | .\.gemini\hooks\my-hook.ps1
+
+# Check exit code
+Write-Host "Exit code: $LASTEXITCODE"
+```
+
+### Check exit codes
+
+Gemini CLI uses exit codes for high-level flow control:
+
+- **Exit 0 (Success)**: The hook ran successfully. The CLI parses `stdout` for
+  JSON decisions.
+- **Exit 2 (System Block)**: A critical block occurred. `stderr` is used as the
+  reason.
+  - For **Agent/Model** events, this aborts the turn.
+  - For **Tool** events, this blocks the tool but allows the agent to continue.
+  - For **AfterAgent**, this triggers an automatic retry turn.
+
+> **TIP**
+>
+> **Blocking vs. Stopping**: Use `decision: "deny"` (or Exit Code 2) to block a
+> **specific action**. Use `{"continue": false}` in your JSON output to **kill
+> the entire agent loop** immediately.
+
+```bash
+#!/usr/bin/env bash
+set -e
+
+# Hook logic
+if process_input; then
+  echo '{"decision": "allow"}'
+  exit 0
+else
+  echo "Critical validation failure" >&2
+  exit 2
+fi
+
+```
+
+### Enable telemetry
+
+Hook execution is logged when `telemetry.logPrompts` is enabled. You can view
+these logs to debug execution flow.
+
+```json
+{
+  "telemetry": {
+    "logPrompts": true
+  }
+}
+```
+
+### Use hook panel
+
+The `/hooks panel` command inside the CLI shows execution status and recent
+output:
+
+```bash
+/hooks panel
+```
+
+Check for:
+
+- Hook execution counts
+- Recent successes/failures
+- Error messages
+- Execution timing
+
+## Development
+
+### Start simple
+
+Begin with basic logging hooks before implementing complex logic:
+
+```bash
+#!/usr/bin/env bash
+# Simple logging hook to understand input structure
+input=$(cat)
+echo "$input" >> .gemini/hook-inputs.log
+# Always return valid JSON
+echo "{}"
+
+```
+
+### Documenting your hooks
+
+Maintainability is critical for complex hook systems. Use descriptions and
+comments to help yourself and others understand why a hook exists.
+
+**Use the `description` field**: This text is displayed in the `/hooks panel` UI
+and helps diagnose issues.
+
+```json
+{
+  "hooks": {
+    "BeforeTool": [
+      {
+        "matcher": "write_file|replace",
+        "hooks": [
+          {
+            "name": "secret-scanner",
+            "type": "command",
+            "command": "$GEMINI_PROJECT_DIR/.gemini/hooks/block-secrets.sh",
+            "description": "Scans code changes for API keys and secrets before writing"
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+**Add comments in hook scripts**: Explain performance expectations and
+dependencies.
+
+```javascript
+#!/usr/bin/env node
+/**
+ * RAG Tool Filter Hook
+ *
+ * Reduces the tool space by extracting keywords from the user's request.
+ *
+ * Performance: ~500ms average
+ * Dependencies: @google/generative-ai
+ */
+```
+
+### Use JSON libraries
+
+Parse JSON with proper libraries instead of text processing.
+
+**Bad:**
+
+```bash
+# Fragile text parsing
+tool_name=$(echo "$input" | grep -oP '"tool_name":\s*"\K[^"]+')
+
+```
+
+**Good:**
+
+```bash
+# Robust JSON parsing
+tool_name=$(echo "$input" | jq -r '.tool_name')
+
+```
+
+### Make scripts executable
+
+Always make hook scripts executable on macOS/Linux:
+
+```bash
+chmod +x .gemini/hooks/*.sh
+chmod +x .gemini/hooks/*.js
+
+```
+
+**Windows Note**: On Windows, PowerShell scripts (`.ps1`) don't use `chmod`, but
+you may need to ensure your execution policy allows them to run (for example,
+`Set-ExecutionPolicy RemoteSigned -Scope CurrentUser`).
+
+### Version control
+
+Commit hooks to share with your team:
+
+```bash
+git add .gemini/hooks/
+git add .gemini/settings.json
+
+```
+
+**`.gitignore` considerations:**
+
+```gitignore
+# Ignore hook cache and logs
+.gemini/hook-cache.json
+.gemini/hook-debug.log
+.gemini/memory/session-*.jsonl
+
+# Keep hook scripts
+!.gemini/hooks/*.sh
+!.gemini/hooks/*.js
+
+```
+
+## Hook security
+
+### Threat Model
+
+Understanding where hooks come from and what they can do is critical for secure
+usage.
+
+| Hook Source                   | Description                                                                                                                       |
+| :---------------------------- | :-------------------------------------------------------------------------------------------------------------------------------- |
+| **System**                    | Configured by system administrators (for example, `/etc/gemini-cli/settings.json`, `/Library/...`). Assumed to be the **safest**. |
+| **User** (`~/.gemini/...`)    | Configured by you. You are responsible for ensuring they are safe.                                                                |
+| **Extensions**                | You explicitly approve and install these. Security depends on the extension source (integrity).                                   |
+| **Project** (`./.gemini/...`) | **Untrusted by default.** Safest in trusted internal repos; higher risk in third-party/public repos.                              |
+
+#### Project Hook Security
+
+When you open a project with hooks defined in `.gemini/settings.json`:
+
+1. **Detection**: Gemini CLI detects the hooks.
+2. **Identification**: A unique identity is generated for each hook based on its
+   `name` and `command`.
+3. **Warning**: If this specific hook identity has not been seen before, a
+   **warning** is displayed.
+4. **Execution**: The hook is executed (unless specific security settings block
+   it).
+5. **Trust**: The hook is marked as "trusted" for this project.
+
+> **Modification detection**: If the `command` string of a project hook is
+> changed (for example, by a `git pull`), its identity changes. Gemini CLI will
+> treat it as a **new, untrusted hook** and warn you again. This prevents
+> malicious actors from silently swapping a verified command for a malicious
+> one.
+
+### Risks
+
+| Risk                         | Description                                                                                                                          |
+| :--------------------------- | :----------------------------------------------------------------------------------------------------------------------------------- |
+| **Arbitrary Code Execution** | Hooks run as your user. They can do anything you can do (delete files, install software).                                            |
+| **Data Exfiltration**        | A hook could read your input (prompts), output (code), or environment variables (`GEMINI_API_KEY`) and send them to a remote server. |
+| **Prompt Injection**         | Malicious content in a file or web page could trick an LLM into running a tool that triggers a hook in an unexpected way.            |
+
+### Mitigation Strategies
+
+#### Verify the source
+
+**Verify the source** of any project hooks or extensions before enabling them.
+
+- For open-source projects, a quick review of the hook scripts is recommended.
+- For extensions, ensure you trust the author or publisher (for example,
+  verified publishers, well-known community members).
+- Be cautious with obfuscated scripts or compiled binaries from unknown sources.
+
+#### Sanitize environment
+
+Hooks inherit the environment of Gemini CLI process, which may include sensitive
+API keys. Gemini CLI provides a
+[redaction system](../reference/configuration.md#environment-variable-redaction)
+that automatically filters variables matching sensitive patterns (for example,
+`KEY`, `TOKEN`).
+
+> **Disabled by Default**: Environment redaction is currently **OFF by
+> default**. We strongly recommend enabling it if you are running third-party
+> hooks or working in sensitive environments.
+
+**Impact on hooks:**
+
+- **Security**: Prevents your hook scripts from accidentally leaking secrets.
+- **Troubleshooting**: If your hook depends on a specific environment variable
+  that is being blocked, you must explicitly allow it in `settings.json`.
+
+```json
+{
+  "security": {
+    "environmentVariableRedaction": {
+      "enabled": true,
+      "allowed": ["MY_REQUIRED_TOOL_KEY"]
+    }
+  }
+}
+```
+
+**System administrators:** You can enforce redaction for all users in the system
+configuration.
+
+## Troubleshooting
+
+### Hook not executing
+
+**Check hook name in `/hooks panel`:** Verify the hook appears in the list and
+is enabled.
+
+**Verify matcher pattern:**
+
+```bash
+# Test regex pattern
+echo "write_file|replace" | grep -E "write_.*|replace"
+
+```
+
+**Check disabled list:** Verify the hook is not listed in your `settings.json`:
+
+```json
+{
+  "hooks": {
+    "disabled": ["my-hook-name"]
+  }
+}
+```
+
+**Ensure script is executable**: For macOS and Linux users, verify the script
+has execution permissions:
+
+```bash
+ls -la .gemini/hooks/my-hook.sh
+chmod +x .gemini/hooks/my-hook.sh
+```
+
+**Windows Note**: On Windows, ensure your execution policy allows running
+scripts (for example, `Get-ExecutionPolicy`).
+
+**Verify script path:** Ensure the path in `settings.json` resolves correctly.
+
+```bash
+# Check path expansion
+echo "$GEMINI_PROJECT_DIR/.gemini/hooks/my-hook.sh"
+
+# Verify file exists
+test -f "$GEMINI_PROJECT_DIR/.gemini/hooks/my-hook.sh" && echo "File exists"
+```
+
+### Hook timing out
+
+**Check configured timeout:** The default is 60000ms (1 minute). You can
+increase this in `settings.json`:
+
+```json
+{
+  "name": "slow-hook",
+  "timeout": 120000
+}
+```
+
+**Optimize slow operations:** Move heavy processing to background tasks or use
+caching.
+
+### Invalid JSON output
+
+**Validate JSON before outputting:**
+
+```bash
+#!/usr/bin/env bash
+output='{"decision": "allow"}'
+
+# Validate JSON
+if echo "$output" | jq empty 2>/dev/null; then
+  echo "$output"
+else
+  echo "Invalid JSON generated" >&2
+  exit 1
+fi
+
+```
+
+### Environment variables not available
+
+**Check if variable is set:**
+
+```bash
+#!/usr/bin/env bash
+if [ -z "$GEMINI_PROJECT_DIR" ]; then
+  echo "GEMINI_PROJECT_DIR not set" >&2
+  exit 1
+fi
+
+```
+
+**Debug available variables:**
+
+```bash
+env > .gemini/hook-env.log
+```
+
+## Authoring secure hooks
+
+When writing your own hooks, follow these practices to ensure they are robust
+and secure.
+
+### Validate all inputs
+
+Never trust data from hooks without validation. Hook inputs often come from the
+LLM or user prompts, which can be manipulated.
+
+```bash
+#!/usr/bin/env bash
+input=$(cat)
+
+# Validate JSON structure
+if ! echo "$input" | jq empty 2>/dev/null; then
+  echo "Invalid JSON input" >&2
+  exit 1
+fi
+
+# Validate tool_name explicitly
+tool_name=$(echo "$input" | jq -r '.tool_name // empty')
+if [[ "$tool_name" != "write_file" && "$tool_name" != "read_file" ]]; then
+  echo "Unexpected tool: $tool_name" >&2
+  exit 1
+fi
+```
+
+### Use timeouts
+
+Prevent denial-of-service (hanging agents) by enforcing timeouts. Gemini CLI
+defaults to 60 seconds, but you should set stricter limits for fast hooks.
+
+```json
+{
+  "hooks": {
+    "BeforeTool": [
+      {
+        "matcher": "*",
+        "hooks": [
+          {
+            "name": "fast-validator",
+            "type": "command",
+            "command": "./hooks/validate.sh",
+            "timeout": 5000 // 5 seconds
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+### Limit permissions
+
+Run hooks with minimal required permissions:
+
+```bash
+#!/usr/bin/env bash
+# Don't run as root
+if [ "$EUID" -eq 0 ]; then
+  echo "Hook should not run as root" >&2
+  exit 1
+fi
+
+# Check file permissions before writing
+if [ -w "$file_path" ]; then
+  # Safe to write
+else
+  echo "Insufficient permissions" >&2
+  exit 1
+fi
+```
+
+### Example: Secret Scanner
+
+Use `BeforeTool` hooks to prevent committing sensitive data. This is a powerful
+pattern for enhancing security in your workflow.
+
+```javascript
+const SECRET_PATTERNS = [
+  /api[_-]?key\s*[:=]\s*['"]?[a-zA-Z0-9_-]{20,}['"]?/i,
+  /password\s*[:=]\s*['"]?[^\s'"]{8,}['"]?/i,
+  /secret\s*[:=]\s*['"]?[a-zA-Z0-9_-]{20,}['"]?/i,
+  /AKIA[0-9A-Z]{16}/, // AWS access key
+  /ghp_[a-zA-Z0-9]{36}/, // GitHub personal access token
+  /sk-[a-zA-Z0-9]{48}/, // OpenAI API key
+];
+
+function containsSecret(content) {
+  return SECRET_PATTERNS.some((pattern) => pattern.test(content));
+}
+```
+
+## Privacy considerations
+
+Hook inputs and outputs may contain sensitive information.
+
+### What data is collected
+
+Hook telemetry may include inputs (prompts, code) and outputs (decisions,
+reasons) unless disabled.
+
+### Privacy settings
+
+**Disable PII logging:** If you are working with sensitive data, disable prompt
+logging in your settings:
+
+```json
+{
+  "telemetry": {
+    "logPrompts": false
+  }
+}
+```
+
+**Suppress Output:** Individual hooks can request their metadata be hidden from
+logs and telemetry by returning `"suppressOutput": true` in their JSON response.
+
+> **Note**
+
+> `suppressOutput` only affects background logging. Any `systemMessage` or
+> `reason` included in the JSON will still be displayed to the user in the
+> terminal.
+
+### Sensitive data in hooks
+
+If your hooks process sensitive data:
+
+1. **Minimize logging:** Don't write sensitive data to log files.
+2. **Sanitize outputs:** Remove sensitive data before outputting JSON or writing
+   to stderr.