# Hooks on Gemini CLI: 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());
const data3 = await fetch(url3).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());
const p3 = fetch(url3).then((r) => r.json());

// Wait for all results
const [data1, data2, data3] = await Promise.all([p1, p2, p3]);
```

### Cache expensive operations

Store results between invocations to avoid repeated computation:

```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]) {
    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 agent loop completion, while `AfterModel` fires
after every LLM call (potentially multiple times per loop):

```json
// If checking final completion, use AfterAgent instead of AfterModel
{
  "hooks": {
    "AfterAgent": [
      {
        "matcher": "*",
        "hooks": [
          {
            "name": "final-checker",
            "command": "./check-completion.sh"
          }
        ]
      }
    ]
  }
}
```

### Filter with matchers

Use specific matchers to avoid unnecessary hook execution. Instead of matching
all tools with `*`, specify only the tools you need:

```json
{
  "matcher": "write_file|replace",
  "hooks": [
    {
      "name": "validate-writes",
      "command": "./validate.sh"
    }
  ]
}
```

### Optimize JSON parsing

For large inputs, use streaming JSON parsers to avoid loading everything into
memory:

```javascript
// Standard approach: parse entire input
const input = JSON.parse(await readStdin());
const content = input.tool_input.content;

// For very large inputs: stream and extract only needed fields
const { createReadStream } = require('fs');
const JSONStream = require('JSONStream');

const stream = createReadStream(0).pipe(JSONStream.parse('tool_input.content'));
let content = '';
stream.on('data', (chunk) => {
  content += chunk;
});
```

## Debugging

### Log to files

Write debug information to dedicated log files:

```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"
```

### 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) {
  console.error(`Hook error: ${error.message}`);
  process.exit(2); // Blocking error
}
```

### Test hooks independently

Run hook scripts manually with sample JSON input:

```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: $?"
```

### Check exit codes

Ensure your script returns the correct exit code:

```bash
#!/usr/bin/env bash
set -e  # Exit on error

# Hook logic
process_input() {
  # ...
}

if process_input; then
  echo "Success message"
  exit 0
else
  echo "Error message" >&2
  exit 2
fi
```

### Enable telemetry

Hook execution is logged when `telemetry.logPrompts` is enabled:

```json
{
  "telemetry": {
    "logPrompts": true
  }
}
```

View hook telemetry in logs to debug execution issues.

### Use hook panel

The `/hooks panel` command 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
echo "Logged input"
```

### 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:

```bash
chmod +x .gemini/hooks/*.sh
chmod +x .gemini/hooks/*.js
```

### Version control

Commit hooks to share with your team:

```bash
git add .gemini/hooks/
git add .gemini/settings.json
git commit -m "Add project hooks for security and testing"
```

**`.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
```

### Document behavior

Add descriptions to help others understand your hooks:

```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, passwords, and other secrets before writing"
          }
        ]
      }
    ]
  }
}
```

Add comments in hook scripts:

```javascript
#!/usr/bin/env node
/**
 * RAG Tool Filter Hook
 *
 * This hook reduces the tool space from 100+ tools to ~15 relevant ones
 * by extracting keywords from the user's request and filtering tools
 * based on semantic similarity.
 *
 * Performance: ~500ms average, cached tool embeddings
 * Dependencies: @google/generative-ai
 */
```

## Troubleshooting

### Hook not executing

**Check hook name in `/hooks panel`:**

```bash
/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:**

```json
{
  "hooks": {
    "disabled": ["my-hook-name"]
  }
}
```

**Ensure script is executable:**

```bash
ls -la .gemini/hooks/my-hook.sh
chmod +x .gemini/hooks/my-hook.sh
```

**Verify script path:**

```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:**

```json
{
  "name": "slow-hook",
  "timeout": 60000
}
```

**Optimize slow operations:**

```javascript
// Before: Sequential operations (slow)
for (const item of items) {
  await processItem(item);
}

// After: Parallel operations (fast)
await Promise.all(items.map((item) => processItem(item)));
```

**Use caching:**

```javascript
const cache = new Map();

async function getCachedData(key) {
  if (cache.has(key)) {
    return cache.get(key);
  }
  const data = await fetchData(key);
  cache.set(key, data);
  return data;
}
```

**Consider splitting into multiple faster hooks:**

```json
{
  "hooks": {
    "BeforeTool": [
      {
        "matcher": "write_file",
        "hooks": [
          {
            "name": "quick-check",
            "command": "./quick-validation.sh",
            "timeout": 1000
          }
        ]
      },
      {
        "matcher": "write_file",
        "hooks": [
          {
            "name": "deep-check",
            "command": "./deep-analysis.sh",
            "timeout": 30000
          }
        ]
      }
    ]
  }
}
```

### 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
```

**Ensure proper quoting and escaping:**

```javascript
// Bad: Unescaped string interpolation
const message = `User said: ${userInput}`;
console.log(JSON.stringify({ message }));

// Good: Automatic escaping
console.log(JSON.stringify({ message: `User said: ${userInput}` }));
```

**Check for binary data or control characters:**

```javascript
function sanitizeForJSON(str) {
  return str.replace(/[\x00-\x1F\x7F-\x9F]/g, ''); // Remove control chars
}

const cleanContent = sanitizeForJSON(content);
console.log(JSON.stringify({ content: cleanContent }));
```

### Exit code issues

**Verify script returns correct codes:**

```bash
#!/usr/bin/env bash
set -e  # Exit on error

# Processing logic
if validate_input; then
  echo "Success"
  exit 0
else
  echo "Validation failed" >&2
  exit 2
fi
```

**Check for unintended errors:**

```bash
#!/usr/bin/env bash
# Don't use 'set -e' if you want to handle errors explicitly
# set -e

if ! command_that_might_fail; then
  # Handle error
  echo "Command failed but continuing" >&2
fi

# Always exit explicitly
exit 0
```

**Use trap for cleanup:**

```bash
#!/usr/bin/env bash

cleanup() {
  # Cleanup logic
  rm -f /tmp/hook-temp-*
}

trap cleanup EXIT

# Hook logic here
```

### 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

if [ -z "$CUSTOM_VAR" ]; then
  echo "Warning: CUSTOM_VAR not set, using default" >&2
  CUSTOM_VAR="default-value"
fi
```

**Debug available variables:**

```bash
#!/usr/bin/env bash

# List all environment variables
env > .gemini/hook-env.log

# Check specific variables
echo "GEMINI_PROJECT_DIR: $GEMINI_PROJECT_DIR" >> .gemini/hook-env.log
echo "GEMINI_SESSION_ID: $GEMINI_SESSION_ID" >> .gemini/hook-env.log
echo "GEMINI_API_KEY: ${GEMINI_API_KEY:+<set>}" >> .gemini/hook-env.log
```

**Use .env files:**

```bash
#!/usr/bin/env bash

# Load .env file if it exists
if [ -f "$GEMINI_PROJECT_DIR/.env" ]; then
  source "$GEMINI_PROJECT_DIR/.env"
fi
```

## Using Hooks Securely

### 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 (e.g., `/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.

> [!IMPORTANT] **Modification Detection**: If the `command` string of a project
> hook is changed (e.g., 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 (e.g., verified
  publishers, well-known community members).
- Be cautious with obfuscated scripts or compiled binaries from unknown sources.

#### Sanitize Environment

Hooks inherit the environment of the Gemini CLI process, which may include
sensitive API keys. Gemini CLI attempts to sanitize sensitive variables, but you
should be cautious.

- **Avoid printing environment variables** to stdout/stderr unless necessary.
- **Use `.env` files** to securely manage sensitive variables, ensuring they are
  excluded from version control.

**System Administrators:** You can enforce environment variable redaction by
default in the system configuration (e.g., `/etc/gemini-cli/settings.json`):

```json
{
  "security": {
    "environmentVariableRedaction": {
      "enabled": true,
      "blocked": ["MY_SECRET_KEY"],
      "allowed": ["SAFE_VAR"]
    }
  }
}
```

## 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",
            "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. Gemini CLI respects
the `telemetry.logPrompts` setting for hook data logging.

### What data is collected

Hook telemetry may include:

- **Hook inputs:** User prompts, tool arguments, file contents
- **Hook outputs:** Hook responses, decision reasons, added context
- **Standard streams:** stdout and stderr from hook processes
- **Execution metadata:** Hook name, event type, duration, success/failure

### Privacy settings

**Enabled (default):**

Full hook I/O is logged to telemetry. Use this when:

- Developing and debugging hooks
- Telemetry is redirected to a trusted enterprise system
- You understand and accept the privacy implications

**Disabled:**

Only metadata is logged (event name, duration, success/failure). Hook inputs and
outputs are excluded. Use this when:

- Sending telemetry to third-party systems
- Working with sensitive data
- Privacy regulations require minimizing data collection

### Configuration

**Disable PII logging in settings:**

```json
{
  "telemetry": {
    "logPrompts": false
  }
}
```

**Disable via environment variable:**

```bash
export GEMINI_TELEMETRY_LOG_PROMPTS=false
```

### 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
3. **Use secure storage:** Encrypt sensitive data at rest
4. **Limit access:** Restrict hook script permissions

**Example sanitization:**

```javascript
function sanitizeOutput(data) {
  const sanitized = { ...data };

  // Remove sensitive fields
  delete sanitized.apiKey;
  delete sanitized.password;

  // Redact sensitive strings
  if (sanitized.content) {
    sanitized.content = sanitized.content.replace(
      /api[_-]?key\s*[:=]\s*['"]?[a-zA-Z0-9_-]{20,}['"]?/gi,
      '[REDACTED]',
    );
  }

  return sanitized;
}

console.log(JSON.stringify(sanitizeOutput(hookOutput)));
```

## Learn more

- [Hooks Reference](index.md) - Complete API reference
- [Writing Hooks](writing-hooks.md) - Tutorial and examples
- [Configuration](../cli/configuration.md) - Gemini CLI settings
- [Hooks Design Document](../hooks-design.md) - Technical architecture