feat(hooks): Hook System Documentation (#14307)

2026-04-29 14:34:55 -07:00 · 2025-12-03 10:57:05 -08:00
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+# 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.
+
+## Security considerations
+
+### Validate all inputs
+
+Never trust data from hooks without validation. Hook inputs may contain
+user-provided data that could be malicious:
+
+```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 required fields
+tool_name=$(echo "$input" | jq -r '.tool_name // empty')
+if [ -z "$tool_name" ]; then
+  echo "Missing tool_name field" >&2
+  exit 1
+fi
+```
+
+### Use timeouts
+
+Set reasonable timeouts to prevent hooks from hanging indefinitely:
+
+```json
+{
+  "hooks": {
+    "BeforeTool": [
+      {
+        "matcher": "*",
+        "hooks": [
+          {
+            "name": "slow-validator",
+            "type": "command",
+            "command": "./hooks/validate.sh",
+            "timeout": 5000
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+**Recommended timeouts:**
+
+- Fast validation: 1000-5000ms
+- Network requests: 10000-30000ms
+- Heavy computation: 30000-60000ms
+
+### Limit permissions
+
+Run hooks with minimal required permissions:
+
+```bash
+#!/usr/bin/env bash
+# Don't run as root
+if [ "$EUID" -eq 0 ]; then
+  echo "Hook should not run as root" >&2
+  exit 1
+fi
+
+# Check file permissions before writing
+if [ -w "$file_path" ]; then
+  # Safe to write
+else
+  echo "Insufficient permissions" >&2
+  exit 1
+fi
+```
+
+### Scan for secrets
+
+Use `BeforeTool` hooks to prevent committing sensitive data:
+
+```javascript
+const SECRET_PATTERNS = [
+  /api[_-]?key\s*[:=]\s*['"]?[a-zA-Z0-9_-]{20,}['"]?/i,
+  /password\s*[:=]\s*['"]?[^\s'"]{8,}['"]?/i,
+  /secret\s*[:=]\s*['"]?[a-zA-Z0-9_-]{20,}['"]?/i,
+  /AKIA[0-9A-Z]{16}/, // AWS access key
+  /ghp_[a-zA-Z0-9]{36}/, // GitHub personal access token
+  /sk-[a-zA-Z0-9]{48}/, // OpenAI API key
+];
+
+function containsSecret(content) {
+  return SECRET_PATTERNS.some((pattern) => pattern.test(content));
+}
+```
+
+### Review external scripts
+
+Always review hook scripts from untrusted sources before enabling them:
+
+```bash
+# Review before installing
+cat third-party-hook.sh | less
+
+# Check for suspicious patterns
+grep -E 'curl|wget|ssh|eval' third-party-hook.sh
+
+# Verify hook source
+ls -la third-party-hook.sh
+```
+
+### Sandbox untrusted hooks
+
+For maximum security, consider running untrusted hooks in isolated environments:
+
+```bash
+# Run hook in Docker container
+docker run --rm \
+  -v "$GEMINI_PROJECT_DIR:/workspace:ro" \
+  -i untrusted-hook-image \
+  /hook-script.sh < input.json
+```
+
+## 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
+const [data1, data2, data3] = await Promise.all([
+  fetch(url1).then((r) => r.json()),
+  fetch(url2).then((r) => r.json()),
+  fetch(url3).then((r) => r.json()),
+]);
+```
+
+### 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": "WriteFile|Edit",
+  "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": "WriteFile",
+  "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 "$input" | jq -r '.tool_name')
+```
+
+### Make scripts executable
+
+Always make hook scripts executable:
+
+```bash
+chmod +x .gemini/hooks/*.sh
+chmod +x .gemini/hooks/*.js
+```
+
+### Version control
+
+Commit hooks to share with your team:
+
+```bash
+git add .gemini/hooks/
+git add .gemini/settings.json
+git commit -m "Add project hooks for security and testing"
+```
+
+**`.gitignore` considerations:**
+
+```gitignore
+# Ignore hook cache and logs
+.gemini/hook-cache.json
+.gemini/hook-debug.log
+.gemini/memory/session-*.jsonl
+
+# Keep hook scripts
+!.gemini/hooks/*.sh
+!.gemini/hooks/*.js
+```
+
+### Document behavior
+
+Add descriptions to help others understand your hooks:
+
+```json
+{
+  "hooks": {
+    "BeforeTool": [
+      {
+        "matcher": "WriteFile|Edit",
+        "hooks": [
+          {
+            "name": "secret-scanner",
+            "type": "command",
+            "command": "$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 "WriteFile" | grep -E "Write.*|Edit"
+```
+
+**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": "WriteFile",
+        "hooks": [
+          {
+            "name": "quick-check",
+            "command": "./quick-validation.sh",
+            "timeout": 1000
+          }
+        ]
+      },
+      {
+        "matcher": "WriteFile",
+        "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
+```
+
+## 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