intermediate⏱45 minLesson 2 of 6

Custom Hooks for Automation

Build custom hooks to automate repetitive tasks in OpenCode. Learn the hook lifecycle, event types, and how to create hooks that validate, transform, and guard your workflow.

Custom Hooks for Automation

What Are Hooks?

Hooks are scripts that run automatically at specific points in the OpenCode lifecycle. They enable:

Validation — Check inputs/outputs before processing
Transformation — Modify data between steps
Guarding — Block dangerous operations
Logging — Track activity for auditing

Hook Events

Event	When It Fires	Use Case
`session.start`	Session begins	Load context, check environment
`session.end`	Session ends	Cleanup, save state
`file.write`	Before file is written	Validate content, format code
`file.read`	Before file is read	Access control, logging
`tool.execute.before`	Before tool runs	Permission checks, validation
`tool.execute.after`	After tool runs	Post-processing, logging

Hook Structure

.opencode/hooks/my-hook/
├── hook.json          # Hook configuration
├── scripts/
│   └── handler.py     # Hook script
└── examples/
    └── sample-input.json

hook.json

json

{
  "name": "validate-frontmatter",
  "description": "Validates YAML frontmatter in lesson files",
  "event": "file.write",
  "pattern": "content/courses/**/*.md",
  "script": ".opencode/hooks/validate-frontmatter/scripts/handler.py",
  "blocking": true
}

Field	Description
`name`	Unique identifier
`description`	What the hook does
`event`	When to trigger
`pattern`	File pattern to match (glob)
`script`	Path to handler script
`blocking`	If true, blocks operation on failure

Creating a Validation Hook

Step 1: Create Directory

bash

mkdir -p .opencode/hooks/validate-frontmatter/scripts

Step 2: Create hook.json

json

{
  "name": "validate-frontmatter",
  "description": "Validates YAML frontmatter in Markdown files",
  "event": "file.write",
  "pattern": "**/*.md",
  "script": ".opencode/hooks/validate-frontmatter/scripts/handler.py",
  "blocking": true
}

Step 3: Create Handler Script

python

#!/usr/bin/env python3
"""Validate YAML frontmatter in Markdown files.

Exit Codes:
    0 - Validation passed
    1 - Warning (non-blocking)
    2 - Validation failed (blocking)
"""
import os
import sys
import yaml
from pathlib import Path


def extract_frontmatter(content: str) -> tuple[str | None, str]:
    """Extract YAML frontmatter from Markdown content."""
    if not content.startswith("---"):
        return None, content

    parts = content.split("---", 2)
    if len(parts) < 3:
        return None, content

    return parts[1].strip(), parts[2]


def validate_frontmatter(frontmatter: str) -> list[str]:
    """Validate frontmatter structure and return errors."""
    errors = []

    try:
        data = yaml.safe_load(frontmatter)
    except yaml.YAMLError as e:
        return [f"Invalid YAML: {e}"]

    required_fields = ["title", "description", "order"]
    for field in required_fields:
        if field not in data:
            errors.append(f"Missing required field: {field}")

    if "order" in data:
        if not isinstance(data["order"], int):
            errors.append("Field 'order' must be an integer")
        elif data["order"] < 1:
            errors.append("Field 'order' must be >= 1")

    return errors


def main() -> int:
    """Main hook entry point."""
    file_path = os.environ.get("OPENCODE_FILE_PATH", "")
    if not file_path:
        print("WARNING: OPENCODE_FILE_PATH not set", file=sys.stderr)
        return 1

    path = Path(file_path)
    if not path.exists():
        print(f"ERROR: File not found: {file_path}", file=sys.stderr)
        return 2

    try:
        content = path.read_text(encoding="utf-8")
    except Exception as e:
        print(f"ERROR: Cannot read file: {e}", file=sys.stderr)
        return 2

    frontmatter, _ = extract_frontmatter(content)
    if frontmatter is None:
        print(f"WARNING: No frontmatter found in {file_path}")
        return 1

    errors = validate_frontmatter(frontmatter)
    if errors:
        for error in errors:
            print(f"VALIDATION ERROR: {error}", file=sys.stderr)
        return 2

    print(f"VALIDATION PASSED: {file_path}")
    return 0


if __name__ == "__main__":
    sys.exit(main())

Step 4: Make Script Executable

bash

chmod +x .opencode/hooks/validate-frontmatter/scripts/handler.py

Registering Hooks

Add hooks to opencode.json:

json

{
  "hooks": {
    "file.write": [
      {
        "name": "validate-frontmatter",
        "pattern": "content/courses/**/*.md",
        "script": ".opencode/hooks/validate-frontmatter/scripts/handler.py",
        "blocking": true
      }
    ],
    "session.start": [
      {
        "name": "load-context",
        "script": ".opencode/hooks/load-context/scripts/handler.py",
        "blocking": false
      }
    ]
  }
}

Guard Hooks

Guard hooks block dangerous operations:

python

#!/usr/bin/env python3
"""Block writes to protected directories."""
import os
import sys
from pathlib import Path


PROTECTED_PATTERNS = [
    ".env",
    "secrets/**",
    "*.key",
    "*.pem",
]


def is_protected(path: Path) -> bool:
    """Check if path matches protected patterns."""
    from fnmatch import fnmatch

    path_str = str(path)
    for pattern in PROTECTED_PATTERNS:
        if fnmatch(path_str, pattern):
            return True
        if fnmatch(path.name, pattern):
            return True
    return False


def main() -> int:
    file_path = os.environ.get("OPENCODE_FILE_PATH", "")
    if not file_path:
        return 0

    path = Path(file_path)
    if is_protected(path):
        print(f"BLOCKED: Cannot write to protected file: {file_path}", file=sys.stderr)
        return 2

    return 0


if __name__ == "__main__":
    sys.exit(main())

Hook Debugging

Enable Debug Logging

bash

export OPENCODE_DEBUG=1
opencode

Test Hooks Manually

bash

OPENCODE_FILE_PATH=test.md python .opencode/hooks/validate-frontmatter/scripts/handler.py

Check Exit Codes

Code	Meaning	Action
`0`	Pass	Continue operation
`1`	Warning	Log, continue
`2`	Block	Stop operation, show error

Practice Questions

Practice Question

What happens when a blocking hook returns exit code 2?

Practice Question

Which hook event fires before a file is written?

Practice Question

What field in hook.json specifies which files trigger the hook?

Practice Question

How do you test a hook manually?

Practice Question

What is a guard hook used for?

✅Success

Key Takeaways

Hooks are scripts that run automatically at specific lifecycle points
The file.write event is ideal for validating content before saving
Use exit code 0 for pass, 1 for warning, 2 for blocking
Guard hooks protect sensitive files and dangerous operations
Hooks are registered in opencode.json under the hooks key
The pattern field uses glob syntax to match files
Test hooks manually by setting OPENCODE_FILE_PATH and running the script

Progress33%

Multi-Agent Orchestration Patterns

Debugging Agent Behavior

Course Content

1. Multi-Agent Orchestration Patterns 2. Custom Hooks for Automation 3. Debugging Agent Behavior 4. Performance Optimization and Caching 5. Memory and Context Management 6. Team Collaboration Workflows