agents-authoring

AGENTS.md and Skill Authoring

Load this skill when creating or updating AGENTS.md files, OpenCode agents, or skills.

Research Foundation

This guidance synthesizes:

• agents.md - Open standard used by 60k+ projects, stewarded by Agentic AI Foundation under Linux Foundation
• GitHub's analysis of 2,500+ repositories with agents.md files
• Anthropic's context engineering documentation
• OpenAI Codex custom instructions guide
• Devin AI's good vs bad instructions documentation
• Builder.io's AGENTS.md best practices guide

Core Concept

AGENTS.md is a "README for agents" - a dedicated, predictable place for AI coding agent context. Supported by: OpenAI Codex, Google Jules, Cursor, VS Code, GitHub Copilot, Devin, Windsurf, OpenCode, Aider, and many others.

Essential Sections

Every AGENTS.md should cover:

1. Dos and Don'ts (Constraints)

Be nitpicky. Clear guidelines prevent repeated mistakes.

### Do
- use TypeScript strict mode
- use functional components with hooks
- default to small, focused diffs

### Don't
- do not hard code colors - use design tokens
- do not add dependencies without approval

2. File-Scoped Commands

Prefer file-specific commands over project-wide. Faster feedback, fewer wasted cycles.

### Commands
# Type check single file (seconds, not minutes)
npm run tsc --noEmit path/to/file.tsx

# Lint single file
npm run eslint --fix path/to/file.tsx

# Test single file
npm run vitest run path/to/file.test.tsx

# Full build only when explicitly requested
npm run build

3. Safety and Permissions

Explicit allow/ask lists prevent surprises.

### Permissions
Allowed without asking:
- read files, list directories
- type check, lint, format single files
- run single unit tests

Ask first:
- package installs
- git push
- deleting files
- full build or E2E suites

4. Project Structure Hints

A tiny index saves exploration time every session.

### Structure
- routes: `src/App.tsx`
- components: `src/components/`
- design tokens: `src/lib/theme/tokens.ts`
- API client: `src/api/client.ts`

5. Good/Bad Example Pointers

Point to real files. Examples beat abstractions.

### Examples
Copy these patterns:
- forms: `src/components/UserForm.tsx`
- data fetching: `src/hooks/useProjects.ts`

Avoid these (legacy):
- class components like `src/legacy/Admin.tsx`

6. When Stuck Guidance

Escape hatch for uncertainty.

### When stuck
- ask a clarifying question
- propose a short plan
- open a draft PR with notes
- do not push large speculative changes

7. PR/Commit Checklist

Define "ready" explicitly.

### PR checklist
- lint, type check, tests: all green
- diff is small and focused
- brief summary of what changed and why

Nested AGENTS.md for Monorepos

Place AGENTS.md in subdirectories for package-specific rules. Agent reads closest file to edited code. Root file provides defaults; nested files override.

/root
  AGENTS.md           # Project-wide defaults
  /packages/legacy
    AGENTS.md         # React 17 rules for this package
  /packages/new-app
    AGENTS.md         # React 18 rules for this package

Anthropic Context Engineering Principles

Clarity at the Right Altitude

• Specific enough to guide behavior effectively
• Flexible enough to provide strong heuristics
• Not so detailed it becomes brittle

Minimality

• Minimal set of information that fully outlines expected behavior
• Minimal does NOT mean short - it means only necessary information
• Remove redundancy ruthlessly

Structure

• Organize into distinct sections
• Group related rules together
• Use consistent formatting

Rule Writing Guidelines

Format: Constraint + Consequence

Bad: "Don't commit directly to main"

Good: "NEVER commit directly to main - use feature branches and PRs"

Prefer Positive Over Negative

Bad: "NEVER use var"

Good: "Use const by default, let when reassignment needed (NEVER var)"

Be Specific, Not Vague

Bad: "Be careful with error handling"

Good: "All async functions MUST have try/catch - unhandled rejections crash the process"

Use Examples Over Adjectives

Bad: "Write concise commit messages"

Good: "Format: fix(auth): handle expired tokens, feat(api): add search endpoint"

Antipatterns

OpenCode Agent Structure

OpenCode agents use markdown files with YAML frontmatter:

---
description: Brief description of agent purpose
mode: subagent
permission:
  skill:
    "*": deny
    specific-skill: allow
---

# Agent Name

Brief intro. Pointer to load relevant skill.

## Workflow

Mandatory steps before starting work.

## Domain Ownership

Paths this agent is responsible for.

## Constraints

NEVER/MUST rules with consequences.

## Verification

Commands to validate work.

## When Stuck

Escape hatch for uncertainty.

Agent vs Skill Separation

Agent (always loaded):

• Workflow/prerequisites (mandatory steps)
• Domain ownership (which paths)
• Hard constraints (NEVER rules)
• Verification commands
• Pointer to skill

Skill (loaded on demand):

• Code examples and patterns
• Step-by-step procedures
• File templates
• Debugging guides
• Comprehensive reference

Decision Heuristic

Ask: "Is this needed in every conversation with this agent?"

• Yes -> Put in agent
• No, only for specific operations -> Put in skill

OpenCode Skill Structure

Location

• Project: .opencode/skills/{name}/SKILL.md
• Global: ~/.config/opencode/skills/{name}/SKILL.md

Frontmatter (Required)

---
name: skill-name
description: 1-1024 chars describing when to use this skill
---

Name Rules

• 1-64 characters
• Lowercase alphanumeric with single hyphen separators
• No leading/trailing hyphens, no consecutive hyphens
• Must match directory name

Body Content

• Start with purpose statement
• Include copy-pasteable examples
• Show pattern variations
• Document common mistakes

Good vs Bad Instructions (Devin Research)

Good Instructions

• Name specific files/components
• Reference existing code as templates
• Include clear success criteria
• Define verification steps

Example: "Create endpoint /users/stats returning JSON. Reference /orders/stats in statsController.js for structure. Add tests to StatsController.test.js."

Bad Instructions

• Vague ("make it user-friendly")
• No specific components mentioned
• Unclear validation criteria
• Open-ended scope

Example: "Add a user stats endpoint." (No format, source, tests, or reference)

Maintenance

When to Update

• AGENTS.md: New constraints, infrastructure changes, command changes
• Skills: New patterns, better examples, common mistakes discovered

Update Process

1. Identify what changed and why
2. Update the authoritative location (not duplicates)
3. Verify no contradictions introduced
4. Validate formatting (markdownlint)

Validation Checklist

Before finalizing changes:

• No duplicate rules across agent and skill
• Each constraint has a consequence
• Commands are copy-pasteable and file-scoped where possible
• Examples reference real files (not invented)
• Skills referenced where detailed patterns live
• "When stuck" guidance included
• Line length <= 100 characters
• Code blocks have language specifiers