AGENTS.md Explained: The Single File That Makes AI Coding Agents Actually Useful

If you’ve ever watched an AI coding agent ignore your project’s conventions — using the wrong naming scheme, adding unnecessary dependencies, writing tests that don’t match your test runner — there’s a simple fix that most developers haven’t heard of yet.

It’s called AGENTS.md, and it’s now natively supported by over 25 AI coding tools.

What Is AGENTS.md?

AGENTS.md is a plain Markdown file you place in the root of your project repository. AI coding agents read it automatically when they start working in your codebase. It tells them everything they need to know about your project’s specific conventions, constraints, and preferences — before they write a single line of code.

Think of it as the onboarding document you’d give a new developer on their first day. Except the agent reads it every time, never forgets it, and never gets bored reading it.

Which Tools Support AGENTS.md?

As of March 2026, AGENTS.md is natively supported by:

• Codex CLI (OpenAI) — the tool that popularized the format
• Cursor — reads it as part of project context
• Windsurf (Codeium) — native support
• Devin (Cognition AI) — reads AGENTS.md for project guidelines
• GitHub Copilot Workspace — uses it for workspace context
• Aider — supports it as a conventions file
• And 20+ other tools in the Codex/open-source ecosystem

Important caveat: Claude Code uses CLAUDE.md, not AGENTS.md. If you’re using Claude Code, create a CLAUDE.md file instead — the content format is the same, just the filename differs.

What to Put in Your AGENTS.md File

Here’s a battle-tested structure:

# Project Overview
[2-3 sentences: what this project does, its tech stack, primary purpose]

# Code Conventions
- Language: TypeScript (strict mode)
- Formatting: Prettier with .prettierrc in root — always run before committing
- Imports: use path aliases from tsconfig.json, never relative ../../ paths
- Naming: camelCase for variables/functions, PascalCase for classes/components

# Testing
- Runner: Vitest
- Location: tests/ alongside source files, named *.test.ts
- Pattern: always mock external HTTP calls with msw
- Coverage threshold: 80% — don't submit PRs that drop below

# Architecture
- State management: Zustand (not Redux, not Context API for global state)
- API calls: all in src/api/ using the typed client in src/api/client.ts
- Components: React functional components only, no class components

# What NOT to Do
- Do NOT add new npm dependencies without asking
- Do NOT modify .env files
- Do NOT commit to main directly — always branch
- Do NOT use console.log in production code (use src/utils/logger.ts)

# Commands
- Dev server: npm run dev
- Tests: npm run test
- Build: npm run build
- Lint: npm run lint (fix before committing)

The Key Sections (and Why They Matter)

Code Conventions

This is where most agents go wrong by default. They’ll use whatever conventions they learned from training data — which may match common patterns but not your specific choices. Explicitly state your formatting tool, naming conventions, and any non-obvious decisions.

Testing Requirements

Agents love to write tests, but they often write them wrong for your specific setup. Naming the test runner, file location pattern, and any required mocking conventions saves enormous back-and-forth.

What NOT to Do

This section has outsized impact. Agents are optimistic problem-solvers — they’ll install a new library to solve a problem when you’d rather they use what’s already there. A clear “don’t do X” list prevents most frustrating surprises.

Architecture Constraints

If your project has specific structural decisions (state management library, API call patterns, folder structure), document them. Agents will infer architecture from your existing code, but they’ll make mistakes when patterns aren’t consistent throughout the codebase.

Tips for Effective AGENTS.md Files

Be specific, not vague. “Follow clean code principles” is useless. “Use descriptive variable names, functions should do one thing, max 30 lines per function” is actionable.

Update it when you make architectural decisions. If you switch from Jest to Vitest, update AGENTS.md the same day. Stale instructions are worse than no instructions.

Keep it focused. Don’t dump your entire engineering wiki into AGENTS.md. Aim for the 10–15 things that trip up a new developer or AI agent most often.

Test it with a fresh agent session. After writing your AGENTS.md, start a new agent session and ask it to make a small change. Watch whether it follows the conventions. If it doesn’t, your instructions need to be clearer.

AGENTS.md vs CLAUDE.md vs .cursorrules

As the ecosystem matures, there’s some fragmentation:

If you’re working with multiple tools, maintain both AGENTS.md and CLAUDE.md with the same content. A one-line symlink or a simple copy-on-update script handles the duplication.

Getting Started

1. Create AGENTS.md in your project root
2. Add sections for conventions, testing, architecture, and anti-patterns
3. Commit it to your repo (agents benefit when it’s in version control)
4. Test with your AI coding tool of choice

The best AGENTS.md is the one you actually maintain. Start small — five clear rules beat a 500-line document you never update.

Sources

1. particula.tech: “AGENTS.md — AI Coding Agent Configuration” — March 16, 2026
2. DeployHQ guide: “20+ tools supporting AGENTS.md”
3. OpenAI Codex CLI documentation

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260318-0800

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