AGENTS.md Best Practices: Clear Instructions for Coding Agents

AI Summary: AGENTS.md is the emerging open standard for repository-level agent governance. Recognized by Cursor, Claude Code, Cline, Codex, and OpenAI Operator, it establishes an authoritative, vendor-neutral contract for build commands, architectural boundaries, testing conventions, and non-negotiable prohibitions, eliminating repetitive prompt boilerplate across developer workflows.

#The Multi-Agent Reality: Why Proprietary Configs Fail

In modern engineering teams, developers use diverse AI tooling: one engineer refactors in Cursor, another reviews diffs using Claude Code CLI in the terminal, while a CI runner executes automated pull-request reviews using an OpenAI o3 agent.

Historically, teams were forced to duplicate instructions across proprietary configuration files:

• .cursorrules or .cursor/rules/*.mdc (Cursor)
• CLAUDE.md (Claude Code)
• .copilot/instructions.md (GitHub Copilot)
• System prompts in CI scripts

AGENTS.md solves this fragmentation by providing an open, Markdown-based specification that all agents parse automatically upon workspace initialization.

#The Structural Hierarchy: Monorepo Inheritance

For monorepo architectures, AGENTS.md supports hierarchical scoping. Agents resolve rules using a nearest-ancestor tree walk:

workspace-root/
├── AGENTS.md                  <-- Global policies (Git, Branching, CI, Security)
├── services/
│   ├── api-gateway/
│   │   └── AGENTS.md          <-- Scoped policies (FastAPI, Python 3.12, Poetry)
│   └── web-frontend/
│       └── AGENTS.md          <-- Scoped policies (Next.js 16, RSC, CSS Modules)

Rules declared in child AGENTS.md files inherit global invariants while overriding directory-specific build, lint, and test commands.

#The 4 Pillars of a Production AGENTS.md

A production-grade AGENTS.md must be concise (under 800 tokens) and strictly partitioned into four operational sections:

# Repository Governance for Autonomous Agents

## 1. Fast Feedback Commands
- Build: `pnpm build`
- Typecheck: `pnpm typecheck`
- Unit Tests: `pnpm test`
- E2E Tests: `pnpm exec playwright test --workers=3`

## 2. Architecture & Code Conventions
- Framework: Next.js 16 App Router + React 19 + TypeScript strict mode.
- Styling: Pure Vanilla CSS; no TailwindCSS. Use CSS variables defined in `src/app/globals.css`.
- Network Boundaries: All outbound HTTP requests must pass through `src/lib/safe-fetch.ts` to prevent SSRF vulnerabilities.

## 3. Hard Prohibitions (Never Violate)
- Never commit directly to `main` — work must occur on `round-N` branches.
- Never run destructive git commands (`git reset --hard`, `git push --force`, `git clean -fd`).
- Never introduce new third-party npm packages without explicit developer permission.
- Never edit files outside the current project boundary.

## 4. Verification Gate Before Task Completion
Before concluding any task, execute:
`pnpm typecheck && pnpm test && pnpm build`
If any command fails, fix the error and re-verify before asking for review.

#The Power of Negative Invariants

LLMs are naturally biased toward aggressive completion. If left unrestricted, an agent attempting to resolve a missing utility will install new dependencies, alter build configurations, or mock out failing tests to achieve an artificial green exit.

To prevent agent drift, write Negative Invariants with Concrete Penalties:

#AGENTS.md vs Adjacent Configuration Files

#Related guidance

To configure editor-specific tooling, study Cursor IDE Optimization, learn terminal governance in Claude Code, and explore What is llms.txt?.

#References

• The AGENTS.md Standard Initiative: Community specifications for machine-readable repository instructions.
• Cursor Documentation: Project Rules: Guidelines on how modern editors parse root and nested instruction files.

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