Agent Instructions

Agent Instructions

Most AI coding tools read project guidance from a file in the repo — historically each looked in a different place (AGENTS.md, CLAUDE.md, .github/copilot-instructions.md, …). Copy the same guidance into each one and the copies drift: a command changes in one file and the others keep telling agents the old way. The fix is one canonical source plus thin tool-specific shims (which include the canonical file). Keep all durable guidance in the canonical file; keep only genuine tool-specific quirks in a tool's own file.

The good news: AGENTS.md is now read by nearly every major tool — including GitHub Copilot — so the "one canonical file" ideal is finally achievable without a per-tool copy.

The canonical file: AGENTS.md

AGENTS.md is the cross-tool standard — a plain-Markdown file at the repo root, read natively by a growing set of agents and editors (Codex, Cursor, Gemini CLI, and GitHub Copilot). Make it the single source of truth for everything durable and tool-neutral:

• What the project is — one-paragraph purpose, the stack, where the important code lives.
• How to build, test, validate, and run — the exact commands an agent should use (and which one gates a PR). Be specific; agents run these verbatim.
• Conventions — code style, naming, commit/PR format, branch model, "definition of done".
• Do / don't — guardrails (never hand-edit generated files, never push to main, secret handling) and the project's non-obvious gotchas.
• Pointers — links to deeper docs rather than inlining them.

Write it tool-neutral: no "when you are Claude…" assumptions. Anything true only for one tool goes in that tool's file, not here. Keep it readable and scoped — an agent re-reads it every turn, so length is a cost.

Nested AGENTS.md. In a monorepo or a large sub-package, drop an AGENTS.md in the subdirectory: tools that support the standard read the nearest one and let it take precedence over the root on conflicts. Reach for this before a path-scoped tool-specific file when the guidance is itself tool-neutral.

Map each tool to its file

Shims need no upkeep. A shim (CLAUDE.md, GEMINI.md) just includes the canonical file, so it never drifts. Everything else reads AGENTS.md directly.

Copilot now reads AGENTS.md

GitHub Copilot used to be the exception that forced a second file — it couldn't read AGENTS.md, so you maintained a .github/copilot-instructions.md. That is no longer true:

• Copilot code review reads AGENTS.md from the repo root (changelog, 2026-06-18).
• Copilot coding agent reads AGENTS.md too (root + nested, nearest-wins).

So AGENTS.md alone now covers Copilot, and a separate .github/copilot-instructions.md is optional, not required:

• Most repos can drop it and rely on AGENTS.md — one canonical file, no subset to keep in sync. Consolidating an existing copilot-instructions.md into AGENTS.md and deleting it is now a legitimate simplification. (It used to silently strip Copilot's guidance, because Copilot couldn't read AGENTS.md; that risk is gone.)
• Keep one only for a concrete reason. Copilot's coding agent treats .github/copilot-instructions.md as always-on context, so a short file there can pin the few rules you want weighted highest. If you keep it, make it a concise subset — short imperative rules, not a copy of AGENTS.md. Brevity is what makes the emphasis work; a long always-on file dilutes it. (Copilot code review's old ~4000-char instruction truncation was removed on 2026-06-12, so length is an emphasis concern now, not a hard limit.)
• Path-scoped rules live in .github/instructions/<area>.instructions.md (with an applyTo: frontmatter glob) — guidance for only part of the tree (tests, infra, a sub-package). Useful with or without a copilot-instructions.md; excludeAgent: "code-review" / "copilot-coding-agent" targets one consumer.

Keep them in sync (definition of done)

Shims (CLAUDE.md/GEMINI.md that @-include) never drift — that's their point. If you keep any hand-maintained second file (a Copilot subset, or a path-scoped .instructions.md), it can drift, so make sync part of done:

A change that touches a command, flag, path, label, generated-file list, validation step, or convention updates every instruction file that referenced it in the same PR — the AGENTS.md canonical text and any subset / path-scoped file.

A stale instruction file is worse than none: it actively misleads every future agent and reviewer. The surest way never to go stale is a single AGENTS.md with no hand-maintained copies.

Recipe: wire up a repo

1. Write AGENTS.md — the canonical, tool-neutral guidance (sections above). This is the bulk of the work; everything else points back to it. Codex, Cursor, Gemini, and Copilot read it directly.
2. Add shims for include-capable tools you support:
   • CLAUDE.md → a single line: @AGENTS.md
   • GEMINI.md → include AGENTS.md the same way.
3. (Optional) .github/copilot-instructions.md — only if you want a few always-on rules weighted highest for Copilot's coding agent; keep it a concise subset, not a dump of AGENTS.md.
4. (Optional) .github/instructions/<area>.instructions.md with applyTo: globs for rules that apply to only part of the tree (tests, infra, a sub-package).
5. Record the sync rule in AGENTS.md if you kept any hand-maintained second file.

Pitfalls checklist

• ❌ Duplicating full AGENTS.md content into CLAUDE.md/GEMINI.md — use a one-line include instead so there is a single source.
• ❌ Keeping a .github/copilot-instructions.md that just duplicates AGENTS.md — now that Copilot reads AGENTS.md, a full copy is pure drift risk; drop it, or trim it to a small focused subset.
• ❌ Putting new canonical guidance in a kept copilot-instructions.md — durable rules belong in AGENTS.md; a subset only re-emphasises a few of them.
• ❌ Letting any kept .github/copilot-instructions.md grow into a parallel rulebook — the old ~4000-char review truncation is gone, but a long always-on file dilutes the emphasis that justified keeping it; split path-specific rules into .instructions.md files.
• ❌ Putting tool-specific assumptions in AGENTS.md — keep it neutral; quirks go in that tool's file.
• ❌ Changing a command/path/convention in AGENTS.md without updating any kept subset in the same PR — that is how the files go stale.