What belongs in a repo's agent instructions

Rules for deciding what goes in the block, for judging what a repo already has, and for explaining both to the user.

The test

Can an agent derive this by reading the repository? If yes, leave it out — a stored copy is a stale duplicate of something the agent reads more accurately first-hand, and it is charged on every session. Write down what the code cannot say.

Admit

Policy the code cannot express — branch rules, frozen and protected paths, generated files, secrets, security and compliance. Stated by a human or read off an enforcing config, never inferred.
What a config file cannot say about running the project — the root test script does nothing in this workspace, integration tests need a service up first, the suite takes eleven minutes so iterate on single files, the Makefile is the real entry point and package.json is vestigial, CI runs a typecheck the test script does not. The invocation itself is already stated in package.json, Makefile, pyproject.toml, or CI config and does not earn a line — the correction or the caveat does.
Conventions that differ from ecosystem defaults. An agent follows the norm unless told otherwise, so only the divergences earn a line. Command invocations count: when the obvious command is wrong here — a bare-repo prefix, a required wrapper — the exact working invocation earns a line, and no observed mistake is needed to admit it.
Pitfalls with observed evidence — a recorded lesson, the maintainer's recollection, the same mistake fixed repeatedly in history, or one this session made and caught. A repo yields hundreds of trap-looking facts and none of them predict real mistakes; only observed behavior does. A surprising scan finding is a question to ask, not a line to write.
Runtime behavior invisible from the repo — replaying webhooks, lying health endpoints, environment quirks — once a human confirms it.
Entry points and pointers to where work lands.

Prefer prohibitions to advice, and name the permitted alternative in the same line.

Exclude

	Why
Repo overviews, directory trees, stack lists	Derived fresh, more accurately; stored copies rot
Anything included for being interesting	Interest is not need
Style rules an agent self-enforces	Belongs in a formatter, linter, hook, or CI check — propose the check instead
Platitudes	Already the default
Commands already stated in `package.json`, a `Makefile`, or CI config	Read from the source of truth; a copy drifts the moment a script is renamed
Pasted code, changelog content, fast-changing facts	Stale immediately
Aspirational state	Describe what is; intent belongs in specs
History and edit narration	Git holds it; state present truth

Retire

A policy or pitfall line goes only when the thing it guards is gone, or the user retires it. Nothing failing lately is not evidence — a working rule erases its own evidence.

Every other line faces one question at each write: would removing it change agent behavior? If no, cut it.

Size

Every line is paid in every session, and instruction-following degrades as the loaded set grows. Count what other always-loaded files add. Over budget means cut the weakest lines or move them behind a trigger — never raise the budget. Ten lines of evidence means ten lines.

Retrieval

An index the agent must choose to fetch gets skipped; one already in context does not. Keep everything load-bearing in the block. A pointer out of it names a trigger the agent can observe — a path, a file type, a named task — never one it must judge ("when the task is complex") or track about itself ("before your first edit").

Rules bounded to a directory go in a nested AGENTS.md there, attached by location rather than by pointer. Use a linked file only when the trigger is not a path.

Maintain

Re-check that caveats still hold — a slow suite that got fast, a workaround for a bug that was fixed.
Diff deletions and renames since the verified SHA against every line.
Record provenance in the block so the next run knows what it is diffing from.
Capture mistakes when they happen, not at review time. One occurrence is a note; recurrence earns a line.
Route anything mechanically preventable to a hook, lint rule, or CI check. A check that lands deletes its line.

Repo or home directory

This block belongs committed: shared by the team, consistent across machines, versioned with the code it constrains.

Two things belong in the user's global agent config instead — rules repeating across all their projects, and personal preferences that are theirs rather than the team's.

Judging an existing file

Report, in this order: what is derivable filler, what is unverifiable or stale, what is missing against the sections above, and what is already good. Keep recorded lessons by default — they are maintainer testimony, and are challenged only with evidence that the thing they name is gone or wrong.

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