`.nvmrc`

One line naming the exact Node version. The guards are dependency-free but not version-agnostic, and CI reads this to pick its runtime.

24.18.0

Exact, not a range. lts/* resolves to something different in six months, which is the whole problem R21 exists to prevent: a runtime that changes under you with no diff to review.

Why exact, when a range would still work today

It would, right up until it does not, and the failure is the confusing kind - the same commit passing yesterday and failing today, with nothing in the history to explain it. The minutes saved by a range are paid back in one afternoon of that.

What does not go in here

A range, an alias, or a codename. lts/*, >=20, iron all defeat the point.

Your local preference. This is what CI runs and what the repository is verified against. If your machine needs something else, that is your machine's problem to reconcile - which is exactly what the file lets you notice.

Decisions behind it

R21 - everything a repository consumes is pinned exact and moves by a reviewed diff. The runtime is consumed like anything else.
It ships as copy rather than fill-from-repo. The standard's guards were tested against a specific runtime, so shipping a version is more useful than shipping a blank - and changing it is one line in a pull request that says why.

If your repository runs a different Node

Then this file is the clearest example of a copy file you are meant to change, and because copy files are content-checked, changing it is drift until you say it was deliberate:

"exceptions": [
  { "kind": "content", "match": ".nvmrc", "reason": "this repo runs Node 22; the shipped guards are dependency-free and pass on it" }
]

That is one line in your manifest copy, it survives updates, and it is what the next agent reads instead of guessing whether the difference was a mistake.

Reference

Status. Optional - part of the core profile.
How it arrives. copy - ship verbatim; the repo gets a byte-for-byte copy (guards, templates that carry no repo-specific content)
Shipped form. read it in the standard's own tree

The rule that requires it

Everything a repo consumes MUST be pinned exact and move only by an

See it in the spec