Short answer: llms.txt is a Markdown file at your site root that points a model at your most useful pages in a compact, stable form. It is an emerging convention, not a standard anyone has promised to honour — and any guide that doesn't tell you that is selling something.
Why ship it anyway
Two reasons that hold regardless of adoption. First, the cost is one file. Second, writing it forces a useful exercise: you have to decide which twenty pages actually represent what you do, in one sentence each. Most teams discover in the process that several of those pages don't exist yet.
The format
Markdown. An H1 with the site name, a blockquote summary, then H2 sections of links with a short description after each. Keep descriptions to one line and factual.
# Example Co
> Example Co makes inventory software for independent retailers in the UK and Ireland.
## Product
- [Pricing](https://example.com/pricing/): Three plans from £29 to £249 per month, billed per location.
- [Integrations](https://example.com/integrations/): Supported POS and accounting systems, updated 2026-09.
## Documentation
- [Getting started](https://example.com/docs/start/): Setup in under an hour, including data import.
## Company
- [About](https://example.com/about/): Founded 2019, 24 staff, based in Bristol.Three mistakes worth avoiding
- Generating it from your sitemap. A list of 400 URLs with no descriptions is worse than nothing; the value is in the selection and the one-line summaries.
- Letting it go stale. A file claiming prices you changed six months ago is a liability. Generate it from the same source of truth as your routes if you can — ours is built from route data at build time, so it cannot drift.
- Treating it as a substitute for crawlability. If your pages return 403 to retrieval agents, a beautiful llms.txt changes nothing.
How to check it
Validate the structure and confirm every link resolves — a file full of 404s is a worse signal than no file. Then re-check after any routing change, because that is when links break silently.