TL;DR. llms.txt is the AEO (Answer Engine Optimization) equivalent of robots.txt + sitemap.xml combined — a Markdown file at your root that tells AI engines what your site is about and where to find structured information. In 2026, ecommerce stores without llms.txt are leaving citation opportunities on the table.
What llms.txt is
A simple Markdown file at https://yourdomain.com/llms.txt. It's a community standard proposed by Jeremy Howard (fast.ai) in 2024 and adopted opportunistically by Anthropic, Perplexity, and other AI engines.
The structure:
# Brand Name
> A 1-2 sentence summary of what the brand does and who it serves.
## About
Brief 200-word description of the brand.
## Products
- [Product category 1](https://example.com/categories/cat-1) — short description
- [Product category 2](https://example.com/categories/cat-2) — short description
## Documentation
- [Help center](https://example.com/help)
- [API docs](https://example.com/docs)
## Pricing
- [Pricing page](https://example.com/pricing)
## Optional
- [Blog](https://example.com/blog)
- [Comparison pages](https://example.com/compare)
- [Glossary](https://example.com/glossary)That's it. Plain Markdown, well under 5KB.
Why AI engines care
Crawling a 50,000-page ecommerce site is expensive. AI engines that index for retrieval (Perplexity, ChatGPT Search) and AI engines that train (Anthropic, OpenAI) both benefit from a curated entry point.