An llms.txt is a small file at a known location on your domain that tells a machine consumer what you offer and where the important things live. It costs almost nothing to publish and it can genuinely help an agent orient — a pointer to your OpenAPI, your key documentation, your terms, your support channel.
Two cautions worth holding.
First, it is not a contract. A machine-readable summary of your documentation does not replace an accurate OpenAPI and good JSON Schema, and a beautiful llms.txt in front of a vague contract does not make an API agent-ready. If you have limited effort, spend it on the schema.
Second, nothing enforces it. It states a preference; the edge decides what actually happens. I have found real cases of providers whose stated policy and deployed behavior disagree, and the deployed behavior is the one that counts. If you publish one, make sure it describes what your infrastructure actually does.
So: publish one, keep it short and accurate, point at the real artifacts, and do not mistake it for the work.
