Definition
.well-known
/.well-known/ is the standard URI prefix, defined by RFC 8615, where a site publishes machine-readable metadata files at predictable paths.
The problem it solves is discovery without guessing. Rather than each service inventing a location for its metadata, RFC 8615 reserves one prefix and an IANA registry records what lives under it. Long-standing examples include `/.well-known/security.txt` and `/.well-known/openid-configuration`.
It is now where the agent standards are landing: `/.well-known/api-catalog` from RFC 9727, MCP server cards at `/.well-known/mcp.json` or `/.well-known/mcp/server-card.json`, and, by convention rather than registration, llms.txt mirrored at `/.well-known/llms.txt`. An agent that has never seen your site has a short list of paths worth trying, and that list is the well-known prefix.
Two operational notes. Publish one copy and redirect the other path to it rather than maintaining duplicates, because two copies of a facts file is one copy plus a future contradiction. And check that your CDN or framework does not intercept the prefix: a well-known path that returns your SPA shell is a well-known path that is not published.
Frequently asked questions
Which .well-known files matter for AI visibility?
api-catalog if you have a public API, an MCP server card if you run an MCP server, and llms.txt by convention. None is required by any engine as of July 2026.
Should I duplicate llms.txt under .well-known?
Serve one canonical file and redirect the other path to it. One document means one thing to keep correct.