The short version: they compose, they don’t compete. Each well-known file answers one question; capabilities.txt answers the one none of the others do — what can this host actually do?
| File | Answers | For | Static? | Job |
|---|---|---|---|---|
robots.txt | What may a crawler access? | Crawlers | Yes | Access control |
sitemap.xml | What pages exist? | Search engines | Yes | Indexing |
llms.txt | What's worth reading? | LLMs reading | Yes | Curated content for models |
OpenAPI | Exact shape of an HTTP API | Developers / clients | Yes | Full API contract |
mcp.json | I run an MCP server | MCP clients | Yes (points to a server) | Advertise an MCP endpoint |
capabilities.txt | What can this host DO? | Agents acting | Yes | Discover invocable capabilities |
llms.txt says what content is worth reading; capabilities.txt says what you can do. Same spirit and format conventions, the next layer — reading vs. acting.
mcp.json advertises that you run an MCP server, so discovery is connection-first (a client connects to learn the tools). capabilities.txt is invocation-agnostic and static — readable with no connection and no MCP client, and it can point at an MCP server, a plain HTTP API, anything. Most sites have a REST API and no MCP server. They compose: capabilities.txt is the catalog; mcp.json is one endpoint it hands off to.
OpenAPI is the detailed contract for an HTTP API. capabilities.txt is a one-screen, well-known, framework-agnostic advertisement of what you can do that can point at any invocation method. You generate a capabilities.txt from your OpenAPI — they are different altitudes of the same goal.
MCP is how an agent invokes a tool over a live connection. capabilities.txt is the static, crawlable discovery layer before invocation; it points at your MCP server (or API) for the actual call. Complementary, not competing.
A site can publish all of them: robots.txt (crawl rules), llms.txt (what to read), and capabilities.txt (what to do) — which points at your OpenAPI or MCP server for invocation, and at CHP if you need governance and provable evidence for the actions. Discovery, invocation, and evidence are different jobs.