If a given server doesn’t have a SRV record instructing where to look, I take it it just queries that domain directly. If so, putting the API at /plan/ is problematic, because in situations where user content is hosted from a domain it can allow any user to control the appearance of the domain by hosting certain stuff under /plan/, and also various domains will already be using /plan/ for something else.<p>The solution for this is well-known URIs <<a href="https://www.rfc-editor.org/rfc/rfc8615.html" rel="nofollow">https://www.rfc-editor.org/rfc/rfc8615.html</a>>: put it under /.well-known/, where all such hosting providers should know not to allow users to place files.<p>So perhaps /.well-known/plan/ or /.well-known/dotplan/. And if you’re serious about this project, once you’ve got the basics ironed out and specified in detail, and probably some degree of interest from others, request registration in the register <<a href="https://github.com/protocol-registries/well-known-uris" rel="nofollow">https://github.com/protocol-registries/well-known-uris</a>>.