v4: a handle with this instance's own host names the local user — another instance writes bob from here as bob@gerbeur.example, so a mention or a link copied from there named no one here: /@bob@gerbeur.example was a 404, and the mention notified nobody. localHandle (shared/validation.ts) drops this instance's host, its API's or its pages', from a handle; resolveUsername reads names by it, so the page, the API and the Open Graph 301 reach bob at /@bob, and so do the mention notifications and the chat's highlight and chime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

API.md

@@ -64,7 +64,9 @@ page to a comment.
 - **A user from another instance** is `name@host` here: their name at home and
   their home's host, with its port if it has one (`alice@music.example`). Their
   name is their home's to change, which leaves the old handle an alias here too.
-  Mentions write the whole handle.
+  Mentions write the whole handle. A handle with this instance's own host, as
+  another instance writes a user from here (`bob@gerbeur.example`), names the
+  local `bob`, in a path, an API key or a mention.
 - **Aliases.** A slug or a name left behind stays an alias for good, which
   always names the item or user itself, so redirects never chain. Nobody else
   can take one. An item can take back one of its own. Deleting an item frees its