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/test/routes/users.test.ts

@@ -59,6 +59,8 @@ test("public profile: by @name in any case or id, without private fields", async
       `/api/users/@${user.username}`,
       `/api/users/@${user.username.toUpperCase()}`,
       `/api/users/${user.id}`,
+      // As another instance writes them.
+      `/api/users/@${user.username}@gerbeur.test`,
     ]
   ) {
     const res = await get<PublicUser>(path);
@@ -68,6 +70,10 @@ test("public profile: by @name in any case or id, without private fields", async
     assertFalse("passwordHash" in res.data);
   }
   assertEquals((await get(`/api/users/@no_one_here`)).status, 404);
+  assertEquals(
+    (await get(`/api/users/@${user.username}@elsewhere.test`)).status,
+    404,
+  );
   // Without the `@`, the key is an id.
   assertEquals((await get(`/api/users/${user.username}`)).status, 404);
   const me = await get<SelfUser>("/api/me", { auth: user.auth });