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>

shared/test/federation.test.ts

@@ -4,7 +4,7 @@ import {
   isFederationEvent,
   parseInstance,
 } from "../federation.ts";
-import { HANDLE_RE, splitHandle } from "../validation.ts";
+import { HANDLE_RE, localHandle, splitHandle } from "../validation.ts";
 
 Deno.test("parseInstance: a host or a URL names an instance's origin", () => {
   assertEquals(parseInstance("music.example"), "https://music.example");
@@ -21,6 +21,13 @@ Deno.test("parseInstance: a host or a URL names an instance's origin", () => {
   assertEquals(instanceHost("http://localhost:8001"), "localhost:8001");
 });
 
+Deno.test("handles: this instance's host is dropped, another's kept", () => {
+  const self = (host: string) => host === "gerbeur.example";
+  assertEquals(localHandle("bob@gerbeur.example", self), "bob");
+  assertEquals(localHandle("bob@music.example", self), "bob@music.example");
+  assertEquals(localHandle("bob", self), "bob");
+});
+
 Deno.test("handles: a name, and a host for a user from another instance", () => {
   assertEquals(splitHandle("bob"), { name: "bob" });
   assertEquals(splitHandle("alice@music.example"), {

shared/validation.ts

@@ -75,6 +75,22 @@ export function splitHandle(handle: string): { name: string; host?: string } {
     : { name: handle.slice(0, at), host: handle.slice(at + 1) };
 }
 
+/**
+ * Returns `handle` as this instance writes it, `isSelfHost` telling its own
+ * host. Another instance writes a user from here with this one's host,
+ * `bob@gerbeur.example`, who is `bob` here: a mention or a link copied from
+ * there names them so. Any other handle is returned as it is. The server
+ * resolves names by it (`resolveUsername`, the mention notifications), and
+ * the chat's highlight (`mentionsUser`) reads mentions by it.
+ */
+export function localHandle(
+  handle: string,
+  isSelfHost: (host: string) => boolean,
+): string {
+  const { name, host } = splitHandle(handle);
+  return host && isSelfHost(host) ? name : handle;
+}
+
 /**
  * How often a dump's or a playlist's slug may change, counted from the aliases
  * the changes left: 3 times a day. Moderators are exempt. slug-service refuses