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/services/notification-service.ts

@@ -25,6 +25,8 @@ import {
 import { type Page, pageOf } from "../lib/pagination.ts";
 import { sendToUser } from "./ws-service.ts";
 import { mentionedUsernames } from "../../shared/mention.ts";
+import { localHandle } from "../../shared/validation.ts";
+import { isSelfHost } from "../lib/self-url.ts";
 
 /** Each id of a user a notification names, beside the field of their name. */
 const USER_FIELDS = [
@@ -359,7 +361,7 @@ export function notifyMentions(
   for (const username of usernames) {
     const mentionedRow = db.prepare(
       `SELECT id FROM users WHERE lower(username) = ?;`,
-    ).get(username) as { id: string } | undefined;
+    ).get(localHandle(username, isSelfHost)) as { id: string } | undefined;
     if (!mentionedRow || mentionedRow.id === mentionerUserId) continue;
     if (skipUserId?.(mentionedRow.id)) continue;