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>

src/test/utils/chat/text.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, test } from "vitest";
 import { mentionsUser } from "../../../utils/chat/text.ts";
+import { API_URL } from "../../../config/api.ts";
 
 describe("mentionsUser", () => {
   test("case-insensitive whole-word match", () => {
@@ -11,6 +12,14 @@ describe("mentionsUser", () => {
     expect(mentionsUser("[@alice](x)", "alice")).toBe(false);
     expect(mentionsUser("(@alice)", "alice")).toBe(false);
   });
+  test("this instance's host, as another instance writes them, is them", () => {
+    const host = new URL(API_URL).host;
+    expect(mentionsUser(`hey @Alice@${host}`, "alice")).toBe(true);
+    expect(mentionsUser("hey @alice@elsewhere.example", "alice")).toBe(false);
+    expect(
+      mentionsUser("hey @alice@elsewhere.example", "alice@elsewhere.example"),
+    ).toBe(true);
+  });
   test("any of several mentions", () => {
     expect(mentionsUser("@bob @carol @alice", "alice")).toBe(true);
   });

src/utils/auth/home.ts

@@ -10,6 +10,17 @@
 import { API_URL } from "../../config/api.ts";
 import { splitHandle } from "../../../shared/validation.ts";
 
+/**
+ * Checks whether `host`, as a handle writes it, is this instance's: its API's,
+ * which handles carry, or its pages'. Another instance writes this one's users
+ * with it.
+ */
+export function isSelfHost(host: string): boolean {
+  const h = host.toLowerCase();
+  return h === new URL(API_URL || location.origin).host ||
+    h === location.host;
+}
+
 /**
  * Returns the host of `handle`'s home instance, or undefined for a local
  * user.

src/utils/chat/text.ts

@@ -5,12 +5,17 @@
  */
 
 import { mentionedUsernames } from "../../../shared/mention.ts";
+import { localHandle } from "../../../shared/validation.ts";
+import { isSelfHost } from "../auth/home.ts";
 
 /**
  * Checks whether `body` @mentions `username`, ignoring case, with the pattern
- * the server uses for mention notifications. `WSProvider` uses it to pick the
- * chime, and `ChatModal` to highlight the message.
+ * the server uses for mention notifications, this instance's own host
+ * included (`localHandle`). `WSProvider` uses it to pick the chime, and
+ * `ChatModal` to highlight the message.
  */
 export function mentionsUser(body: string, username: string): boolean {
-  return mentionedUsernames(body).includes(username.toLowerCase());
+  return mentionedUsernames(body)
+    .map((name) => localHandle(name, isSelfHost))
+    .includes(username.toLowerCase());
 }