v4: a moderator's or an admin's @everyone in the chat notifies every user — there was no way to reach everyone at once. The new chat:everyone permission, held by moderators and admins, lets a chat message saying @everyone notify every user but its author and those reading the chat, through notifyEveryone, after notifyMentions, so a user also named is told once, by name; the notification is a user_mentioned with everyone set, which the notifications page and the digest word as "mentioned everyone in the chat". chat_messages.mentions_everyone, sent as ChatMessage.mentionsEveryone, is set from the author's role each time the body is written, so a user's @everyone reaches no one, a moderator's edit of a user's message can't make it reach everyone, and a demoted author's next edit loses it. Only an edit that adds @everyone notifies, so fixing an announcement's typo reaches no one new. The client highlights and chimes by the flag, never by the text; Markdown shows @everyone as a mention only in a flagged message, and never links it to a profile. The chat composer offers @everyone first to those who may say it. No account may be named everyone, in any case: migration 0023 adds the column, flags the moderators' and admins' past @everyone messages, renames a user named everyone to everyone_N and drops an alias everyone. 265 backend tests and 681 frontend tests pass, and deno check, tsc, eslint, doc:lint and i18n:compile --strict are clean; checked in screenshots of the chat at 1280 and 390px, with a moderator's and a user's @everyone and the suggestions open.

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

shared/mention.ts

@@ -31,3 +31,20 @@ export function mentionedUsernames(body: string): string[] {
     ...new Set([...prose.matchAll(MENTION_RE)].map((m) => m[1].toLowerCase())),
   ];
 }
+
+/**
+ * The name `@everyone` reaches. Said by a role with `chat:everyone` in the
+ * chat, it notifies every user, so no account may take it
+ * (`assertUsernameAvailable`).
+ */
+export const EVERYONE = "everyone";
+
+/**
+ * Checks whether `body` says `@everyone`, in any case, by the rule of
+ * {@link mentionedUsernames}. Only the chat service reads it, to set a
+ * message's `mentionsEveryone` when its author may; the client reads that
+ * flag, not the text.
+ */
+export function mentionsEveryone(body: string): boolean {
+  return mentionedUsernames(body).includes(EVERYONE);
+}