diff --git a/web/chat/chat-input-bar.react.js b/web/chat/chat-input-bar.react.js
--- a/web/chat/chat-input-bar.react.js
+++ b/web/chat/chat-input-bar.react.js
@@ -608,7 +608,7 @@
           ? {
               textBeforeAtSymbol:
                 typeaheadRegexMatches.groups?.textPrefix ?? '',
-              query: typeaheadRegexMatches.groups?.username ?? '',
+              query: typeaheadRegexMatches.groups?.mentionText ?? '',
             }
           : null,
       [typeaheadRegexMatches],
diff --git a/web/utils/typeahead-utils.js b/web/utils/typeahead-utils.js
--- a/web/utils/typeahead-utils.js
+++ b/web/utils/typeahead-utils.js
@@ -3,12 +3,12 @@
 import classNames from 'classnames';
 import * as React from 'react';
 
-import { oldValidUsernameRegexString } from 'lib/shared/account-utils.js';
 import {
   getNewTextAndSelection,
   type MentionTypeaheadSuggestionItem,
   type TypeaheadTooltipActionItem,
 } from 'lib/shared/mention-utils.js';
+import { validChatNameRegexString } from 'lib/shared/thread-utils.js';
 import { stringForUserExplicit } from 'lib/shared/user-utils.js';
 import type { SetState } from 'lib/types/hook-types.js';
 
@@ -18,7 +18,7 @@
 import Button from '../components/button.react.js';
 
 const webMentionTypeaheadRegex: RegExp = new RegExp(
-  `(?<textPrefix>(?:^(?:.|\n)*\\s+)|^)@(?<username>${oldValidUsernameRegexString})?$`,
+  `(?<textPrefix>(?:^(?:.|\n)*\\s+)|^)@(?<mentionText>${validChatNameRegexString})?$`,
 );
 
 export type TooltipPosition = {