diff --git a/app/assets/javascripts/discourse/app/components/d-editor.gjs b/app/assets/javascripts/discourse/app/components/d-editor.gjs
index 3608a7c6904..0d726921235 100644
--- a/app/assets/javascripts/discourse/app/components/d-editor.gjs
+++ b/app/assets/javascripts/discourse/app/components/d-editor.gjs
@@ -38,7 +38,7 @@ import loadEmojiSearchAliases from "discourse/lib/load-emoji-search-aliases";
 import loadRichEditor from "discourse/lib/load-rich-editor";
 import { rovingButtonBar } from "discourse/lib/roving-button-bar";
 import { emojiUrlFor, generateCookFunction } from "discourse/lib/text";
-import userSearch from "discourse/lib/user-search";
+import userSearch, { validateSearchResult } from "discourse/lib/user-search";
 import {
   destroyUserStatuses,
   initUserStatusHtml,
@@ -463,7 +463,10 @@ export default class DEditor extends Component {
       },
       onRender: (options) => renderUserStatusHtml(options),
       key: "@",
-      transformComplete: (v) => v.username || v.name,
+      transformComplete: (v) => {
+        validateSearchResult(v);
+        return v.username || v.name;
+      },
       afterComplete: () => {
         schedule(
           "afterRender",
diff --git a/app/assets/javascripts/discourse/app/lib/search.js b/app/assets/javascripts/discourse/app/lib/search.js
index 272694cdc84..0b5139be92a 100644
--- a/app/assets/javascripts/discourse/app/lib/search.js
+++ b/app/assets/javascripts/discourse/app/lib/search.js
@@ -7,7 +7,7 @@ import getURL from "discourse/lib/get-url";
 import { deepMerge } from "discourse/lib/object";
 import { emojiUnescape } from "discourse/lib/text";
 import { userPath } from "discourse/lib/url";
-import userSearch from "discourse/lib/user-search";
+import userSearch, { validateSearchResult } from "discourse/lib/user-search";
 import { escapeExpression } from "discourse/lib/utilities";
 import Category from "discourse/models/category";
 import Post from "discourse/models/post";
@@ -26,6 +26,7 @@ const logSearchLinkClickedCallbacks = [];
 export function addLogSearchLinkClickedCallbacks(fn) {
   logSearchLinkClickedCallbacks.push(fn);
 }
+
 export function resetLogSearchLinkClickedCallbacks() {
   logSearchLinkClickedCallbacks.clear();
 }
@@ -244,7 +245,10 @@ export function applySearchAutocomplete($input, siteSettings) {
         width: "100%",
         treatAsTextarea: true,
         autoSelectFirstSuggestion: false,
-        transformComplete: (v) => v.username || v.name,
+        transformComplete: (v) => {
+          validateSearchResult(v);
+          return v.username || v.name;
+        },
         dataSource: (term) => userSearch({ term, includeGroups: true }),
       })
     );
diff --git a/app/assets/javascripts/discourse/app/lib/user-search.js b/app/assets/javascripts/discourse/app/lib/user-search.js
index 0f88f6018e6..41dc7014489 100644
--- a/app/assets/javascripts/discourse/app/lib/user-search.js
+++ b/app/assets/javascripts/discourse/app/lib/user-search.js
@@ -9,6 +9,7 @@ import { cloneJSON } from "discourse/lib/object";
 import { userPath } from "discourse/lib/url";
 import { emailValid } from "discourse/lib/utilities";
 import { CANCELLED_STATUS } from "discourse/modifiers/d-autocomplete";
+import { i18n } from "discourse-i18n";
 
 let cache = {},
   cacheKey,
@@ -373,3 +374,27 @@ export default function userSearch(options) {
     );
   });
 }
+
+export function validateSearchResult(obj) {
+  const expectedPropertiesMap = {
+    isUser: {
+      nameProperty: "username",
+      translateKey: "composer.autocomplete.username_missing",
+    },
+    isEmail: {
+      nameProperty: "username",
+      translateKey: "composer.autocomplete.username_missing",
+    },
+    isGroup: {
+      nameProperty: "name",
+      translateKey: "composer.autocomplete.name_missing",
+    },
+  };
+
+  for (const [isEntity, props] of Object.entries(expectedPropertiesMap)) {
+    if (obj[isEntity] && !obj[props.nameProperty]) {
+      throw new Error(i18n(props.translateKey));
+    }
+  }
+  return true;
+}
diff --git a/app/assets/javascripts/discourse/app/modifiers/d-autocomplete.js b/app/assets/javascripts/discourse/app/modifiers/d-autocomplete.js
index 25bd9c7b4ea..220ab628b5b 100644
--- a/app/assets/javascripts/discourse/app/modifiers/d-autocomplete.js
+++ b/app/assets/javascripts/discourse/app/modifiers/d-autocomplete.js
@@ -5,6 +5,7 @@ import { cancel } from "@ember/runloop";
 import { service } from "@ember/service";
 import Modifier from "ember-modifier";
 import DAutocompleteResults from "discourse/components/d-autocomplete-results";
+import { extractError } from "discourse/lib/ajax-error";
 import discourseDebounce from "discourse/lib/debounce";
 import { INPUT_DELAY } from "discourse/lib/environment";
 import { VISIBILITY_OPTIMIZERS } from "float-kit/lib/constants";
@@ -54,6 +55,7 @@ export default class DAutocompleteModifier extends Modifier {
   }
 
   @service menu;
+  @service toasts;
 
   @tracked expanded = false;
   @tracked results = [];
@@ -488,7 +490,17 @@ export default class DAutocompleteModifier extends Modifier {
 
     // Transform if needed
     if (this.options.transformComplete) {
-      term = await this.options.transformComplete(term, event);
+      try {
+        term = await this.options.transformComplete(term, event);
+      } catch (e) {
+        this.toasts.error({
+          duration: "short",
+          data: {
+            message: extractError(e, e.message),
+          },
+        });
+        return;
+      }
     }
 
     if (!term) {
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index ff04013dd7e..31f416a77cf 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -2880,6 +2880,10 @@ en:
       publishing: "Publishing Topic…"
 
     composer:
+      autocomplete:
+        username_missing: "Selected autocomplete result is missing username."
+        name_missing: "Selected autocomplete result is missing name."
+
       emoji: "Emoji :)"
       more_emoji: "more…"
       options: "Options"
diff --git a/plugins/chat/assets/javascripts/discourse/components/chat-composer.gjs b/plugins/chat/assets/javascripts/discourse/components/chat-composer.gjs
index 3f98d2a0aa2..990b88d7860 100644
--- a/plugins/chat/assets/javascripts/discourse/components/chat-composer.gjs
+++ b/plugins/chat/assets/javascripts/discourse/components/chat-composer.gjs
@@ -33,7 +33,7 @@ import { cloneJSON } from "discourse/lib/object";
 import optionalService from "discourse/lib/optional-service";
 import { emojiUrlFor } from "discourse/lib/text";
 import { TextareaAutocompleteHandler } from "discourse/lib/textarea-text-manipulation";
-import userSearch from "discourse/lib/user-search";
+import userSearch, { validateSearchResult } from "discourse/lib/user-search";
 import {
   destroyUserStatuses,
   initUserStatusHtml,
@@ -482,7 +482,7 @@ export default class ChatComposer extends Component {
         if (obj.isUser) {
           this.#addMentionedUser(cloneJSON(obj));
         }
-
+        validateSearchResult(obj);
         return obj.username || obj.name;
       },
       dataSource: (term) => {
diff --git a/plugins/discourse-ai/assets/javascripts/discourse/components/ai-bot-conversations.gjs b/plugins/discourse-ai/assets/javascripts/discourse/components/ai-bot-conversations.gjs
index 5604681ac6a..905cef33ec8 100644
--- a/plugins/discourse-ai/assets/javascripts/discourse/components/ai-bot-conversations.gjs
+++ b/plugins/discourse-ai/assets/javascripts/discourse/components/ai-bot-conversations.gjs
@@ -19,7 +19,7 @@ import userAutocomplete from "discourse/lib/autocomplete/user";
 import { setupHashtagAutocomplete } from "discourse/lib/hashtag-autocomplete";
 import UppyUpload from "discourse/lib/uppy/uppy-upload";
 import UppyMediaOptimization from "discourse/lib/uppy-media-optimization-plugin";
-import userSearch from "discourse/lib/user-search";
+import userSearch, { validateSearchResult } from "discourse/lib/user-search";
 import {
   destroyUserStatuses,
   initUserStatusHtml,
@@ -204,7 +204,10 @@ export default class AiBotConversations extends Component {
       width: "100%",
       treatAsTextarea: true,
       autoSelectFirstSuggestion: true,
-      transformComplete: (obj) => obj.username || obj.name,
+      transformComplete: (obj) => {
+        validateSearchResult(obj);
+        return obj.username || obj.name;
+      },
       afterComplete: (text) => {
         this.textarea.value = text;
         this.focusTextarea();