diff --git a/lib/shared/search-utils.js b/lib/shared/search-utils.js
--- a/lib/shared/search-utils.js
+++ b/lib/shared/search-utils.js
@@ -4,7 +4,11 @@
 
 import { messageID } from './message-utils.js';
 import SearchIndex from './search-index.js';
-import { getContainingThreadID, userIsMember } from './thread-utils.js';
+import {
+  getContainingThreadID,
+  userIsMember,
+  userSupportsThickThreads,
+} from './thread-utils.js';
 import {
   searchMessagesActionTypes,
   useSearchMessages as useSearchMessagesAction,
@@ -24,6 +28,7 @@
 } from '../selectors/chat-selectors.js';
 import { useUserSearchIndex } from '../selectors/nav-selectors.js';
 import { relationshipBlockedInEitherDirection } from '../shared/relationship-utils.js';
+import type { AuxUserInfos } from '../types/aux-user-types.js';
 import type { MessageInfo, RawMessageInfo } from '../types/message-types.js';
 import type {
   RoleInfo,
@@ -36,6 +41,7 @@
 import {
   type ThreadType,
   threadTypeIsSidebar,
+  threadTypeIsThick,
 } from '../types/thread-types-enum.js';
 import type {
   AccountUserInfo,
@@ -52,7 +58,7 @@
 
 function appendUserInfo({
   results,
-  excludeUserIDs,
+  shouldExcludeUserFromResult,
   userInfo,
   parentThreadInfo,
   communityThreadInfo,
@@ -65,14 +71,14 @@
       isMemberOfContainingThread: boolean,
     },
   },
-  +excludeUserIDs: $ReadOnlyArray<string>,
+  +shouldExcludeUserFromResult: (userID: string) => boolean,
   +userInfo: AccountUserInfo | GlobalAccountUserInfo,
   +parentThreadInfo: ?ThreadInfo,
   +communityThreadInfo: ?ThreadInfo,
   +containingThreadInfo: ?ThreadInfo,
 }) {
   const { id } = userInfo;
-  if (excludeUserIDs.includes(id) || id in results) {
+  if (id in results || shouldExcludeUserFromResult(id)) {
     return;
   }
 
@@ -104,6 +110,7 @@
 function usePotentialMemberItems({
   text,
   userInfos,
+  auxUserInfos,
   excludeUserIDs,
   includeServerSearchUsers,
   inputParentThreadInfo,
@@ -112,6 +119,7 @@
 }: {
   +text: string,
   +userInfos: { +[id: string]: AccountUserInfo },
+  +auxUserInfos: AuxUserInfos,
   +excludeUserIDs: $ReadOnlyArray<string>,
   +includeServerSearchUsers?: $ReadOnlyArray<GlobalAccountUserInfo>,
   +inputParentThreadInfo?: ?ThreadInfo,
@@ -149,6 +157,20 @@
     return null;
   }, [containingThreadID, communityThreadInfo, parentThreadInfo]);
 
+  const shouldExcludeUserFromResult = React.useCallback(
+    (userID: string) => {
+      if (excludeUserIDs.includes(userID)) {
+        return true;
+      }
+      return !!(
+        threadType &&
+        threadTypeIsThick(threadType) &&
+        !userSupportsThickThreads(userID, auxUserInfos)
+      );
+    },
+    [auxUserInfos, excludeUserIDs, threadType],
+  );
+
   const filteredUserResults = React.useMemo(() => {
     const results: {
       [id: string]: {
@@ -161,7 +183,7 @@
       for (const id in userInfos) {
         appendUserInfo({
           results,
-          excludeUserIDs,
+          shouldExcludeUserFromResult,
           userInfo: userInfos[id],
           parentThreadInfo,
           communityThreadInfo,
@@ -173,7 +195,7 @@
       for (const id of ids) {
         appendUserInfo({
           results,
-          excludeUserIDs,
+          shouldExcludeUserFromResult,
           userInfo: userInfos[id],
           parentThreadInfo,
           communityThreadInfo,
@@ -186,7 +208,7 @@
       for (const userInfo of includeServerSearchUsers) {
         appendUserInfo({
           results,
-          excludeUserIDs,
+          shouldExcludeUserFromResult,
           userInfo,
           parentThreadInfo,
           communityThreadInfo,
@@ -214,14 +236,14 @@
 
     return userResults;
   }, [
-    text,
-    userInfos,
-    searchIndex,
-    excludeUserIDs,
+    communityThreadInfo,
+    containingThreadInfo,
     includeServerSearchUsers,
     parentThreadInfo,
-    containingThreadInfo,
-    communityThreadInfo,
+    searchIndex,
+    shouldExcludeUserFromResult,
+    text,
+    userInfos,
   ]);
 
   const sortedMembers = React.useMemo(() => {
diff --git a/lib/shared/thread-actions-utils.js b/lib/shared/thread-actions-utils.js
--- a/lib/shared/thread-actions-utils.js
+++ b/lib/shared/thread-actions-utils.js
@@ -6,6 +6,7 @@
   threadIsPending,
   threadOtherMembers,
   pendingThreadType,
+  userSupportsThickThreads,
 } from './thread-utils.js';
 import {
   newThreadActionTypes,
@@ -136,10 +137,8 @@
       otherMemberIDs.length > 0,
       'otherMemberIDs should not be empty for threads',
     );
-    const allUsersSupportThickThreads = otherMemberIDs.every(
-      memberID =>
-        auxUserInfos[memberID]?.deviceList &&
-        auxUserInfos[memberID].deviceList.devices.length > 0,
+    const allUsersSupportThickThreads = otherMemberIDs.every(memberID =>
+      userSupportsThickThreads(memberID, auxUserInfos),
     );
     if (threadTypeIsThick(threadInfo.type) && allUsersSupportThickThreads) {
       const type = assertThickThreadType(
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -40,6 +40,7 @@
   getRelativeMemberInfos,
   usersWithPersonalThreadSelector,
 } from '../selectors/user-selectors.js';
+import type { AuxUserInfos } from '../types/aux-user-types.js';
 import type {
   RelativeMemberInfo,
   RawThreadInfo,
@@ -1795,6 +1796,16 @@
   };
 }
 
+function userSupportsThickThreads(
+  userID: string,
+  auxUserInfos: AuxUserInfos,
+): boolean {
+  return (
+    !!auxUserInfos[userID]?.deviceList &&
+    auxUserInfos[userID].deviceList.devices.length > 0
+  );
+}
+
 export {
   threadHasPermission,
   useCommunityRootMembersToRole,
@@ -1861,4 +1872,5 @@
   extractMentionedMembers,
   isMemberActive,
   createThreadTimestamps,
+  userSupportsThickThreads,
 };
diff --git a/native/chat/compose-subchannel.react.js b/native/chat/compose-subchannel.react.js
--- a/native/chat/compose-subchannel.react.js
+++ b/native/chat/compose-subchannel.react.js
@@ -196,9 +196,11 @@
   const communityThreadInfo = useSelector(state =>
     community ? threadInfoSelector(state)[community] : null,
   );
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
   const userSearchResults = usePotentialMemberItems({
     text: usernameInputText,
     userInfos: otherUserInfos,
+    auxUserInfos,
     excludeUserIDs: userInfoInputIDs,
     inputParentThreadInfo: parentThreadInfo,
     inputCommunityThreadInfo: communityThreadInfo,
diff --git a/native/chat/message-list-container.react.js b/native/chat/message-list-container.react.js
--- a/native/chat/message-list-container.react.js
+++ b/native/chat/message-list-container.react.js
@@ -267,9 +267,11 @@
 
     const serverSearchResults = useSearchUsers(usernameInputText);
 
+    const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
     const userSearchResults = usePotentialMemberItems({
       text: usernameInputText,
       userInfos: otherUserInfos,
+      auxUserInfos,
       excludeUserIDs: userInfoInputArray.map(userInfo => userInfo.id),
       includeServerSearchUsers: serverSearchResults,
     });
diff --git a/native/chat/settings/add-users-modal.react.js b/native/chat/settings/add-users-modal.react.js
--- a/native/chat/settings/add-users-modal.react.js
+++ b/native/chat/settings/add-users-modal.react.js
@@ -187,9 +187,11 @@
   const communityThreadInfo = useSelector(state =>
     community ? threadInfoSelector(state)[community] : null,
   );
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
   const userSearchResults = usePotentialMemberItems({
     text: usernameInputText,
     userInfos: otherUserInfos,
+    auxUserInfos,
     excludeUserIDs,
     inputParentThreadInfo: parentThreadInfo,
     inputCommunityThreadInfo: communityThreadInfo,
diff --git a/web/chat/chat-thread-composer.react.js b/web/chat/chat-thread-composer.react.js
--- a/web/chat/chat-thread-composer.react.js
+++ b/web/chat/chat-thread-composer.react.js
@@ -31,6 +31,7 @@
 import type { InputState } from '../input/input-state.js';
 import Alert from '../modals/alert.react.js';
 import { updateNavInfoActionType } from '../redux/action-types.js';
+import { useSelector } from '../redux/redux-utils.js';
 
 type Props = {
   +userInfoInputArray: $ReadOnlyArray<AccountUserInfo>,
@@ -57,9 +58,11 @@
 
   const searchResults = useSearchUsers(usernameInputText);
 
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
   const userListItems = usePotentialMemberItems({
     text: usernameInputText,
     userInfos: otherUserInfos,
+    auxUserInfos,
     excludeUserIDs: userInfoInputIDs,
     includeServerSearchUsers: searchResults,
   });
diff --git a/web/settings/relationship/add-users-utils.js b/web/settings/relationship/add-users-utils.js
--- a/web/settings/relationship/add-users-utils.js
+++ b/web/settings/relationship/add-users-utils.js
@@ -147,9 +147,11 @@
     [previouslySelectedUsers, threadInfo.members],
   );
 
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
   const userSearchResults = usePotentialMemberItems({
     text: searchText,
     userInfos: otherUserInfos,
+    auxUserInfos,
     excludeUserIDs,
     inputParentThreadInfo: parentThreadInfo,
     inputCommunityThreadInfo: communityThreadInfo,
@@ -213,9 +215,11 @@
     [previouslySelectedUsers],
   );
 
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
   const userSearchResults = usePotentialMemberItems({
     text: searchText,
     userInfos: otherUserInfos,
+    auxUserInfos,
     excludeUserIDs: previouslySelectedUserIDs,
     inputParentThreadInfo: parentThreadInfo,
     inputCommunityThreadInfo: communityThreadInfo,