diff --git a/keyserver/src/responders/relationship-responders.js b/keyserver/src/responders/relationship-responders.js
--- a/keyserver/src/responders/relationship-responders.js
+++ b/keyserver/src/responders/relationship-responders.js
@@ -9,7 +9,7 @@
   type RelationshipRequest,
   farcasterRelationshipRequestValidator,
 } from 'lib/types/relationship-types.js';
-import { tShape } from 'lib/utils/validation-utils.js';
+import { tShape, tUserID } from 'lib/utils/validation-utils.js';
 
 import type { Viewer } from '../session/viewer.js';
 import { updateRelationships } from '../updaters/relationship-updaters.js';
@@ -20,7 +20,7 @@
       traditionalRelationshipActionsList,
       'relationship action',
     ),
-    userIDs: t.list(t.String),
+    userIDs: t.list(tUserID),
   });
 
 export const updateRelationshipInputValidator: TUnion<RelationshipRequest> =
diff --git a/keyserver/src/responders/thread-responders.js b/keyserver/src/responders/thread-responders.js
--- a/keyserver/src/responders/thread-responders.js
+++ b/keyserver/src/responders/thread-responders.js
@@ -34,6 +34,7 @@
   tColor,
   tPassword,
   tID,
+  tUserID,
 } from 'lib/utils/validation-utils.js';
 
 import {
@@ -71,7 +72,7 @@
 export const roleChangeRequestInputValidator: TInterface<RoleChangeRequest> =
   tShape<RoleChangeRequest>({
     threadID: tID,
-    memberIDs: t.list(t.String),
+    memberIDs: t.list(tUserID),
     role: t.refinement(tID, str => {
       if (str.indexOf('|') !== -1) {
         str = str.split('|')[1];
@@ -91,7 +92,7 @@
 export const removeMembersRequestInputValidator: TInterface<RemoveMembersRequest> =
   tShape<RemoveMembersRequest>({
     threadID: tID,
-    memberIDs: t.list(t.String),
+    memberIDs: t.list(tUserID),
   });
 
 async function memberRemovalResponder(
@@ -122,7 +123,7 @@
       description: t.maybe(t.String),
       color: t.maybe(tColor),
       parentThreadID: t.maybe(tID),
-      newMemberIDs: t.maybe(t.list(t.String)),
+      newMemberIDs: t.maybe(t.list(tUserID)),
       avatar: t.maybe(updateUserAvatarRequestValidator),
     }),
     accountPassword: t.maybe(tPassword),
@@ -140,7 +141,7 @@
   description: t.maybe(t.String),
   color: t.maybe(tColor),
   parentThreadID: t.maybe(tID),
-  initialMemberIDs: t.maybe(t.list(t.String)),
+  initialMemberIDs: t.maybe(t.list(tUserID)),
   calendarQuery: t.maybe(entryQueryInputValidator),
 };
 const newThreadRequestInputValidator: TUnion<ServerNewThreadRequest> = t.union([
diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -82,6 +82,7 @@
   tOldValidUsername,
   tRegex,
   tID,
+  tUserID,
 } from 'lib/utils/validation-utils.js';
 
 import {
@@ -695,7 +696,7 @@
 
 export const keyserverAuthRequestInputValidator: TInterface<KeyserverAuthRequest> =
   tShape<KeyserverAuthRequest>({
-    userID: t.String,
+    userID: tUserID,
     deviceID: t.String,
     calendarQuery: entryQueryInputValidator,
     deviceTokenUpdateRequest: t.maybe(deviceTokenUpdateRequestInputValidator),