diff --git a/lib/types/identity-search/auth-message-types.js b/lib/types/identity-search/auth-message-types.js
--- a/lib/types/identity-search/auth-message-types.js
+++ b/lib/types/identity-search/auth-message-types.js
@@ -15,7 +15,7 @@
 import type { TInterface } from 'tcomb';
 import t from 'tcomb';
 
-import { tShape, tString } from '../../utils/validation-utils.js';
+import { tShape, tString, tUserID } from '../../utils/validation-utils.js';
 
 export type IdentitySearchAuthMessage = {
   +type: 'IdentitySearchAuthMessage',
@@ -27,7 +27,7 @@
 export const identityAuthMessageValidator: TInterface<IdentitySearchAuthMessage> =
   tShape<IdentitySearchAuthMessage>({
     type: tString('IdentitySearchAuthMessage'),
-    userID: t.String,
+    userID: tUserID,
     deviceID: t.String,
     accessToken: t.String,
   });
diff --git a/lib/types/identity-search/search-response-types.js b/lib/types/identity-search/search-response-types.js
--- a/lib/types/identity-search/search-response-types.js
+++ b/lib/types/identity-search/search-response-types.js
@@ -15,7 +15,7 @@
 import type { TInterface, TUnion } from 'tcomb';
 import t from 'tcomb';
 
-import { tShape, tString } from '../../utils/validation-utils.js';
+import { tShape, tString, tUserID } from '../../utils/validation-utils.js';
 
 export type IdentitySearchFailure = {
   +id: string,
@@ -35,7 +35,7 @@
 
 export const identitySearchUserValidator: TInterface<IdentitySearchUser> =
   tShape<IdentitySearchUser>({
-    userID: t.String,
+    userID: tUserID,
     username: t.String,
   });
 
diff --git a/lib/types/identity-service-types.js b/lib/types/identity-service-types.js
--- a/lib/types/identity-service-types.js
+++ b/lib/types/identity-service-types.js
@@ -17,7 +17,7 @@
   currentUserInfoValidator,
   type CurrentUserInfo,
 } from './user-types.js';
-import { tShape } from '../utils/validation-utils.js';
+import { tUserID, tShape } from '../utils/validation-utils.js';
 
 export type UserAuthMetadata = {
   +userID: string,
@@ -99,7 +99,7 @@
 
 export const farcasterUserValidator: TInterface<FarcasterUser> =
   tShape<FarcasterUser>({
-    userID: t.String,
+    userID: tUserID,
     username: t.String,
     farcasterID: t.String,
   });
@@ -188,7 +188,7 @@
 };
 export const identityAuthResultValidator: TInterface<IdentityAuthResult> =
   tShape<IdentityAuthResult>({
-    userID: t.String,
+    userID: tUserID,
     accessToken: t.String,
     username: t.String,
     preRequestUserState: t.maybe(currentUserInfoValidator),
diff --git a/lib/types/relationship-types.js b/lib/types/relationship-types.js
--- a/lib/types/relationship-types.js
+++ b/lib/types/relationship-types.js
@@ -5,7 +5,12 @@
 
 import type { AccountUserInfo } from './user-types.js';
 import { values } from '../utils/objects.js';
-import { tNumEnum, tShape, tString } from '../utils/validation-utils.js';
+import {
+  tUserID,
+  tNumEnum,
+  tShape,
+  tString,
+} from '../utils/validation-utils.js';
 
 export const undirectedStatus = Object.freeze({
   KNOW_OF: 0,
@@ -86,7 +91,7 @@
 export const farcasterRelationshipRequestValidator: TInterface<FarcasterRelationshipRequest> =
   tShape<FarcasterRelationshipRequest>({
     action: tString('farcaster'),
-    userIDsToFID: t.dict(t.String, t.String),
+    userIDsToFID: t.dict(tUserID, t.String),
   });
 
 type SharedRelationshipRow = {
diff --git a/lib/types/report-types.js b/lib/types/report-types.js
--- a/lib/types/report-types.js
+++ b/lib/types/report-types.js
@@ -9,7 +9,11 @@
 import type { AppState, BaseAction } from './redux-types.js';
 import { type MixedRawThreadInfos } from './thread-types.js';
 import type { UserInfo, UserInfos } from './user-types.js';
-import { tPlatformDetails, tShape } from '../utils/validation-utils.js';
+import {
+  tPlatformDetails,
+  tShape,
+  tUserID,
+} from '../utils/validation-utils.js';
 
 export type EnabledReports = {
   +crashReports: boolean,
@@ -218,7 +222,7 @@
 
 export const reportInfoValidator: TInterface<ReportInfo> = tShape<ReportInfo>({
   id: t.String,
-  viewerID: t.String,
+  viewerID: tUserID,
   platformDetails: tPlatformDetails,
   creationTime: t.Number,
 });
diff --git a/lib/types/request-types.js b/lib/types/request-types.js
--- a/lib/types/request-types.js
+++ b/lib/types/request-types.js
@@ -28,7 +28,7 @@
   accountUserInfoValidator,
 } from './user-types.js';
 import { mixedRawThreadInfoValidator } from '../permissions/minimally-encoded-raw-thread-info-validators.js';
-import { tNumber, tShape, tID } from '../utils/validation-utils.js';
+import { tNumber, tShape, tID, tUserID } from '../utils/validation-utils.js';
 
 // "Server requests" are requests for information that the server delivers to
 // clients. Clients then respond to those requests with a "client response".
@@ -138,7 +138,7 @@
         userInfos: t.maybe(t.list(accountUserInfoValidator)),
         deleteThreadIDs: t.maybe(t.list(tID)),
         deleteEntryIDs: t.maybe(t.list(tID)),
-        deleteUserInfoIDs: t.maybe(t.list(t.String)),
+        deleteUserInfoIDs: t.maybe(t.list(tUserID)),
       }),
     ),
   });
diff --git a/lib/types/tunnelbroker/peer-to-peer-message-types.js b/lib/types/tunnelbroker/peer-to-peer-message-types.js
--- a/lib/types/tunnelbroker/peer-to-peer-message-types.js
+++ b/lib/types/tunnelbroker/peer-to-peer-message-types.js
@@ -3,7 +3,7 @@
 import type { TInterface, TUnion } from 'tcomb';
 import t from 'tcomb';
 
-import { tShape, tString } from '../../utils/validation-utils.js';
+import { tShape, tString, tUserID } from '../../utils/validation-utils.js';
 import { type EncryptedData, encryptedDataValidator } from '../crypto-types.js';
 import {
   signedDeviceListValidator,
@@ -15,7 +15,7 @@
   +deviceID: string,
 };
 const senderInfoValidator: TInterface<SenderInfo> = tShape<SenderInfo>({
-  userID: t.String,
+  userID: tUserID,
   deviceID: t.String,
 });
 
@@ -84,7 +84,7 @@
 export const deviceListUpdatedValidator: TInterface<DeviceListUpdated> =
   tShape<DeviceListUpdated>({
     type: tString(peerToPeerMessageTypes.DEVICE_LIST_UPDATED),
-    userID: t.String,
+    userID: tUserID,
     signedDeviceList: signedDeviceListValidator,
   });
 
diff --git a/lib/types/tunnelbroker/qr-code-auth-message-types.js b/lib/types/tunnelbroker/qr-code-auth-message-types.js
--- a/lib/types/tunnelbroker/qr-code-auth-message-types.js
+++ b/lib/types/tunnelbroker/qr-code-auth-message-types.js
@@ -3,7 +3,7 @@
 import type { TInterface, TUnion } from 'tcomb';
 import t from 'tcomb';
 
-import { tShape, tString } from '../../utils/validation-utils.js';
+import { tShape, tString, tUserID } from '../../utils/validation-utils.js';
 
 export const qrCodeAuthMessageTypes = Object.freeze({
   DEVICE_LIST_UPDATE_SUCCESS: 'DeviceListUpdateSuccess',
@@ -19,7 +19,7 @@
 export const deviceListUpdateSuccessValidator: TInterface<DeviceListUpdateSuccess> =
   tShape<DeviceListUpdateSuccess>({
     type: tString(qrCodeAuthMessageTypes.DEVICE_LIST_UPDATE_SUCCESS),
-    userID: t.String,
+    userID: tUserID,
     primaryDeviceID: t.String,
   });
 
diff --git a/lib/types/tunnelbroker/session-types.js b/lib/types/tunnelbroker/session-types.js
--- a/lib/types/tunnelbroker/session-types.js
+++ b/lib/types/tunnelbroker/session-types.js
@@ -3,7 +3,7 @@
 import type { TInterface } from 'tcomb';
 import t from 'tcomb';
 
-import { tShape, tString } from '../../utils/validation-utils.js';
+import { tShape, tString, tUserID } from '../../utils/validation-utils.js';
 
 export type TunnelbrokerDeviceTypes = 'mobile' | 'web' | 'keyserver';
 
@@ -35,7 +35,7 @@
     type: tString('ConnectionInitializationMessage'),
     deviceID: t.String,
     accessToken: t.String,
-    userID: t.String,
+    userID: tUserID,
     notifyToken: t.maybe(t.String),
     deviceType: t.enums.of(['mobile', 'web', 'keyserver']),
     deviceAppVersion: t.maybe(t.String),
diff --git a/lib/utils/entity-text.js b/lib/utils/entity-text.js
--- a/lib/utils/entity-text.js
+++ b/lib/utils/entity-text.js
@@ -6,7 +6,7 @@
 
 import type { GetENSNames } from './ens-helpers.js';
 import type { GetFCNames } from './farcaster-helpers.js';
-import { tID, tShape, tString } from './validation-utils.js';
+import { tID, tShape, tString, tUserID } from './validation-utils.js';
 import { useENSNames } from '../hooks/ens-cache.js';
 import { useFCNames } from '../hooks/fc-cache.js';
 import { threadNoun } from '../shared/thread-utils.js';
@@ -31,7 +31,7 @@
 };
 export const userEntityValidator: TInterface<UserEntity> = tShape<UserEntity>({
   type: tString('user'),
-  id: t.String,
+  id: tUserID,
   username: t.maybe(t.String),
   isViewer: t.maybe(t.Boolean),
   possessive: t.maybe(t.Boolean),
diff --git a/lib/utils/url-utils.js b/lib/utils/url-utils.js
--- a/lib/utils/url-utils.js
+++ b/lib/utils/url-utils.js
@@ -7,6 +7,7 @@
   tID,
   tShape,
   pendingThreadIDRegex,
+  tUserID,
 } from './validation-utils.js';
 
 type MutableURLInfo = {
@@ -50,7 +51,7 @@
     ]),
   ),
   threadCreation: t.maybe(t.Boolean),
-  selectedUserList: t.maybe(t.list(t.String)),
+  selectedUserList: t.maybe(t.list(tUserID)),
   inviteSecret: t.maybe(t.String),
   qrCode: t.maybe(t.Boolean),
 });