diff --git a/lib/actions/user-actions.js b/lib/actions/user-actions.js
--- a/lib/actions/user-actions.js
+++ b/lib/actions/user-actions.js
@@ -52,7 +52,7 @@
 } from '../types/avatar-types.js';
 import type { RawEntryInfo, CalendarQuery } from '../types/entry-types.js';
 import type {
-  Identities,
+  UserIdentitiesResponse,
   IdentityAuthResult,
 } from '../types/identity-service-types.js';
 import type {
@@ -1381,7 +1381,7 @@
 
 function useFindUserIdentities(): (
   userIDs: $ReadOnlyArray<string>,
-) => Promise<Identities> {
+) => Promise<UserIdentitiesResponse> {
   const client = React.useContext(IdentityClientContext);
   const identityClient = client?.identityClient;
   invariant(identityClient, 'Identity client should be set');
diff --git a/lib/components/farcaster-data-handler.react.js b/lib/components/farcaster-data-handler.react.js
--- a/lib/components/farcaster-data-handler.react.js
+++ b/lib/components/farcaster-data-handler.react.js
@@ -124,7 +124,8 @@
 
     const userStoreIDs = Object.keys(userInfos);
 
-    const userIdentities = await findUserIdentities(userStoreIDs);
+    const { identities: userIdentities } =
+      await findUserIdentities(userStoreIDs);
 
     const userStoreFarcasterUsers = Object.entries(userIdentities)
       .filter(([, identity]) => identity.farcasterID !== null)
diff --git a/lib/handlers/user-infos-handler.react.js b/lib/handlers/user-infos-handler.react.js
--- a/lib/handlers/user-infos-handler.react.js
+++ b/lib/handlers/user-infos-handler.react.js
@@ -70,7 +70,7 @@
       // 1. Fetch usernames from identity
       const promise = (async () => {
         newUserIDs.forEach(id => requestedIDsRef.current.add(id));
-        const identities = await findUserIdentities(newUserIDs);
+        const { identities } = await findUserIdentities(newUserIDs);
         newUserIDs.forEach(id => requestedIDsRef.current.delete(id));
 
         const newUserInfos = [];
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
@@ -190,7 +190,9 @@
   ) => Promise<$ReadOnlyArray<FarcasterUser>>;
   +linkFarcasterAccount: (farcasterID: string) => Promise<void>;
   +unlinkFarcasterAccount: () => Promise<void>;
-  +findUserIdentities: (userIDs: $ReadOnlyArray<string>) => Promise<Identities>;
+  +findUserIdentities: (
+    userIDs: $ReadOnlyArray<string>,
+  ) => Promise<UserIdentitiesResponse>;
   +versionSupported: () => Promise<boolean>;
   +changePassword: (oldPassword: string, newPassword: string) => Promise<void>;
 }
@@ -282,6 +284,22 @@
   identityValidator,
 );
 
+export type ReservedUserIdentifiers = {
+  +[userID: string]: string,
+};
+export const reservedIdentifiersValidator: TDict<ReservedUserIdentifiers> =
+  t.dict(t.String, t.String);
+
+export type UserIdentitiesResponse = {
+  +identities: Identities,
+  +reservedUserIdentifiers: ReservedUserIdentifiers,
+};
+export const userIdentitiesResponseValidator: TInterface<UserIdentitiesResponse> =
+  tShape<UserIdentitiesResponse>({
+    identities: identitiesValidator,
+    reservedUserIdentifiers: reservedIdentifiersValidator,
+  });
+
 export type SignedDeviceList = {
   // JSON-stringified RawDeviceList
   +rawDeviceList: string,
diff --git a/native/identity-service/identity-service-context-provider.react.js b/native/identity-service/identity-service-context-provider.react.js
--- a/native/identity-service/identity-service-context-provider.react.js
+++ b/native/identity-service/identity-service-context-provider.react.js
@@ -27,7 +27,7 @@
   type UserDevicesOlmInboundKeys,
   type UserDevicesOlmOutboundKeys,
   type UsersSignedDeviceLists,
-  identitiesValidator,
+  userIdentitiesResponseValidator,
   type UsersDevicesPlatformDetails,
   peersDeviceListsValidator,
 } from 'lib/types/identity-service-types.js';
@@ -666,8 +666,10 @@
           token,
           userIDs,
         );
-        const { identities } = JSON.parse(result);
-        return assertWithValidator(identities, identitiesValidator);
+        return assertWithValidator(
+          JSON.parse(result),
+          userIdentitiesResponseValidator,
+        );
       },
       versionSupported: () => {
         return commRustModule.versionSupported();
diff --git a/web/grpc/identity-service-client-wrapper.js b/web/grpc/identity-service-client-wrapper.js
--- a/web/grpc/identity-service-client-wrapper.js
+++ b/web/grpc/identity-service-client-wrapper.js
@@ -29,7 +29,8 @@
   farcasterUsersValidator,
   type UsersSignedDeviceLists,
   type Identities,
-  identitiesValidator,
+  type UserIdentitiesResponse,
+  userIdentitiesResponseValidator,
   type PeersDeviceLists,
   peersDeviceListsValidator,
   type IdentityPlatformDetails,
@@ -649,31 +650,41 @@
     await client.unlinkFarcasterAccount(new Empty());
   };
 
-  findUserIdentities: (userIDs: $ReadOnlyArray<string>) => Promise<Identities> =
-    async userIDs => {
-      const client = this.authClient;
-      if (!client) {
-        throw new Error('Identity service client is not initialized');
-      }
-      const request = new IdentityAuthStructs.UserIdentitiesRequest();
-      request.setUserIdsList([...userIDs]);
-      const response = await client.findUserIdentities(request);
-      const identityObjects = response.toObject()?.identitiesMap;
-
-      let identities: Identities = {};
-      identityObjects.forEach(([userID, identityObject]) => {
-        identities = {
-          ...identities,
-          [userID]: {
-            ethIdentity: identityObject.ethIdentity,
-            username: identityObject.username,
-            farcasterID: identityObject.farcasterId,
-          },
-        };
-      });
+  findUserIdentities: (
+    userIDs: $ReadOnlyArray<string>,
+  ) => Promise<UserIdentitiesResponse> = async userIDs => {
+    const client = this.authClient;
+    if (!client) {
+      throw new Error('Identity service client is not initialized');
+    }
+    const request = new IdentityAuthStructs.UserIdentitiesRequest();
+    request.setUserIdsList([...userIDs]);
+    const response = await client.findUserIdentities(request);
+    const responseObject = response.toObject();
+    const identityObjects = responseObject?.identitiesMap;
+    const reservedUserEntries = responseObject?.reservedUserIdentifiersMap;
+
+    let identities: Identities = {};
+    identityObjects.forEach(([userID, identityObject]) => {
+      identities = {
+        ...identities,
+        [userID]: {
+          ethIdentity: identityObject.ethIdentity,
+          username: identityObject.username,
+          farcasterID: identityObject.farcasterId,
+        },
+      };
+    });
 
-      return assertWithValidator(identities, identitiesValidator);
+    const userIdentitiesResponse: UserIdentitiesResponse = {
+      identities,
+      reservedUserIdentifiers: Object.fromEntries(reservedUserEntries),
     };
+    return assertWithValidator(
+      userIdentitiesResponse,
+      userIdentitiesResponseValidator,
+    );
+  };
 
   versionSupported: () => Promise<boolean> = async () => {
     const client = this.unauthClient;