diff --git a/src/client/Transport.ts b/src/client/Transport.ts
index c13cbbeba..0e7a07030 100644
--- a/src/client/Transport.ts
+++ b/src/client/Transport.ts
@@ -26,10 +26,6 @@ import {
 } from "../core/Schemas";
 import { LobbyConfig } from "./ClientGameRunner";
 import { LocalServer } from "./LocalServer";
-import { UsernameInput } from "./UsernameInput";
-import { HostLobbyModal as HostPrivateLobbyModal } from "./HostLobbyModal";
-import { JoinPrivateLobbyModal } from "./JoinPrivateLobbyModal";
-import { SinglePlayerModal } from "./SinglePlayerModal";
 import { PlayerView } from "../core/game/GameView";
 
 export class PauseGameEvent implements GameEvent {
@@ -175,6 +171,7 @@ export class Transport {
               ClientPingMessageSchema.parse({
                 type: "ping",
                 clientID: this.lobbyConfig.clientID,
+                persistentID: this.lobbyConfig.persistentID,
                 gameID: this.lobbyConfig.gameID,
               }),
             ),
@@ -437,6 +434,7 @@ export class Transport {
       const msg = ClientSendWinnerSchema.parse({
         type: "winner",
         clientID: this.lobbyConfig.clientID,
+        persistentID: this.lobbyConfig.persistentID,
         gameID: this.lobbyConfig.gameID,
         winner: event.winner,
       });
@@ -455,6 +453,7 @@ export class Transport {
       const msg = ClientIntentMessageSchema.parse({
         type: "intent",
         clientID: this.lobbyConfig.clientID,
+        persistentID: this.lobbyConfig.persistentID,
         gameID: this.lobbyConfig.gameID,
         intent: intent,
       });
diff --git a/src/core/Schemas.ts b/src/core/Schemas.ts
index 6ebcc2d7d..f8796c725 100644
--- a/src/core/Schemas.ts
+++ b/src/core/Schemas.ts
@@ -267,6 +267,7 @@ export const ServerMessageSchema = z.union([
 const ClientBaseMessageSchema = z.object({
   type: z.enum(["winner", "join", "intent", "ping", "log"]),
   clientID: ID,
+  persistentID: SafeString.nullable(), // WARNING: persistent id is private.
   gameID: ID,
 });
 
@@ -294,7 +295,6 @@ export const ClientIntentMessageSchema = ClientBaseMessageSchema.extend({
 // WARNING: never send this message to clients.
 export const ClientJoinMessageSchema = ClientBaseMessageSchema.extend({
   type: z.literal("join"),
-  persistentID: SafeString, // WARNING: persistent id is private.
   lastTurn: z.number(), // The last turn the client saw.
   username: SafeString,
 });
diff --git a/src/server/GameServer.ts b/src/server/GameServer.ts
index 1b77e274b..e7b2acafc 100644
--- a/src/server/GameServer.ts
+++ b/src/server/GameServer.ts
@@ -103,6 +103,19 @@ export class GameServer {
         const clientMsg: ClientMessage = ClientMessageSchema.parse(
           JSON.parse(message),
         );
+        if (this.allClients.has(clientMsg.clientID)) {
+          const client = this.allClients.get(clientMsg.clientID);
+          if (client.persistentID != clientMsg.persistentID) {
+            console.warn(
+              `Client ID ${clientMsg.clientID} sent incorrect id ${clientMsg.persistentID}, does not match persistent id ${client.persistentID}`,
+            );
+            return;
+          }
+        }
+
+        // Clear out persistent id to make sure it doesn't get sent to other clients.
+        clientMsg.persistentID = null;
+
         if (clientMsg.type == "intent") {
           if (clientMsg.gameID == this.id) {
             this.addIntent(clientMsg.intent);