diff --git a/src/client/ClientGameRunner.ts b/src/client/ClientGameRunner.ts
index 5cc88c3eb..09271afda 100644
--- a/src/client/ClientGameRunner.ts
+++ b/src/client/ClientGameRunner.ts
@@ -194,7 +194,7 @@ export function joinLobby(
 
 async function createClientGame(
   lobbyConfig: LobbyConfig,
-  clientID: ClientID,
+  clientID: ClientID | undefined,
   eventBus: EventBus,
   transport: Transport,
   userSettings: UserSettings,
@@ -267,7 +267,7 @@ export class ClientGameRunner {
 
   constructor(
     private lobby: LobbyConfig,
-    private clientID: ClientID,
+    private clientID: ClientID | undefined,
     private eventBus: EventBus,
     private renderer: GameRenderer,
     private input: InputHandler,
@@ -294,7 +294,7 @@ export class ClientGameRunner {
   }
 
   private async saveGame(update: WinUpdate) {
-    if (this.myPlayer === null) {
+    if (!this.clientID) {
       return;
     }
     const players: PlayerRecord[] = [
@@ -544,6 +544,7 @@ export class ClientGameRunner {
       return;
     }
     if (this.myPlayer === null) {
+      if (!this.clientID) return;
       const myPlayer = this.gameView.playerByClientID(this.clientID);
       if (myPlayer === null) return;
       this.myPlayer = myPlayer;
@@ -578,6 +579,7 @@ export class ClientGameRunner {
     const tile = this.gameView.ref(cell.x, cell.y);
 
     if (this.myPlayer === null) {
+      if (!this.clientID) return;
       const myPlayer = this.gameView.playerByClientID(this.clientID);
       if (myPlayer === null) return;
       this.myPlayer = myPlayer;
@@ -639,6 +641,7 @@ export class ClientGameRunner {
     }
 
     if (this.myPlayer === null) {
+      if (!this.clientID) return;
       const myPlayer = this.gameView.playerByClientID(this.clientID);
       if (myPlayer === null) return;
       this.myPlayer = myPlayer;
@@ -664,6 +667,7 @@ export class ClientGameRunner {
     }
 
     if (this.myPlayer === null) {
+      if (!this.clientID) return;
       const myPlayer = this.gameView.playerByClientID(this.clientID);
       if (myPlayer === null) return;
       this.myPlayer = myPlayer;
diff --git a/src/client/LocalServer.ts b/src/client/LocalServer.ts
index 90712b868..58c43306c 100644
--- a/src/client/LocalServer.ts
+++ b/src/client/LocalServer.ts
@@ -113,7 +113,8 @@ export class LocalServer {
       gameStartInfo: this.lobbyConfig.gameStartInfo,
       turns: [],
       lobbyCreatedAt: this.lobbyConfig.gameStartInfo.lobbyCreatedAt,
-      myClientID: this.clientID,
+      // Don't send myClientID for replays — viewer has no player identity.
+      myClientID: this.lobbyConfig.gameRecord ? undefined : this.clientID,
     } satisfies ServerStartGameMessage);
   }
 
@@ -127,7 +128,7 @@ export class LocalServer {
         gameStartInfo: this.lobbyConfig.gameStartInfo!,
         turns: this.turns,
         lobbyCreatedAt: this.lobbyConfig.gameStartInfo!.lobbyCreatedAt,
-        myClientID: this.clientID,
+        myClientID: this.lobbyConfig.gameRecord ? undefined : this.clientID,
       } satisfies ServerStartGameMessage);
     }
     if (clientMsg.type === "intent") {
diff --git a/src/core/GameRunner.ts b/src/core/GameRunner.ts
index 9019a78e5..427a32781 100644
--- a/src/core/GameRunner.ts
+++ b/src/core/GameRunner.ts
@@ -33,7 +33,7 @@ import { simpleHash } from "./Util";
 
 export async function createGameRunner(
   gameStart: GameStartInfo,
-  clientID: ClientID,
+  clientID: ClientID | undefined,
   mapLoader: GameMapLoader,
   callBack: (gu: GameUpdateViewData | ErrorUpdate) => void,
 ): Promise<GameRunner> {
diff --git a/src/core/Schemas.ts b/src/core/Schemas.ts
index 067e83069..a42738774 100644
--- a/src/core/Schemas.ts
+++ b/src/core/Schemas.ts
@@ -552,8 +552,9 @@ export const ServerStartGameMessageSchema = z.object({
   turns: TurnSchema.array(),
   gameStartInfo: GameStartInfoSchema,
   lobbyCreatedAt: z.number(),
-  // The clientID assigned to this connection by the server
-  myClientID: ID,
+  // The clientID assigned to this connection by the server.
+  // Absent for replays where the viewer has no player identity.
+  myClientID: ID.optional(),
 });
 
 export const ServerDesyncSchema = z.object({
diff --git a/src/core/execution/ExecutionManager.ts b/src/core/execution/ExecutionManager.ts
index d50aaadb7..9b61629d9 100644
--- a/src/core/execution/ExecutionManager.ts
+++ b/src/core/execution/ExecutionManager.ts
@@ -36,7 +36,7 @@ export class Executor {
   constructor(
     private mg: Game,
     private gameID: GameID,
-    private clientID: ClientID,
+    private clientID: ClientID | undefined,
   ) {
     // Add one to avoid id collisions with bots.
     this.random = new PseudoRandom(simpleHash(gameID) + 1);
diff --git a/src/core/game/GameView.ts b/src/core/game/GameView.ts
index bdd9f2092..d3e3ad87e 100644
--- a/src/core/game/GameView.ts
+++ b/src/core/game/GameView.ts
@@ -657,7 +657,7 @@ export class GameView implements GameMap {
     public worker: WorkerClient,
     private _config: Config,
     private _mapData: TerrainMapData,
-    private _myClientID: ClientID,
+    private _myClientID: ClientID | undefined,
     private _myUsername: string,
     private _gameID: GameID,
     private humans: Player[],
@@ -785,7 +785,9 @@ export class GameView implements GameMap {
       }
     });
 
-    this._myPlayer ??= this.playerByClientID(this._myClientID);
+    if (this._myClientID) {
+      this._myPlayer ??= this.playerByClientID(this._myClientID);
+    }
 
     for (const unit of this._units.values()) {
       unit._wasUpdated = false;
@@ -1103,7 +1105,7 @@ export class GameView implements GameMap {
     );
   }
 
-  myClientID(): ClientID {
+  myClientID(): ClientID | undefined {
     return this._myClientID;
   }
 
diff --git a/src/core/worker/WorkerClient.ts b/src/core/worker/WorkerClient.ts
index 7655f9050..372e4af96 100644
--- a/src/core/worker/WorkerClient.ts
+++ b/src/core/worker/WorkerClient.ts
@@ -23,7 +23,7 @@ export class WorkerClient {
 
   constructor(
     private gameStartInfo: GameStartInfo,
-    private clientID: ClientID,
+    private clientID: ClientID | undefined,
   ) {
     this.worker = new Worker(new URL("./Worker.worker.ts", import.meta.url), {
       type: "module",
diff --git a/src/core/worker/WorkerMessages.ts b/src/core/worker/WorkerMessages.ts
index b8b04740d..7d75f1882 100644
--- a/src/core/worker/WorkerMessages.ts
+++ b/src/core/worker/WorkerMessages.ts
@@ -39,7 +39,7 @@ interface BaseWorkerMessage {
 export interface InitMessage extends BaseWorkerMessage {
   type: "init";
   gameStartInfo: GameStartInfo;
-  clientID: ClientID;
+  clientID: ClientID | undefined;
 }
 
 export interface TurnMessage extends BaseWorkerMessage {