diff --git a/.changeset/violet-clouds-smile.md b/.changeset/violet-clouds-smile.md
new file mode 100644
index 0000000000..c56a3f711b
--- /dev/null
+++ b/.changeset/violet-clouds-smile.md
@@ -0,0 +1,5 @@
+---
+"@assistant-ui/react": patch
+---
+
+fix: external store thread list should not crash
diff --git a/packages/react/src/runtimes/external-store/ExternalStoreThreadListRuntimeCore.tsx b/packages/react/src/runtimes/external-store/ExternalStoreThreadListRuntimeCore.tsx
index fc1a6c26b2..47c971653b 100644
--- a/packages/react/src/runtimes/external-store/ExternalStoreThreadListRuntimeCore.tsx
+++ b/packages/react/src/runtimes/external-store/ExternalStoreThreadListRuntimeCore.tsx
@@ -1,18 +1,26 @@
 import type { Unsubscribe } from "../../types";
 import { ExternalStoreThreadRuntimeCore } from "./ExternalStoreThreadRuntimeCore";
 import { ThreadListRuntimeCore } from "../core/ThreadListRuntimeCore";
-import { ExternalStoreThreadListAdapter } from "./ExternalStoreAdapter";
+import {
+  ExternalStoreThreadData,
+  ExternalStoreThreadListAdapter,
+} from "./ExternalStoreAdapter";
 
 export type ExternalStoreThreadFactory = () => ExternalStoreThreadRuntimeCore;
 
 const EMPTY_ARRAY = Object.freeze([]);
 const DEFAULT_THREAD_ID = "DEFAULT_THREAD_ID";
+const DEFAULT_THREADS = Object.freeze([DEFAULT_THREAD_ID]);
+const DEFAULT_THREAD: ExternalStoreThreadData<"regular"> = Object.freeze({
+  threadId: DEFAULT_THREAD_ID,
+  state: "regular",
+});
 
 export class ExternalStoreThreadListRuntimeCore
   implements ThreadListRuntimeCore
 {
   private _mainThreadId: string = DEFAULT_THREAD_ID;
-  private _threads: readonly string[] = EMPTY_ARRAY;
+  private _threads: readonly string[] = DEFAULT_THREADS;
   private _archivedThreads: readonly string[] = EMPTY_ARRAY;
 
   public get newThreadId() {
@@ -51,6 +59,7 @@ export class ExternalStoreThreadListRuntimeCore
     for (const thread of this.adapter.archivedThreads ?? []) {
       if (thread.threadId === threadId) return thread;
     }
+    if (threadId === DEFAULT_THREAD_ID) return DEFAULT_THREAD;
     return undefined;
   }