feat(chat): persist model per thread and auto-load on thread switch

studio/frontend/src/features/chat/chat-page.tsx

@@ -358,6 +358,41 @@ export function ChatPage(): ReactElement {
     setViewBeforeCompare(null);
   }, [viewBeforeCompare]);
 
+  const handleThreadSelect = useCallback(
+    (nextView: ChatView) => {
+      setView(nextView);
+
+      const threadId =
+        nextView.mode === "single" ? nextView.threadId : undefined;
+      const pairId =
+        nextView.mode === "compare" ? nextView.pairId : undefined;
+
+      void (async () => {
+        let thread: import("./types").ThreadRecord | undefined;
+        if (threadId) {
+          thread = await db.threads.get(threadId);
+        } else if (pairId) {
+          thread = await db.threads
+            .where("pairId")
+            .equals(pairId)
+            .first();
+        }
+        const threadModelId = thread?.modelId;
+        if (!threadModelId) return;
+
+        const currentCheckpoint =
+          useChatRuntimeStore.getState().params.checkpoint;
+        if (threadModelId === currentCheckpoint) return;
+
+        if (currentCheckpoint) {
+          await ejectModel();
+        }
+        await selectModel({ id: threadModelId });
+      })();
+    },
+    [ejectModel, selectModel],
+  );
+
   const models = useMemo<ModelOption[]>(
     () =>
       modelsFromStore.map((model) => ({
@@ -500,7 +535,7 @@ export function ChatPage(): ReactElement {
         <InlineSidebar>
           <ThreadSidebar
             view={view}
-            onSelect={setView}
+            onSelect={handleThreadSelect}
             onNewThread={handleNewThread}
             onNewCompare={handleNewCompare}
             showCompare={canCompare}

studio/frontend/src/features/chat/db.ts

@@ -19,6 +19,20 @@ db.version(2)
   })
   .upgrade((tx) => tx.table("messages").clear());
 
+db.version(3)
+  .stores({
+    threads: "id, modelType, pairId, archived, createdAt",
+    messages: "id, threadId, createdAt",
+  })
+  .upgrade((tx) =>
+    tx
+      .table("threads")
+      .toCollection()
+      .modify((thread) => {
+        if (!thread.modelId) thread.modelId = "";
+      }),
+  );
+
 export { db };
 
 export function useLiveQuery<T>(

studio/frontend/src/features/chat/runtime-provider.tsx

@@ -323,10 +323,13 @@ function createDexieAdapter(
     },
 
     async initialize(threadId: string) {
+      const currentModelId =
+        useChatRuntimeStore.getState().params.checkpoint ?? "";
       await db.threads.add({
         id: threadId,
         title: "New Chat",
         modelType,
+        modelId: currentModelId,
         pairId,
         archived: false,
         createdAt: Date.now(),

studio/frontend/src/features/chat/types.ts

@@ -8,6 +8,7 @@ export interface ThreadRecord {
   id: string;
   title: string;
   modelType: ModelType;
+  modelId?: string;
   pairId?: string;
   archived: boolean;
   createdAt: number;