Persist last selected model when opening a new thread

- Track `lastSelectedModel` and `lastSelectedProvider` globally in `composerDraftStore`
- Update `setModel`/`setProvider` to write these globals on explicit non-null selection
- Pre-populate new thread drafts with the last selected model/provider in `useHandleNewThread`Persist last selected model when opening a new thread

Author: WeldFire

apps/web/src/composerDraftStore.test.ts

@@ -372,6 +372,8 @@ describe("composerDraftStore setModel", () => {
       draftsByThreadId: {},
       draftThreadsByThreadId: {},
       projectDraftThreadIdByProjectId: {},
+      lastSelectedModel: null,
+      lastSelectedProvider: null,
     });
   });
 
@@ -384,6 +386,23 @@ describe("composerDraftStore setModel", () => {
       "gpt-5.3-codex",
     );
   });
+
+  it("updates lastSelectedModel when a non-null model is set", () => {
+    const store = useComposerDraftStore.getState();
+
+    store.setModel(threadId, "gpt-5.3-codex");
+
+    expect(useComposerDraftStore.getState().lastSelectedModel).toBe("gpt-5.3-codex");
+  });
+
+  it("does not update lastSelectedModel when model is set to null", () => {
+    const store = useComposerDraftStore.getState();
+
+    store.setModel(threadId, "gpt-5.3-codex");
+    store.setModel(threadId, null);
+
+    expect(useComposerDraftStore.getState().lastSelectedModel).toBe("gpt-5.3-codex");
+  });
 });
 
 describe("composerDraftStore setProvider", () => {
@@ -394,6 +413,8 @@ describe("composerDraftStore setProvider", () => {
       draftsByThreadId: {},
       draftThreadsByThreadId: {},
       projectDraftThreadIdByProjectId: {},
+      lastSelectedModel: null,
+      lastSelectedProvider: null,
     });
   });
 
@@ -413,6 +434,14 @@ describe("composerDraftStore setProvider", () => {
 
     expect(useComposerDraftStore.getState().draftsByThreadId[threadId]).toBeUndefined();
   });
+
+  it("updates lastSelectedProvider when a non-null provider is set", () => {
+    const store = useComposerDraftStore.getState();
+
+    store.setProvider(threadId, "codex");
+
+    expect(useComposerDraftStore.getState().lastSelectedProvider).toBe("codex");
+  });
 });
 
 describe("composerDraftStore runtime and interaction settings", () => {

apps/web/src/composerDraftStore.ts

@@ -97,6 +97,8 @@ interface PersistedComposerDraftStoreState {
   draftsByThreadId: Record<ThreadId, PersistedComposerThreadDraftState>;
   draftThreadsByThreadId: Record<ThreadId, PersistedDraftThreadState>;
   projectDraftThreadIdByProjectId: Record<ProjectId, ThreadId>;
+  lastSelectedModel: string | null;
+  lastSelectedProvider: ProviderKind | null;
 }
 
 interface ComposerThreadDraftState {
@@ -130,6 +132,8 @@ interface ComposerDraftStoreState {
   draftsByThreadId: Record<ThreadId, ComposerThreadDraftState>;
   draftThreadsByThreadId: Record<ThreadId, DraftThreadState>;
   projectDraftThreadIdByProjectId: Record<ProjectId, ThreadId>;
+  lastSelectedModel: string | null;
+  lastSelectedProvider: ProviderKind | null;
   getDraftThreadByProjectId: (projectId: ProjectId) => ProjectDraftThread | null;
   getDraftThread: (threadId: ThreadId) => DraftThreadState | null;
   setProjectDraftThreadId: (
@@ -185,6 +189,8 @@ const EMPTY_PERSISTED_DRAFT_STORE_STATE: PersistedComposerDraftStoreState = {
   draftsByThreadId: {},
   draftThreadsByThreadId: {},
   projectDraftThreadIdByProjectId: {},
+  lastSelectedModel: null,
+  lastSelectedProvider: null,
 };
 
 const EMPTY_IMAGES: ComposerImageAttachment[] = [];
@@ -450,10 +456,17 @@ function normalizePersistedComposerDraftState(value: unknown): PersistedComposer
       ...(codexFastMode ? { codexFastMode } : {}),
     };
   }
+  const lastSelectedModel =
+    typeof candidate.lastSelectedModel === "string"
+      ? (normalizeModelSlug(candidate.lastSelectedModel) ?? null)
+      : null;
+  const lastSelectedProvider = normalizeProviderKind(candidate.lastSelectedProvider);
   return {
     draftsByThreadId: nextDraftsByThreadId,
     draftThreadsByThreadId,
     projectDraftThreadIdByProjectId,
+    lastSelectedModel,
+    lastSelectedProvider,
   };
 }
 
@@ -563,6 +576,8 @@ export const useComposerDraftStore = create<ComposerDraftStoreState>()(
       draftsByThreadId: {},
       draftThreadsByThreadId: {},
       projectDraftThreadIdByProjectId: {},
+      lastSelectedModel: null,
+      lastSelectedProvider: null,
       getDraftThreadByProjectId: (projectId) => {
         if (projectId.length === 0) {
           return null;
@@ -841,7 +856,13 @@ export const useComposerDraftStore = create<ComposerDraftStoreState>()(
           } else {
             nextDraftsByThreadId[threadId] = nextDraft;
           }
-          return { draftsByThreadId: nextDraftsByThreadId };
+          const nextState: Partial<ComposerDraftStoreState> = {
+            draftsByThreadId: nextDraftsByThreadId,
+          };
+          if (normalizedProvider !== null) {
+            nextState.lastSelectedProvider = normalizedProvider;
+          }
+          return nextState;
         });
       },
       setModel: (threadId, model) => {
@@ -868,7 +889,13 @@ export const useComposerDraftStore = create<ComposerDraftStoreState>()(
           } else {
             nextDraftsByThreadId[threadId] = nextDraft;
           }
-          return { draftsByThreadId: nextDraftsByThreadId };
+          const nextState: Partial<ComposerDraftStoreState> = {
+            draftsByThreadId: nextDraftsByThreadId,
+          };
+          if (normalizedModel !== null) {
+            nextState.lastSelectedModel = normalizedModel;
+          }
+          return nextState;
         });
       },
       setRuntimeMode: (threadId, runtimeMode) => {

apps/web/src/hooks/useHandleNewThread.ts

@@ -89,6 +89,9 @@ export function useHandleNewThread() {
       const threadId = newThreadId();
       const createdAt = new Date().toISOString();
       return (async () => {
+        const { lastSelectedModel, lastSelectedProvider, setModel, setProvider } =
+          useComposerDraftStore.getState();
+
         setProjectDraftThreadId(projectId, threadId, {
           createdAt,
           branch: options?.branch ?? null,
@@ -97,6 +100,13 @@ export function useHandleNewThread() {
           runtimeMode: DEFAULT_RUNTIME_MODE,
         });
 
+        if (lastSelectedModel) {
+          setModel(threadId, lastSelectedModel);
+        }
+        if (lastSelectedProvider) {
+          setProvider(threadId, lastSelectedProvider);
+        }
+
         await navigate({
           to: "/$threadId",
           params: { threadId },