diff --git a/frontend/src/spa/vertical-slice.ts b/frontend/src/spa/vertical-slice.ts
index a63bf93..a89b18b 100644
--- a/frontend/src/spa/vertical-slice.ts
+++ b/frontend/src/spa/vertical-slice.ts
@@ -41,10 +41,10 @@ export function createVerticalSliceController(
   api: ApiClient,
   sessionContextStore: SessionContextStore = NOOP_SESSION_CONTEXT_STORE
 ): VerticalSliceController {
-  const persistedContext = sessionContextStore.get();
+  let contextState = sessionContextStore.get();
 
   const state: VerticalSliceState = {
-    sessionCode: persistedContext?.sessionCode ?? '',
+    sessionCode: contextState?.sessionCode ?? '',
     session: null,
     gameplayPhase: null,
     joinState: 'idle',
@@ -55,13 +55,22 @@ export function createVerticalSliceController(
 
   const normalizeCode = (value: string): string => value.trim().toUpperCase();
 
+  const syncContext = (next: SessionContext): void => {
+    contextState = next;
+    sessionContextStore.set(next);
+  };
+
+  const resolveSessionCode = (sessionCode: string): string => {
+    const normalizedRequestedCode = normalizeCode(sessionCode);
+    const fallbackCode = normalizeCode(state.sessionCode || contextState?.sessionCode || '');
+    return normalizedRequestedCode || fallbackCode;
+  };
+
   async function hydrateLobby(sessionCode: string): Promise<VerticalSliceState> {
     state.loadingSession = true;
     state.errorMessage = null;
 
-    const normalizedRequestedCode = normalizeCode(sessionCode);
-    const fallbackCode = normalizeCode(state.sessionCode || persistedContext?.sessionCode || '');
-    state.sessionCode = normalizedRequestedCode || fallbackCode;
+    state.sessionCode = resolveSessionCode(sessionCode);
 
     const result = await api.getSession(state.sessionCode);
     state.loadingSession = false;
@@ -76,8 +85,8 @@ export function createVerticalSliceController(
     state.gameplayPhase = deriveGameplayPhase(result.data);
     state.sessionCode = normalizeCode(result.data.session.code);
 
-    if (persistedContext && state.sessionCode === normalizeCode(persistedContext.sessionCode)) {
-      sessionContextStore.set({ ...persistedContext, sessionCode: state.sessionCode });
+    if (contextState) {
+      syncContext({ ...contextState, sessionCode: state.sessionCode });
     }
 
     return { ...state };
@@ -87,9 +96,7 @@ export function createVerticalSliceController(
     state.joinState = 'loading';
     state.errorMessage = null;
 
-    const normalizedRequestedCode = normalizeCode(sessionCode);
-    const fallbackCode = normalizeCode(state.sessionCode || persistedContext?.sessionCode || '');
-    const requestCode = normalizedRequestedCode || fallbackCode;
+    const requestCode = resolveSessionCode(sessionCode);
 
     const join = await api.joinSession({ code: requestCode, nickname });
     if (!join.ok) {
@@ -101,7 +108,7 @@ export function createVerticalSliceController(
     state.joinState = 'success';
     state.sessionCode = normalizeCode(join.data.session.code || requestCode);
 
-    sessionContextStore.set({
+    syncContext({
       sessionCode: state.sessionCode,
       playerToken: join.data.player.session_token,
       nickname: join.data.player.nickname
@@ -114,9 +121,7 @@ export function createVerticalSliceController(
     state.startRoundState = 'loading';
     state.errorMessage = null;
 
-    const normalizedRequestedCode = normalizeCode(sessionCode);
-    const fallbackCode = normalizeCode(state.sessionCode || persistedContext?.sessionCode || '');
-    const codeToUse = normalizedRequestedCode || fallbackCode;
+    const codeToUse = resolveSessionCode(sessionCode);
 
     const start = await api.startRound(codeToUse, { category_slug: categorySlug });
     if (!start.ok) {
diff --git a/frontend/tests/vertical-slice.test.ts b/frontend/tests/vertical-slice.test.ts
index ef1e004..5f9c4ad 100644
--- a/frontend/tests/vertical-slice.test.ts
+++ b/frontend/tests/vertical-slice.test.ts
@@ -124,6 +124,67 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
     expect(api.getSession).toHaveBeenCalledWith('ABCD12');
   });
 
+  it('uses stored session code for startRound when input code is empty after join', async () => {
+    const api = makeApiMock();
+    const sessionContextStore = makeSessionContextStore();
+    const controller = createVerticalSliceController(api, sessionContextStore);
+
+    await controller.joinLobby('abcd12', 'Maja');
+    await controller.startRound('', 'history');
+
+    expect(api.startRound).toHaveBeenCalledWith('ABCD12', { category_slug: 'history' });
+  });
+
+  it('keeps joined player context when hydrate syncs normalized session code', async () => {
+    const api = makeApiMock({
+      getSession: vi.fn().mockResolvedValue({
+        ok: true,
+        status: 200,
+        data: {
+          session: { code: 'abcd12', status: 'lobby', host_id: 1, current_round: 1, players_count: 3 },
+          players: [],
+          round_question: null,
+          phase_view_model: {
+            status: 'lobby',
+            round_number: 1,
+            players_count: 3,
+            constraints: {
+              min_players_to_start: 3,
+              max_players_mvp: 5,
+              min_players_reached: true,
+              max_players_allowed: true
+            },
+            host: {
+              can_start_round: true,
+              can_show_question: false,
+              can_mix_answers: false,
+              can_calculate_scores: false,
+              can_reveal_scoreboard: false,
+              can_start_next_round: false,
+              can_finish_game: false
+            },
+            player: {
+              can_join: true,
+              can_submit_lie: false,
+              can_submit_guess: false,
+              can_view_final_result: false
+            }
+          }
+        }
+      })
+    });
+    const sessionContextStore = makeSessionContextStore();
+    const controller = createVerticalSliceController(api, sessionContextStore);
+
+    await controller.joinLobby('abcd12', 'Maja');
+
+    expect(sessionContextStore.set).toHaveBeenLastCalledWith({
+      sessionCode: 'ABCD12',
+      playerToken: 'token-1',
+      nickname: 'Maja'
+    });
+  });
+
   it('surfaces a friendly error when join fails', async () => {
     const api = makeApiMock({
       joinSession: vi.fn().mockResolvedValue({