fix(ui): keep the selected model when switching agent modes (#2690)

fix(ui): keep the selected model when switching agent modes

packages/ui/src/stores/useConfigStore.test.ts

@@ -698,6 +698,30 @@ describe('useConfigStore provider persistence', () => {
     expect(state.currentModelId).toBe('model-a');
   });
 
+  test('[issue-2531] setAgent keeps the manual model when switching to an agent without an override', () => {
+    const sessionId = 'ses_2531_mode_switch';
+    useSessionUIStore.setState({ currentSessionId: sessionId });
+    useConfigStore.setState({
+      activeDirectoryKey: DIRECTORY,
+      providers: [provider('deepseek', 'deepseek-v4-pro'), provider('kimi', 'kimi-k3')],
+      agents: [testAgent('build'), testAgent('plan')],
+      settingsDefaultModel: 'deepseek/deepseek-v4-pro',
+      currentProviderId: 'kimi',
+      currentModelId: 'kimi-k3',
+      currentAgentName: 'build',
+      selectionSource: 'manual',
+      currentVariant: undefined,
+      directoryScoped: {},
+    });
+
+    useConfigStore.getState().setAgent('plan');
+
+    const state = useConfigStore.getState();
+    expect(state.currentAgentName).toBe('plan');
+    expect(state.currentProviderId).toBe('kimi');
+    expect(state.currentModelId).toBe('kimi-k3');
+  });
+
   test('loadAgents does not fetch OpenCode config directly', async () => {
     useConfigStore.setState({
       activeDirectoryKey: DIRECTORY,

packages/ui/src/stores/useConfigStore.ts

@@ -2435,6 +2435,9 @@ export const useConfigStore = create<ConfigStore>()(
                         currentProviderId,
                         currentModelId,
                     } = get();
+                    // Captured before the first set below, which unconditionally
+                    // marks the selection as manual.
+                    const hadManualSelection = get().selectionSource === "manual";
 
                     set((state) => {
                         const directoryKey = state.activeDirectoryKey;
@@ -2554,8 +2557,7 @@ export const useConfigStore = create<ConfigStore>()(
                         // Prefer a session-level manual override for this agent over the
                         // agent's configured default. Re-applying setAgent after subtask
                         // completion / rematerialization must not clobber the override
-                        // (issue #2404). Explicit agent-picker switches still force the
-                        // agent default via ModelControls' shouldPreferAgentModel path.
+                        // (issue #2404).
                         if (currentSessionId) {
                             const existingAgentModel = useSelectionStore.getState().getAgentModelForSession(currentSessionId, agentName);
                             if (existingAgentModel && hasProviderModel(providers, existingAgentModel.providerId, existingAgentModel.modelId)) {
@@ -2584,6 +2586,14 @@ export const useConfigStore = create<ConfigStore>()(
                             }
                         }
 
+                        // The user has a live manual model selection and the target
+                        // agent configures no model of its own. Switching modes or
+                        // agents must not reset the selection to the settings default
+                        // (issue #2531) — mode switches are not model changes.
+                        if (hadManualSelection && currentProviderId && currentModelId) {
+                            return;
+                        }
+
                         // If the agent has no preferred model, use settings default.
                         if (settingsDefaultModel) {
                             const parsed = parseModelString(settingsDefaultModel);