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

Switching between Build and Plan modes reset the model selector to the
settings default because setAgent fell through to the settings-default
fallback whenever the target agent had no saved override, and the
explicit-switch path in ModelControls force-applied the agent's default
model, overwriting any per-agent override.

setAgent now keeps the current model selection when the user has a live
manual selection and the target agent configures no model of its own,
and the explicit-switch handler no longer clobbers saved per-agent
overrides with the agent default. Startup and pin behavior are
unchanged: the settings-default and agent-pin cascade still applies
when no manual selection exists yet.

Fixes #2531

packages/ui/src/components/chat/ModelControls.tsx

@@ -641,7 +641,6 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
     ];
 
     const prevAgentNameRef = React.useRef<string | undefined>(undefined);
-    const explicitAgentSwitchRef = React.useRef<string | null>(null);
     const latestLoadedUserChoiceRestoreRef = React.useRef<string | null>(null);
 
     const currentSessionDirectory = currentSessionId ? getDirectoryForSession(currentSessionId) : undefined;
@@ -1032,9 +1031,6 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
                     prevAgentNameRef.current = currentAgentName;
 
                     if (currentAgentName && currentSessionId) {
-                        const shouldPreferAgentModel = explicitAgentSwitchRef.current === currentAgentName;
-                        explicitAgentSwitchRef.current = null;
-
                         await new Promise<void>((resolve) => {
                             const timer = setTimeout(resolve, 50);
                             abortController.signal.addEventListener('abort', () => {
@@ -1047,33 +1043,6 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
                             return;
                         }
 
-                        const selectedAgent = shouldPreferAgentModel
-                            ? agents.find((agent) => agent.name === currentAgentName)
-                            : undefined;
-                        if (selectedAgent?.model?.providerID && selectedAgent.model.modelID) {
-                            const result = tryApplyModelSelection(
-                                selectedAgent.model.providerID,
-                                selectedAgent.model.modelID,
-                                currentAgentName,
-                            );
-                            if (result === 'applied' || result === 'provider-missing') {
-                                if (result === 'applied') {
-                                    saveSessionModelSelection(
-                                        currentSessionId,
-                                        selectedAgent.model.providerID,
-                                        selectedAgent.model.modelID,
-                                    );
-                                    saveAgentModelForSession(
-                                        currentSessionId,
-                                        currentAgentName,
-                                        selectedAgent.model.providerID,
-                                        selectedAgent.model.modelID,
-                                    );
-                                }
-                                return;
-                            }
-                        }
-
                         const persistedChoice = getAgentModelForSession(currentSessionId, currentAgentName);
 
                         if (persistedChoice) {
@@ -1099,12 +1068,9 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
             abortController.abort();
         };
     }, [
-        agents,
         currentAgentName,
         currentSessionId,
         getAgentModelForSession,
-        saveAgentModelForSession,
-        saveSessionModelSelection,
         tryApplyModelSelection,
         contextHydrated,
     ]);
@@ -1185,7 +1151,6 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
 
     const handleAgentChange = React.useCallback((agentName: string, options?: { closeModelSelector?: boolean }) => {
         try {
-            explicitAgentSwitchRef.current = agentName;
             setAgent(agentName);
             addRecentAgent(agentName);
             if (options?.closeModelSelector ?? true) {

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

@@ -589,6 +589,30 @@ 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

@@ -2387,6 +2387,9 @@ 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;
@@ -2508,8 +2511,7 @@ 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
+                        // (issue #2404).
-                        // agent default via ModelControls' shouldPreferAgentModel path.
                         if (currentSessionId) {
                             const existingAgentModel = useSelectionStore.getState().getAgentModelForSession(currentSessionId, agentName);
                             if (existingAgentModel && hasProviderModel(providers, existingAgentModel.providerId, existingAgentModel.modelId)) {
@@ -2538,6 +2540,14 @@ 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);