diff --git a/apps/frontend/src/features/settings/hooks/useSettings.test.ts b/apps/frontend/src/features/settings/hooks/useSettings.test.ts
index 6dde595..fb0604d 100644
--- a/apps/frontend/src/features/settings/hooks/useSettings.test.ts
+++ b/apps/frontend/src/features/settings/hooks/useSettings.test.ts
@@ -136,3 +136,52 @@ describe('useSettings tab deep-linking', () => {
     expect(result.current.activeTab).toBe('Notifications');
   });
 });
+
+describe('useSettings base URL handling across providers', () => {
+  const OLLAMA_SAVED: AiProviderPublicConfig = {
+    provider: 'ollama',
+    model: 'llama3.2',
+    baseUrl: 'http://localhost:11434',
+    hasApiKey: false,
+    apiKeyLast4: null,
+  };
+
+  it('does not carry a saved Ollama base URL into a fixed-endpoint provider', async () => {
+    // The reported sequence: Ollama is saved, the user switches to a hosted
+    // provider, and the backend answers "AI provider destination is not
+    // permitted." because a fixed destination rejects any base URL -- while
+    // the form shows no base URL field to clear, so the value is invisible.
+    vi.mocked(aiService.getConfig).mockResolvedValue(OLLAMA_SAVED);
+    vi.mocked(aiService.testConfig).mockResolvedValue({ ok: true, message: 'Connected.' });
+    vi.mocked(aiService.saveConfig).mockResolvedValue(EMPTY_CONFIG);
+
+    const { result } = renderHook(() => useSettings(), { wrapper: wrapper() });
+    await waitFor(() => expect(result.current.baseUrl).toBe('http://localhost:11434'));
+
+    act(() => result.current.setProvider('openai'));
+    expect(result.current.baseUrl).toBe('');
+
+    await act(async () => {
+      await result.current.testAiConfig();
+    });
+    expect(vi.mocked(aiService.testConfig).mock.lastCall?.[0].baseUrl).toBeUndefined();
+
+    await act(async () => {
+      await result.current.saveAiConfig();
+    });
+    expect(vi.mocked(aiService.saveConfig).mock.lastCall?.[0].baseUrl).toBeUndefined();
+  });
+
+  it('still sends the base URL for a provider that requires one', async () => {
+    vi.mocked(aiService.getConfig).mockResolvedValue(OLLAMA_SAVED);
+    vi.mocked(aiService.saveConfig).mockResolvedValue(OLLAMA_SAVED);
+
+    const { result } = renderHook(() => useSettings(), { wrapper: wrapper() });
+    await waitFor(() => expect(result.current.baseUrl).toBe('http://localhost:11434'));
+
+    await act(async () => {
+      await result.current.saveAiConfig();
+    });
+    expect(vi.mocked(aiService.saveConfig).mock.lastCall?.[0].baseUrl).toBe('http://localhost:11434');
+  });
+});
diff --git a/apps/frontend/src/features/settings/hooks/useSettings.ts b/apps/frontend/src/features/settings/hooks/useSettings.ts
index 83c97d3..43fd119 100644
--- a/apps/frontend/src/features/settings/hooks/useSettings.ts
+++ b/apps/frontend/src/features/settings/hooks/useSettings.ts
@@ -88,12 +88,27 @@ export function useSettings() {
     (nextProvider: AiProvider) => {
       setProvider(nextProvider);
       setModel(capabilityByProvider.get(nextProvider)?.defaultModel ?? '');
+      // A provider with a fixed endpoint has no base URL field, so a value
+      // left over from a previously selected provider would be invisible here
+      // and still be sent -- and the backend denies any base URL on a fixed
+      // destination, which surfaced as "AI provider destination is not
+      // permitted." with nothing on screen to correct.
+      if (!capabilityByProvider.get(nextProvider)?.requiresBaseUrl) {
+        setBaseUrl('');
+      }
       setStatusMessage(null);
       setError(null);
     },
     [capabilityByProvider],
   );
 
+  // Belt and braces: never send a base URL for a provider that does not take
+  // one, whatever the field happens to hold.
+  const baseUrlForRequest = useCallback(() => {
+    if (!capabilityByProvider.get(provider)?.requiresBaseUrl) return undefined;
+    return baseUrl.trim() || undefined;
+  }, [baseUrl, capabilityByProvider, provider]);
+
   const saveAiConfig = useCallback(async () => {
     setLoading(true);
     setError(null);
@@ -104,7 +119,7 @@ export function useSettings() {
         provider,
         apiKey: apiKey.trim() || undefined,
         model: model.trim() || defaultModel,
-        baseUrl: baseUrl.trim() || undefined,
+        baseUrl: baseUrlForRequest(),
       });
       setAiConfig(config);
       setApiKey('');
@@ -114,7 +129,7 @@ export function useSettings() {
     } finally {
       setLoading(false);
     }
-  }, [apiKey, baseUrl, capabilityByProvider, model, provider]);
+  }, [apiKey, baseUrlForRequest, capabilityByProvider, model, provider]);
 
   const testAiConfig = useCallback(async () => {
     setTesting(true);
@@ -126,7 +141,7 @@ export function useSettings() {
         provider,
         apiKey: apiKey.trim() || undefined,
         model: model.trim() || defaultModel,
-        baseUrl: baseUrl.trim() || undefined,
+        baseUrl: baseUrlForRequest(),
       });
       setStatusMessage(response.message);
     } catch (caught) {
@@ -134,7 +149,7 @@ export function useSettings() {
     } finally {
       setTesting(false);
     }
-  }, [apiKey, baseUrl, capabilityByProvider, model, provider]);
+  }, [apiKey, baseUrlForRequest, capabilityByProvider, model, provider]);
 
   return {
     tabs: settingsTabs,