diff --git a/client/src/components/Settings/IntegrationsTab.test.tsx b/client/src/components/Settings/IntegrationsTab.test.tsx
index e04a95619..e3ce2e6e9 100644
--- a/client/src/components/Settings/IntegrationsTab.test.tsx
+++ b/client/src/components/Settings/IntegrationsTab.test.tsx
@@ -644,11 +644,11 @@ describe('IntegrationsTab', () => {
     expect(createBtn).toBeDisabled();
   });
 
-  it('FE-COMP-INTEGRATIONS-032: error toast shown when create OAuth client fails', async () => {
+  it('FE-COMP-INTEGRATIONS-032: a refused client registration toasts the reason the server gave', async () => {
     const user = userEvent.setup();
     server.use(
       http.post('/api/oauth/clients', () =>
-        HttpResponse.json({ error: 'server error' }, { status: 500 })
+        HttpResponse.json({ error: 'Redirect URI must use HTTPS, loopback HTTP, or a private custom scheme: http://192.168.1.5/cb' }, { status: 400 })
       )
     );
     enableMcp();
@@ -657,9 +657,24 @@ describe('IntegrationsTab', () => {
     await user.click(screen.getByRole('button', { name: /New Client/i }));
     await screen.findByText('Register OAuth Client');
     await user.type(screen.getByPlaceholderText(/Claude Web, My MCP App/i), 'Fail Client');
+    await user.type(screen.getByPlaceholderText(/https:\/\/your-app/i), 'http://192.168.1.5/cb');
+    await user.click(screen.getByRole('button', { name: /Register Client/i }));
+    expect(await screen.findByText(/Redirect URI must use HTTPS/)).toBeInTheDocument();
+    expect(screen.queryByText('Failed to register OAuth client')).not.toBeInTheDocument();
+  });
+
+  it('FE-COMP-INTEGRATIONS-032b: a failure the server says nothing about keeps the generic text', async () => {
+    const user = userEvent.setup();
+    server.use(http.post('/api/oauth/clients', () => HttpResponse.error()));
+    enableMcp();
+    render(<><ToastContainer /><IntegrationsTab /></>);
+    await screen.findByText('MCP Configuration');
+    await user.click(screen.getByRole('button', { name: /New Client/i }));
+    await screen.findByText('Register OAuth Client');
+    await user.type(screen.getByPlaceholderText(/Claude Web, My MCP App/i), 'Fail Client');
     await user.type(screen.getByPlaceholderText(/https:\/\/your-app/i), 'http://localhost');
     await user.click(screen.getByRole('button', { name: /Register Client/i }));
-    expect(await screen.findByText(/Failed to register/i)).toBeInTheDocument();
+    expect(await screen.findByText('Failed to register OAuth client')).toBeInTheDocument();
   });
 });
 
diff --git a/client/src/components/Settings/IntegrationsTab.tsx b/client/src/components/Settings/IntegrationsTab.tsx
index 23085f799..4310717a9 100644
--- a/client/src/components/Settings/IntegrationsTab.tsx
+++ b/client/src/components/Settings/IntegrationsTab.tsx
@@ -13,6 +13,7 @@ import ApiKeysSection from './ApiKeysSection'
 import { PRESET_SCOPES_DEFAULT, PRESET_SCOPES_READONLY } from '../../api/oauthScopes'
 import ScopeGroupPicker from '../OAuth/ScopeGroupPicker'
 import { useAuthStore } from '../../store/authStore'
+import { getApiErrorMessage } from '../../utils/apiError'
 
 interface OAuthPreset {
   id: string
@@ -266,8 +267,11 @@ function useIntegrations() {
       setOauthNewUris('')
       setOauthNewScopes([])
       setOauthIsMachine(false)
-    } catch {
-      toast.error(t('settings.oauth.toast.createError'))
+    } catch (err) {
+      // The server names the rule that refused the client (a scope, the
+      // ten-client cap, a redirect URI it will not take); swallowing it left
+      // "Failed to register OAuth client" and nothing to act on.
+      toast.error(getApiErrorMessage(err, t('settings.oauth.toast.createError')))
     } finally {
       setOauthCreating(false)
     }
diff --git a/client/src/mobile/screens/settings/MSettingsMcp.tsx b/client/src/mobile/screens/settings/MSettingsMcp.tsx
index b4304cf6e..873837007 100644
--- a/client/src/mobile/screens/settings/MSettingsMcp.tsx
+++ b/client/src/mobile/screens/settings/MSettingsMcp.tsx
@@ -11,6 +11,7 @@ import MChip from '../../components/MChip'
 import { MSetCard, MSetEyebrow, MSetInput, MSetTextarea, MSetButton, MSetHint, MSetRow } from './MSettingsUi'
 import MConfirmSheet from './MConfirmSheet'
 import MSegmented from '../../components/MSegmented'
+import { getApiErrorMessage } from '../../../utils/apiError'
 
 interface OAuthPreset {
   id: string
@@ -188,8 +189,11 @@ export default function MSettingsMcp() {
       setNewUris('')
       setNewScopes([])
       setIsMachine(false)
-    } catch {
-      toast.error(t('settings.oauth.toast.createError'))
+    } catch (err) {
+      // The server names the rule that refused the client (a scope, the
+      // ten-client cap, a redirect URI it will not take); swallowing it left
+      // "Failed to register OAuth client" and nothing to act on.
+      toast.error(getApiErrorMessage(err, t('settings.oauth.toast.createError')))
     } finally {
       setCreating(false)
     }
diff --git a/client/tests/unit/mobile/settings/MSettingsMcp.test.tsx b/client/tests/unit/mobile/settings/MSettingsMcp.test.tsx
index 7839c7599..fbacac3c3 100644
--- a/client/tests/unit/mobile/settings/MSettingsMcp.test.tsx
+++ b/client/tests/unit/mobile/settings/MSettingsMcp.test.tsx
@@ -265,17 +265,19 @@ describe('MSettingsMcp', () => {
     expect(writeText).toHaveBeenNthCalledWith(2, 'sec-copy');
   });
 
-  it('FE-MOB-SETMCP-011: a failed registration surfaces an error toast and keeps the form open', async () => {
+  it('FE-MOB-SETMCP-011: a refused registration surfaces the reason the server gave and keeps the form open', async () => {
     const user = userEvent.setup();
-    server.use(http.post('/api/oauth/clients', () => HttpResponse.json({ error: 'boom' }, { status: 500 })));
+    server.use(http.post('/api/oauth/clients', () =>
+      HttpResponse.json({ error: 'Redirect URI must use HTTPS, loopback HTTP, or a private custom scheme: http://192.168.1.5/cb' }, { status: 400 })));
     renderScreen();
 
     await user.click(screen.getByRole('button', { name: 'New Client' }));
     await user.type(await screen.findByPlaceholderText(/Claude Web, My MCP App/), 'Broken');
-    fireEvent.change(screen.getByPlaceholderText(/your-app/), { target: { value: 'http://localhost' } });
+    fireEvent.change(screen.getByPlaceholderText(/your-app/), { target: { value: 'http://192.168.1.5/cb' } });
     await user.click(screen.getByRole('button', { name: 'Register Client' }));
 
-    await screen.findByText('Failed to register OAuth client');
+    await screen.findByText(/Redirect URI must use HTTPS/);
+    expect(screen.queryByText('Failed to register OAuth client')).toBeNull();
     expect(screen.getByText('Register OAuth Client')).toBeInTheDocument();
   });