diff --git a/src/main/services/runtime/ClaudeMultimodelBridgeService.ts b/src/main/services/runtime/ClaudeMultimodelBridgeService.ts
index 98b6063f..53a6bb17 100644
--- a/src/main/services/runtime/ClaudeMultimodelBridgeService.ts
+++ b/src/main/services/runtime/ClaudeMultimodelBridgeService.ts
@@ -1,7 +1,10 @@
 import { execCli } from '@main/utils/childProcess';
 import { resolveInteractiveShellEnv } from '@main/utils/shellEnv';
 import { createLogger } from '@shared/utils/logger';
-import { createDefaultCliExtensionCapabilities } from '@shared/utils/providerExtensionCapabilities';
+import {
+  createDefaultCliExtensionCapabilities,
+  createLegacyRuntimeFallbackCliExtensionCapabilities,
+} from '@shared/utils/providerExtensionCapabilities';
 
 import { resolveGeminiRuntimeAuth } from './geminiRuntimeAuth';
 import { buildProviderAwareCliEnv } from './providerAwareCliEnv';
@@ -145,7 +148,7 @@ function createDefaultProviderStatus(providerId: CliProviderId): CliProviderStat
     capabilities: {
       teamLaunch: false,
       oneShot: false,
-      extensions: createDefaultCliExtensionCapabilities(),
+      extensions: createLegacyRuntimeFallbackCliExtensionCapabilities(),
     },
     selectedBackendId: null,
     resolvedBackendId: null,
@@ -159,7 +162,9 @@ function createDefaultProviderStatus(providerId: CliProviderId): CliProviderStat
 function mapRuntimeExtensionCapabilities(
   capabilities?: RuntimeExtensionCapabilitiesResponse
 ): CliProviderStatus['capabilities']['extensions'] {
-  const defaults = createDefaultCliExtensionCapabilities();
+  const defaults = capabilities
+    ? createDefaultCliExtensionCapabilities()
+    : createLegacyRuntimeFallbackCliExtensionCapabilities();
 
   return {
     plugins: {
diff --git a/src/shared/utils/providerExtensionCapabilities.ts b/src/shared/utils/providerExtensionCapabilities.ts
index c6d03b49..4d5d0c90 100644
--- a/src/shared/utils/providerExtensionCapabilities.ts
+++ b/src/shared/utils/providerExtensionCapabilities.ts
@@ -10,6 +10,27 @@ const SUPPORTED_SHARED_CAPABILITY: CliExtensionCapability = {
   reason: null,
 };
 
+const LEGACY_MULTIMODEL_FALLBACK_CAPABILITIES: CliExtensionCapabilities = {
+  plugins: {
+    status: 'unsupported',
+    ownership: 'shared',
+    reason:
+      'This runtime does not declare plugin capability support. Upgrade the runtime to manage plugins here.',
+  },
+  mcp: {
+    status: 'read-only',
+    ownership: 'shared',
+    reason:
+      'This runtime does not declare MCP management support. Upgrade the runtime to install or remove MCP servers here.',
+  },
+  skills: {
+    ...SUPPORTED_SHARED_CAPABILITY,
+  },
+  apiKeys: {
+    ...SUPPORTED_SHARED_CAPABILITY,
+  },
+};
+
 export function createDefaultCliExtensionCapabilities(
   overrides?: Partial<CliExtensionCapabilities>
 ): CliExtensionCapabilities {
@@ -22,10 +43,22 @@ export function createDefaultCliExtensionCapabilities(
   };
 }
 
+export function createLegacyRuntimeFallbackCliExtensionCapabilities(
+  overrides?: Partial<CliExtensionCapabilities>
+): CliExtensionCapabilities {
+  return {
+    plugins: { ...LEGACY_MULTIMODEL_FALLBACK_CAPABILITIES.plugins },
+    mcp: { ...LEGACY_MULTIMODEL_FALLBACK_CAPABILITIES.mcp },
+    skills: { ...LEGACY_MULTIMODEL_FALLBACK_CAPABILITIES.skills },
+    apiKeys: { ...LEGACY_MULTIMODEL_FALLBACK_CAPABILITIES.apiKeys },
+    ...overrides,
+  };
+}
+
 export function getCliProviderExtensionCapabilities(
   provider: Pick<CliProviderStatus, 'capabilities'>
 ): CliExtensionCapabilities {
-  return provider.capabilities.extensions ?? createDefaultCliExtensionCapabilities();
+  return provider.capabilities.extensions ?? createLegacyRuntimeFallbackCliExtensionCapabilities();
 }
 
 export function getCliProviderExtensionCapability(
diff --git a/test/main/services/runtime/ClaudeMultimodelBridgeService.test.ts b/test/main/services/runtime/ClaudeMultimodelBridgeService.test.ts
index d6cde5c7..d69ea045 100644
--- a/test/main/services/runtime/ClaudeMultimodelBridgeService.test.ts
+++ b/test/main/services/runtime/ClaudeMultimodelBridgeService.test.ts
@@ -254,4 +254,43 @@ describe('ClaudeMultimodelBridgeService', () => {
     });
     expect(provider.statusMessage).toContain('ANTHROPIC_API_KEY');
   });
+
+  it('falls back conservatively when the runtime omits extension capability metadata', async () => {
+    execCliMock.mockResolvedValue({
+      stdout: JSON.stringify({
+        providers: {
+          codex: {
+            supported: true,
+            authenticated: true,
+            verificationState: 'verified',
+            canLoginFromUi: true,
+            capabilities: {
+              teamLaunch: true,
+              oneShot: true,
+            },
+          },
+        },
+      }),
+      stderr: '',
+      exitCode: 0,
+    });
+
+    const { ClaudeMultimodelBridgeService } =
+      await import('@main/services/runtime/ClaudeMultimodelBridgeService');
+    const service = new ClaudeMultimodelBridgeService();
+
+    const provider = await service.getProviderStatus('/mock/agent_teams_orchestrator', 'codex');
+
+    expect(provider).toMatchObject({
+      providerId: 'codex',
+      capabilities: {
+        extensions: {
+          plugins: { status: 'unsupported' },
+          mcp: { status: 'read-only' },
+          skills: { status: 'supported' },
+          apiKeys: { status: 'supported' },
+        },
+      },
+    });
+  });
 });
diff --git a/test/shared/utils/providerExtensionCapabilities.test.ts b/test/shared/utils/providerExtensionCapabilities.test.ts
new file mode 100644
index 00000000..da8dd4a6
--- /dev/null
+++ b/test/shared/utils/providerExtensionCapabilities.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  createLegacyRuntimeFallbackCliExtensionCapabilities,
+  getCliProviderExtensionCapabilities,
+} from '@shared/utils/providerExtensionCapabilities';
+
+import type { CliProviderStatus } from '@shared/types';
+
+function makeProvider(
+  overrides?: Partial<CliProviderStatus>
+): Pick<CliProviderStatus, 'capabilities'> {
+  return {
+    capabilities: {
+      teamLaunch: false,
+      oneShot: false,
+      ...(overrides?.capabilities ?? {}),
+    } as CliProviderStatus['capabilities'],
+  };
+}
+
+describe('providerExtensionCapabilities', () => {
+  it('returns conservative fallback capabilities when runtime omits extension metadata', () => {
+    const capabilities = getCliProviderExtensionCapabilities(
+      makeProvider({
+        capabilities: {
+          teamLaunch: true,
+          oneShot: true,
+        } as CliProviderStatus['capabilities'],
+      })
+    );
+
+    expect(capabilities).toEqual(createLegacyRuntimeFallbackCliExtensionCapabilities());
+  });
+
+  it('keeps plugins unsupported and mcp read-only in the legacy multimodel fallback', () => {
+    const capabilities = createLegacyRuntimeFallbackCliExtensionCapabilities();
+
+    expect(capabilities.plugins.status).toBe('unsupported');
+    expect(capabilities.mcp.status).toBe('read-only');
+    expect(capabilities.skills.status).toBe('supported');
+    expect(capabilities.apiKeys.status).toBe('supported');
+  });
+});