feat(backends): add external-to-external failover before native fallback

src/audit/types.ts

@@ -223,7 +223,7 @@ export interface BackendFallbackEvent {
   channel: string;
   sender: string;
   from_backend: 'claude_code' | 'opencode' | 'codex' | 'gemini';
-  to_backend: 'native';
+  to_backend: 'native' | 'claude_code' | 'opencode' | 'codex' | 'gemini';
   reason: string;
 }
 

src/daemon/routing.test.ts

@@ -1197,6 +1197,85 @@ describe('daemon auto-escalate integration', () => {
     expect(reply).toHaveBeenCalledWith(expect.objectContaining({ text: 'native fallback response' }));
   });
 
+  it('fails over to another enabled external backend before native fallback', async () => {
+    const processSpy = vi.spyOn(AgentOrchestrator.prototype, 'process')
+      .mockResolvedValue('native should not be used');
+    const session = {
+      id: 'telegram:external-failover',
+      addMessage: vi.fn(),
+      getHistory: vi.fn(() => []),
+      clear: vi.fn(),
+      replaceHistory: vi.fn(),
+      getConfig: vi.fn(() => undefined),
+      setConfig: vi.fn(),
+      deleteConfig: vi.fn(),
+    };
+
+    const codexBackend = {
+      name: 'codex',
+      process: vi.fn(async () => {
+        throw new Error('codex failed');
+      }),
+    };
+    const geminiBackend = {
+      name: 'gemini',
+      process: vi.fn(async () => 'gemini recovered'),
+    };
+
+    const router = createMessageRouter({
+      sessionManager: {
+        getSession: vi.fn(() => session),
+      } as unknown as MessageRouterDeps['sessionManager'],
+      modelRouter: {
+        getAvailableTiers: () => ['fast', 'default', 'complex', 'local'],
+        getAllLabels: () => ({ fast: 'fast', default: 'default', complex: 'complex', local: 'local' }),
+        getLabel: (tier: string) => tier,
+      } as unknown as MessageRouterDeps['modelRouter'],
+      systemPrompt: 'test prompt',
+      toolRegistry: {
+        clone() { return this; },
+        register: vi.fn(),
+      } as unknown as MessageRouterDeps['toolRegistry'],
+      toolExecutor: {} as unknown as MessageRouterDeps['toolExecutor'],
+      config: {
+        agents: {
+          primary_tier: 'default',
+          delegation: {
+            compaction: 'fast',
+            memory_extraction: 'fast',
+            classification: 'fast',
+            tool_summarisation: 'fast',
+            complex_reasoning: 'complex',
+          },
+          max_delegation_depth: 3,
+          max_iterations: 10,
+          auto_escalate: false,
+        },
+        compaction: { enabled: false },
+        models: { default: { provider: 'anthropic', model: 'claude' } },
+      } as unknown as MessageRouterDeps['config'],
+      externalBackends: {
+        codex: codexBackend,
+        gemini: geminiBackend,
+      } as unknown as MessageRouterDeps['externalBackends'],
+      defaultExternalBackendName: 'codex',
+    });
+
+    const reply = vi.fn(async (_message: OutboundMessage) => {});
+    await router.handler({
+      id: 'm-external-failover',
+      channel: 'telegram',
+      senderId: 'external-failover',
+      text: 'hello failover',
+      timestamp: Date.now(),
+    } as MessageRouterInput, reply);
+
+    expect(codexBackend.process).toHaveBeenCalled();
+    expect(geminiBackend.process).toHaveBeenCalled();
+    expect(processSpy).not.toHaveBeenCalled();
+    expect(reply).toHaveBeenCalledWith(expect.objectContaining({ text: 'gemini recovered' }));
+  });
+
   it('uses per-agent backend override instead of default external backend', async () => {
     const processSpy = vi.spyOn(AgentOrchestrator.prototype, 'process');
     const session = {