fix(providers): fail over across provider failure domains

nanobot/providers/fallback_provider.py

@@ -291,6 +291,8 @@ class FallbackProvider(LLMProvider):
 
     @staticmethod
     def _should_fallback(response: LLMResponse) -> bool:
+        if LLMProvider.is_arrearage_response(response):
+            return True
         if response.error_should_retry is False:
             return False
         status = response.error_status_code

tests/agent/test_runner_fallback.py

@@ -436,6 +436,53 @@ class TestFailoverOnTransientError:
         factory.assert_called_once_with(_fallback("fallback-a"))
 
 
+class TestFailoverOnArrearageError:
+    @pytest.mark.asyncio
+    async def test_non_retryable_quota_tries_configured_fallback(self) -> None:
+        arrearage = _make_response(
+            "insufficient quota",
+            finish_reason="error",
+            error_status_code=429,
+            error_type="insufficient_quota",
+            error_should_retry=False,
+        )
+        primary = _FakeProvider("primary", arrearage)
+        fallback = _FakeProvider("fallback", _make_response("fallback ok"))
+        fallback_preset = _fallback("fallback-a")
+        factory = MagicMock(return_value=fallback)
+        fb = FallbackProvider(
+            primary=primary,
+            fallback_presets=[fallback_preset],
+            provider_factory=factory,
+        )
+
+        result = await fb.chat(messages=[{"role": "user", "content": "hi"}])
+
+        assert result.content == "fallback ok"
+        factory.assert_called_once_with(fallback_preset)
+
+    @pytest.mark.asyncio
+    async def test_without_fallback_presets_returns_original_error(self) -> None:
+        arrearage = _make_response(
+            "payment required",
+            finish_reason="error",
+            error_status_code=402,
+            error_should_retry=False,
+        )
+        primary = _FakeProvider("primary", arrearage)
+        factory = MagicMock()
+        fb = FallbackProvider(
+            primary=primary,
+            fallback_presets=[],
+            provider_factory=factory,
+        )
+
+        result = await fb.chat(messages=[{"role": "user", "content": "hi"}])
+
+        assert result is arrearage
+        factory.assert_not_called()
+
+
 class TestNoFallbackOnNonRetryableError:
     @pytest.mark.asyncio
     async def test_bad_request(self) -> None: