fix(proxy): keep config-defined deployments when a config read returns no model_list

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/proxy/proxy_server.py

@@ -6491,10 +6491,27 @@ class ProxyConfig:
         router_model_ids: Final = llm_router.get_model_ids()
         # Check for model IDs in llm_router not present in combined_id_list and delete them
 
+        kept_config_ids: Final[frozenset[str]] = (
+            frozenset(
+                model_id
+                for model_id in router_model_ids
+                if (deployment := llm_router.get_deployment(model_id=model_id)) is not None
+                and deployment.model_info.db_model is False
+            )
+            if not model_list
+            else frozenset()
+        )
+        if kept_config_ids:
+            verbose_proxy_logger.warning(
+                "Config read in _delete_deployment returned no model_list. "
+                "Keeping %d config-defined deployments to avoid removing valid models.",
+                len(kept_config_ids),
+            )
+
         for model_id in router_model_ids:
-            if model_id not in combined_id_list:
+            if model_id not in combined_id_list and model_id not in kept_config_ids:
                 llm_router.delete_deployment(id=model_id)
-        return frozenset(combined_id_list)
+        return frozenset(combined_id_list) | kept_config_ids
 
     def _resolve_db_litellm_param(self, key: str, value: object) -> object:
         if not isinstance(value, str):

tests/test_litellm/proxy/test_update_llm_router_resilience.py

@@ -290,3 +290,96 @@ class TestDeleteDeploymentKeepsPluginConfigModels:
         entry = {"model_name": "gpt-4o-mini", "litellm_params": {"model": "gpt-4o-mini"}}
         pin_complexity_router_model_id(entry)
         assert "model_info" not in entry
+
+
+class TestDeleteDeploymentKeepsConfigModelsOnEmptyConfigRead:
+    """Regression: a config read that succeeds but returns no model_list (e.g. a
+    partially written file) must not evict config-sourced deployments, because
+    nothing re-adds config models at runtime. DB-sourced deployments missing from
+    db_models must still be evicted."""
+
+    @staticmethod
+    def _router(model_list):
+        from litellm import Router
+        from litellm.types.router import RouterGeneralSettings
+
+        return Router(
+            model_list=model_list,
+            router_general_settings=RouterGeneralSettings(async_only_mode=True),
+        )
+
+    @pytest.mark.asyncio
+    async def test_delete_deployment_keeps_config_models_when_config_read_has_no_model_list(self, tmp_path):
+        config_file_path = str(tmp_path / "config.yaml")
+        (tmp_path / "config.yaml").write_text("general_settings:\n  master_key: sk-1234\n")
+
+        router = self._router(
+            [
+                {
+                    "model_name": "config-model",
+                    "litellm_params": {"model": "gpt-4o-mini"},
+                    "model_info": {"id": "config-model-1"},
+                },
+                {
+                    "model_name": "db-model",
+                    "litellm_params": {"model": "gpt-4o-mini"},
+                    "model_info": {"id": "db-model-1", "db_model": True},
+                },
+            ]
+        )
+        proxy_config = ProxyConfig()
+        with (
+            patch("litellm.proxy.proxy_server.llm_router", router),  # test-quality-ok: reads module global
+            patch(  # test-quality-ok: reads module global
+                "litellm.proxy.proxy_server.user_config_file_path",
+                config_file_path,
+            ),
+        ):
+            result = await proxy_config._delete_deployment(db_models=[])
+
+        model_ids = router.get_model_ids()
+        assert "config-model-1" in model_ids
+        assert "db-model-1" not in model_ids
+        assert result is not None
+        assert "config-model-1" in result
+
+    @pytest.mark.asyncio
+    async def test_delete_deployment_still_evicts_config_model_removed_from_non_empty_model_list(self, tmp_path):
+        config_file_path = str(tmp_path / "config.yaml")
+        (tmp_path / "config.yaml").write_text(
+            "model_list:\n"
+            "  - model_name: model-a\n"
+            "    litellm_params:\n"
+            "      model: gpt-4o-mini\n"
+            "    model_info:\n"
+            "      id: model-a-id\n"
+        )
+
+        router = self._router(
+            [
+                {
+                    "model_name": "model-a",
+                    "litellm_params": {"model": "gpt-4o-mini"},
+                    "model_info": {"id": "model-a-id"},
+                },
+                {
+                    "model_name": "model-b",
+                    "litellm_params": {"model": "gpt-4o-mini"},
+                    "model_info": {"id": "model-b-id"},
+                },
+            ]
+        )
+        proxy_config = ProxyConfig()
+        with (
+            patch("litellm.proxy.proxy_server.llm_router", router),  # test-quality-ok: reads module global
+            patch(  # test-quality-ok: reads module global
+                "litellm.proxy.proxy_server.user_config_file_path",
+                config_file_path,
+            ),
+        ):
+            result = await proxy_config._delete_deployment(db_models=[])
+
+        model_ids = router.get_model_ids()
+        assert "model-a-id" in model_ids
+        assert "model-b-id" not in model_ids
+        assert result == frozenset({"model-a-id"})