fix(proxy): initialize string success/failure callbacks at startup after config load (#38226)

* fix(proxy): eagerly initialize string callbacks

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

* fix(proxy): materialize string callbacks after load_config so later litellm_settings keys are applied

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

* fix(proxy): drop casts when snapshotting string callbacks so LIT006 stays at base

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

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: yucheng <yucheng@berri.ai>

litellm/proxy/utils.py

@@ -177,6 +177,9 @@ from litellm.types.mcp import (
 )
 from litellm.types.proxy.policy_engine.pipeline_types import PipelineExecutionResult
 from litellm.types.utils import LLMResponseTypes, LoggedLiteLLMParams
+from litellm.utils import (
+    _add_custom_logger_callback_to_specific_event,  # pyright: ignore[reportPrivateUsage]  # only string-to-logger helper
+)
 
 if TYPE_CHECKING:
     from mcp.types import CallToolResult
@@ -857,6 +860,14 @@ class ProxyLogging:
                 litellm.logging_callback_manager.add_litellm_async_success_callback(callback)
                 litellm.logging_callback_manager.add_litellm_async_failure_callback(callback)
 
+        # Runs after load_config applied every litellm_settings key: logger __init__s read e.g. s3_callback_params
+        success_callbacks: Final = tuple(cb for cb in litellm.success_callback if isinstance(cb, str))
+        failure_callbacks: Final = tuple(cb for cb in litellm.failure_callback if isinstance(cb, str))
+        for callback in success_callbacks:
+            _add_custom_logger_callback_to_specific_event(callback, "success")
+        for callback in failure_callbacks:
+            _add_custom_logger_callback_to_specific_event(callback, "failure")
+
     async def update_request_status(self, litellm_call_id: str, status: Literal["success", "fail"]):
         # only use this if slack alerting is being used
         if self.alerting is None:

tests/test_litellm/proxy/test_proxy_server.py

@@ -3166,6 +3166,47 @@ async def test_custom_ui_sso_sign_in_handler_config_loading():
         os.unlink(config_file_path)
 
 
+@pytest.mark.asyncio
+async def test_startup_initializes_string_callbacks_after_all_litellm_settings_load(tmp_path, monkeypatch):
+    from litellm.integrations.s3_v2 import S3Logger
+    from litellm.litellm_core_utils import litellm_logging
+    from litellm.proxy.proxy_server import ProxyConfig
+    from litellm.proxy.utils import ProxyLogging
+
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text(
+        "model_list: []\n"
+        "litellm_settings:\n"
+        "  success_callback:\n"
+        "    - s3_v2\n"
+        "  failure_callback:\n"
+        "    - s3_v2\n"
+        "  s3_callback_params:\n"
+        "    s3_bucket_name: ordering-regression-bucket\n"
+        "    s3_region_name: us-west-2\n"
+    )
+
+    monkeypatch.setattr(litellm, "success_callback", [])
+    monkeypatch.setattr(litellm, "_async_success_callback", [])
+    monkeypatch.setattr(litellm, "failure_callback", [])
+    monkeypatch.setattr(litellm, "_async_failure_callback", [])
+    monkeypatch.setattr(litellm, "callbacks", [])
+    monkeypatch.setattr(litellm, "s3_callback_params", None)
+    monkeypatch.setattr(litellm_logging, "_in_memory_loggers", [])
+
+    await ProxyConfig().load_config(router=MagicMock(), config_file_path=str(config_file))
+    ProxyLogging(user_api_key_cache=MagicMock())._init_litellm_callbacks(llm_router=None)
+
+    success_loggers = [cb for cb in litellm._async_success_callback if isinstance(cb, S3Logger)]
+    failure_loggers = [cb for cb in litellm._async_failure_callback if isinstance(cb, S3Logger)]
+    assert len(success_loggers) == 1
+    assert len(failure_loggers) == 1
+    assert success_loggers[0].s3_bucket_name == "ordering-regression-bucket"
+    assert success_loggers[0].s3_region_name == "us-west-2"
+    assert "s3_v2" not in litellm.success_callback
+    assert "s3_v2" not in litellm.failure_callback
+
+
 @pytest.mark.asyncio
 async def test_load_config_max_budget_env_var_coerced_to_float(tmp_path, monkeypatch):
     """