fix: match exact class in callback dedup so a custom subclass does not block a built-in logger (#34804)

litellm/utils.py

@@ -558,10 +558,10 @@ def _custom_logger_class_exists_in_success_callbacks(
     e.g if `LangfusePromptManagement` is passed in, it will return True if an instance of `LangfusePromptManagement` exists in litellm.success_callback or litellm._async_success_callback
 
     Prevents double adding a custom logger callback to the litellm callbacks
+
+    Matches on the exact class; an instance of a subclass does not count as registered
     """
-    return any(
-        isinstance(cb, type(callback_class)) for cb in litellm.success_callback + litellm._async_success_callback
-    )
+    return any(type(cb) is type(callback_class) for cb in litellm.success_callback + litellm._async_success_callback)
 
 
 def _custom_logger_class_exists_in_failure_callbacks(
@@ -573,10 +573,10 @@ def _custom_logger_class_exists_in_failure_callbacks(
     e.g if `LangfusePromptManagement` is passed in, it will return True if an instance of `LangfusePromptManagement` exists in litellm.failure_callback or litellm._async_failure_callback
 
     Prevents double adding a custom logger callback to the litellm callbacks
+
+    Matches on the exact class; an instance of a subclass does not count as registered
     """
-    return any(
-        isinstance(cb, type(callback_class)) for cb in litellm.failure_callback + litellm._async_failure_callback
-    )
+    return any(type(cb) is type(callback_class) for cb in litellm.failure_callback + litellm._async_failure_callback)
 
 
 def get_request_guardrails(kwargs: Dict[str, Any]) -> List[str]:
@@ -766,7 +766,7 @@ def function_setup(
                         llm_router=None,  # type: ignore
                     )
                     if callback is None or any(
-                        isinstance(cb, type(callback)) for cb in litellm._async_success_callback
+                        type(cb) is type(callback) for cb in litellm._async_success_callback
                     ):  # don't double add a callback
                         continue
                 if callback not in litellm.input_callback:

tests/test_litellm/test_utils.py

@@ -4916,3 +4916,94 @@ def test_is_prompt_caching_valid_prompt_explicit_min_token_count_overrides_model
         is_prompt_caching_valid_prompt(model="claude-opus-4-8", messages=PROMPT_CACHE_MESSAGES, min_token_count=8192)
         is False
     )
+
+
+def test_custom_logger_guards_ignore_subclass_instances(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Regression LIT-4392: the success/failure existence guards used isinstance, so a user
+    subclass of a built-in logger already promoted into the callback lists made the guard
+    report the built-in itself as registered and the configured logger was silently skipped.
+    The exact-class assertions must hold alongside the subclass assertions: the guards still
+    have to dedup a second instance of the same class, only a subclass must stop matching."""
+    from litellm.integrations.custom_logger import CustomLogger
+    from litellm.utils import (
+        _custom_logger_class_exists_in_failure_callbacks,
+        _custom_logger_class_exists_in_success_callbacks,
+    )
+
+    class BuiltinLogger(CustomLogger):
+        pass
+
+    class UserSubclassLogger(BuiltinLogger):
+        pass
+
+    builtin_instance = BuiltinLogger()
+
+    monkeypatch.setattr(litellm, "success_callback", [UserSubclassLogger()])
+    monkeypatch.setattr(litellm, "failure_callback", [UserSubclassLogger()])
+    monkeypatch.setattr(litellm, "_async_success_callback", [])
+    monkeypatch.setattr(litellm, "_async_failure_callback", [])
+    assert _custom_logger_class_exists_in_success_callbacks(builtin_instance) is False
+    assert _custom_logger_class_exists_in_failure_callbacks(builtin_instance) is False
+
+    monkeypatch.setattr(litellm, "success_callback", [BuiltinLogger()])
+    monkeypatch.setattr(litellm, "failure_callback", [BuiltinLogger()])
+    assert _custom_logger_class_exists_in_success_callbacks(builtin_instance) is True
+    assert _custom_logger_class_exists_in_failure_callbacks(builtin_instance) is True
+
+
+@pytest.mark.asyncio
+async def test_s3_v2_success_callback_registers_alongside_user_subclass(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Regression LIT-4392: with a user S3Logger subclass registered via litellm_settings.callbacks
+    and success_callback ["s3_v2"], the built-in s3_v2 logger was never added and S3 logs were
+    silently dropped while requests kept returning 200."""
+    from litellm.integrations.s3_v2 import S3Logger
+    from litellm.utils import _add_custom_logger_callback_to_specific_event
+
+    class UserS3Logger(S3Logger):
+        async def async_log_success_event(self, kwargs, response_obj, start_time, end_time):
+            pass
+
+    user_logger = UserS3Logger()
+    monkeypatch.setattr(litellm, "success_callback", [user_logger, "s3_v2"])
+    monkeypatch.setattr(litellm, "_async_success_callback", [user_logger])
+    monkeypatch.setattr(litellm, "failure_callback", [])
+    monkeypatch.setattr(litellm, "_async_failure_callback", [])
+
+    _add_custom_logger_callback_to_specific_event("s3_v2", "success")
+
+    assert any(type(cb) is S3Logger for cb in litellm.success_callback)
+    assert any(type(cb) is S3Logger for cb in litellm._async_success_callback)
+    assert "s3_v2" not in litellm.success_callback
+    assert user_logger in litellm.success_callback
+
+
+@pytest.mark.asyncio
+async def test_builtin_string_callback_registers_when_subclass_already_active(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Regression LIT-4392, litellm.callbacks path: the inline dedup in function_setup also
+    matched subclass instances, so a built-in name in litellm.callbacks was dropped whenever a
+    user subclass was already promoted into _async_success_callback."""
+    from litellm.integrations.s3_v2 import S3Logger
+
+    class UserS3Logger(S3Logger):
+        async def async_log_success_event(self, kwargs, response_obj, start_time, end_time):
+            pass
+
+    user_logger = UserS3Logger()
+    monkeypatch.setattr(litellm, "callbacks", ["s3_v2"])
+    monkeypatch.setattr(litellm, "input_callback", [])
+    monkeypatch.setattr(litellm, "success_callback", [user_logger])
+    monkeypatch.setattr(litellm, "failure_callback", [])
+    monkeypatch.setattr(litellm, "_async_success_callback", [user_logger])
+    monkeypatch.setattr(litellm, "_async_failure_callback", [])
+
+    await litellm.acompletion(
+        model="gpt-5.6",
+        messages=[{"role": "user", "content": "hi"}],
+        mock_response="ok",
+    )
+
+    assert any(type(cb) is S3Logger for cb in litellm._async_success_callback)