fix(proxy): answer the Anthropic error envelope on the /anthropic pass-through

The pass-through relays upstream errors in Anthropic's shape, but the proxy's
own refusals there (bad key, model access, budget) still answered the OpenAI
body, so an Anthropic SDK pointed at /anthropic could not read them

litellm/proxy/common_utils/anthropic_error_payload.py

@@ -1,8 +1,8 @@
-"""Renders the Anthropic error envelope for the Messages routes, wherever the failure is raised.
+"""Renders the Anthropic error envelope for the Anthropic routes, wherever the failure is raised.
 
 The central ``ProxyException`` handler answers every route with OpenAI's ``{"error": {...}}``
 body, which an Anthropic SDK cannot read. Auth, key, budget and model-access failures are
-raised by ``user_api_key_auth`` before the Messages handler runs, so the envelope is picked
+raised by ``user_api_key_auth`` before the route handler runs, so the envelope is picked
 from the request route rather than inside the handler.
 """
 
@@ -29,15 +29,17 @@ ANTHROPIC_MESSAGES_ROUTES: Final[frozenset[str]] = frozenset(
         "/claude_code_gateway/v1/messages/count_tokens",
     }
 )
+ANTHROPIC_PASS_THROUGH_PREFIX: Final = "/anthropic/"
 
 
-def is_anthropic_messages_route(route: str) -> bool:
-    """True for the Messages routes, whose callers read the Anthropic error envelope.
+def uses_anthropic_error_envelope(route: str) -> bool:
+    """True for the Messages routes and the Anthropic pass-through, whose callers read the Anthropic envelope.
 
     Narrower than ``LiteLLMRoutes.anthropic_routes``: the Skills and marketplace routes there
-    answer their own body shape, and ``/anthropic/v1/messages`` relays the upstream response.
+    answer their own body shape. The pass-through relays upstream errors as Anthropic sent them,
+    so the proxy's own refusals there answer the same envelope.
     """
-    return route in ANTHROPIC_MESSAGES_ROUTES
+    return route in ANTHROPIC_MESSAGES_ROUTES or route.startswith(ANTHROPIC_PASS_THROUGH_PREFIX)
 
 
 def anthropic_error_status_code(exc: ProxyException) -> int:

litellm/proxy/proxy_server.py

@@ -471,7 +471,7 @@ from litellm.proxy.common_request_processing import (  # noqa: F401, RUF100  # l
 )
 from litellm.proxy.common_utils.anthropic_error_payload import (
     anthropic_error_json_response,
-    is_anthropic_messages_route,
+    uses_anthropic_error_envelope,
 )
 from litellm.proxy.common_utils.auth_cache_invalidation_pubsub import (
     AuthCacheInvalidationSubscriber,
@@ -2192,7 +2192,7 @@ async def openai_exception_handler(request: Request, exc: ProxyException):
     # NOTE: DO NOT MODIFY THIS, its crucial to map to Openai exceptions
     log_model_access_denial(exc)
     headers: Final = exc.headers
-    if is_anthropic_messages_route(get_request_route(request)):
+    if uses_anthropic_error_envelope(get_request_route(request)):
         return anthropic_error_json_response(exc, request)
     error_dict: Final = with_call_id(
         JSON_OBJECT.validate_python(exc.to_dict()),

tests/integration/authorization/test_unknown_key_generic_401.py

@@ -66,6 +66,7 @@ class _AnthropicErrorResponse(BaseModel):
 
 GENERIC_ERROR: Final = _ProxyError(message=GENERIC_MESSAGE, type="token_not_found_in_db", param="key", code="401")
 MESSAGES_PATH: Final = "/v1/messages"
+ANTHROPIC_ENVELOPE_PATHS: Final = frozenset({MESSAGES_PATH, "/anthropic/v1/messages"})
 ANTHROPIC_GENERIC_ERROR: Final = _AnthropicErrorDetail(type="authentication_error", message=GENERIC_MESSAGE)
 
 
@@ -145,8 +146,8 @@ def _assert_messages_refusal(response: httpx.Response, *secrets: str) -> None:
 
 
 def _assert_refusal(path: str, response: httpx.Response, *secrets: str) -> None:
-    """The Messages routes answer the Anthropic envelope, every other route the OpenAI one."""
-    if path == MESSAGES_PATH:
+    """The Messages and Anthropic pass-through routes answer the Anthropic envelope, every other route the OpenAI one."""
+    if path in ANTHROPIC_ENVELOPE_PATHS:
         _assert_messages_refusal(response, *secrets)
     else:
         _assert_generic_refusal(response, *secrets)

tests/unit/proxy/proxy_server/test_exception_handlers.py

@@ -189,10 +189,15 @@ _MESSAGES_ROUTES: Final = (
     "/claude_code_gateway/v1/messages",
     "/claude_code_gateway/v1/messages/count_tokens",
 )
+_ANTHROPIC_PASS_THROUGH_ROUTES: Final = (
+    "/anthropic/v1/messages",
+    "/anthropic/v1/messages/count_tokens",
+    "/anthropic/v1/models",
+)
 
 
 @pytest.mark.asyncio
-@pytest.mark.parametrize("path", _MESSAGES_ROUTES)
+@pytest.mark.parametrize("path", (*_MESSAGES_ROUTES, *_ANTHROPIC_PASS_THROUGH_ROUTES))
 async def test_openai_exception_handler_answers_the_anthropic_envelope_on_the_messages_routes(path):
     """An auth failure raised before the Messages handler (user_api_key_auth) must reach an
     Anthropic SDK as {"type": "error", "error": {...}}, not the OpenAI {"error": {...}} body."""
@@ -305,14 +310,15 @@ async def test_openai_exception_handler_messages_route_copies_the_call_id_when_o
 
 
 @pytest.mark.asyncio
-async def test_a_messages_request_with_no_key_answers_the_anthropic_authentication_error(client, monkeypatch):
+@pytest.mark.parametrize("path", ("/v1/messages", "/anthropic/v1/messages"))
+async def test_a_messages_request_with_no_key_answers_the_anthropic_authentication_error(client, monkeypatch, path):
     """End to end through the real user_api_key_auth: a /v1/messages request with no
     credential is rejected before the handler, and the SDK must see Anthropic's
     authentication_error envelope, not OpenAI's {"error": {...}} body."""
     monkeypatch.setattr("litellm.proxy.proxy_server.master_key", "test-master-key")
     monkeypatch.setattr("litellm.proxy.proxy_server.general_settings", {})
 
-    response = client.post("/v1/messages", json={"model": "gpt-4", "messages": [{"role": "user", "content": "hi"}]})
+    response = client.post(path, json={"model": "gpt-4", "messages": [{"role": "user", "content": "hi"}]})
     body = response.json()
 
     assert response.status_code == 401