[Bugfix][Anthropic] Return 4xx for client-caused errors in /v1/messages (#52246)

Signed-off-by: Do_it_now_! <lizhengcheng233@gmail.com>

tests/entrypoints/anthropic/test_anthropic_messages_conversion.py

@@ -15,12 +15,14 @@ Also covers cache usage computation in ``_build_anthropic_usage``.
 import json
 from argparse import Namespace
 from http import HTTPStatus
+from typing import Annotated
 from unittest.mock import MagicMock
 
 import pytest
 from fastapi import FastAPI
 from fastapi.exceptions import RequestValidationError
 from fastapi.testclient import TestClient
+from pydantic import BaseModel, Field, ValidationError
 
 from vllm.entrypoints.anthropic.api_router import attach_router
 from vllm.entrypoints.anthropic.protocol import (
@@ -47,6 +49,7 @@ from vllm.entrypoints.openai.engine.protocol import (
 from vllm.entrypoints.serve.exception_handling.handlers.validation import (
     validation_exception_handler,
 )
+from vllm.exceptions import VLLMValidationError
 
 _convert = AnthropicServingMessages._convert_anthropic_to_openai_request
 _img_url = AnthropicServingMessages._convert_image_source_to_url
@@ -1458,3 +1461,95 @@ class TestCacheSalt:
 
         assert response.status_code == HTTPStatus.BAD_REQUEST
         handler.create_messages.assert_not_awaited()
+
+
+# ======================================================================
+# Client-caused errors are 4xx, not 500 (Issue #52088)
+# ======================================================================
+
+
+class TestClientErrorResponses:
+    @staticmethod
+    def _make_api_app(handler: MagicMock):
+        app = FastAPI()
+        attach_router(app)
+        app.state.args = Namespace(log_error_stack=False)
+        app.exception_handler(RequestValidationError)(validation_exception_handler)
+        app.state.anthropic_serving_messages = handler
+        return app
+
+    @staticmethod
+    def _request_body() -> dict:
+        return {
+            "model": "test-model",
+            "max_tokens": 1,
+            "messages": [{"role": "user", "content": "Hello"}],
+        }
+
+    @staticmethod
+    def _conversion_error() -> ValidationError:
+        """A real pydantic ValidationError like the one ChatCompletionRequest
+        construction raises when Anthropic input violates the OpenAI schema."""
+
+        class _StubRequest(BaseModel):
+            stop: Annotated[list[str], Field(max_length=4)] | None = None
+
+        with pytest.raises(ValidationError) as exc_info:
+            _StubRequest(stop=["a"] * 6)
+        return exc_info.value
+
+    def test_validation_error_returns_bad_request(self):
+        """A pydantic ValidationError during Anthropic->OpenAI conversion is
+        surfaced as a 400 BadRequestError, not a 500."""
+        handler = MagicMock(spec=AnthropicServingMessages)
+        handler.create_messages.side_effect = self._conversion_error()
+
+        app = self._make_api_app(handler)
+        with TestClient(app, raise_server_exceptions=False) as client:
+            response = client.post("/v1/messages", json=self._request_body())
+
+        assert response.status_code == HTTPStatus.BAD_REQUEST
+        body = response.json()
+        assert body["type"] == "error"
+        assert body["error"]["type"] == "BadRequestError"
+        assert "at most 4 items" in body["error"]["message"]
+
+    def test_vllm_client_error_returns_bad_request(self):
+        """VLLMClientError raised by the serving layer maps to 400."""
+        handler = MagicMock(spec=AnthropicServingMessages)
+        handler.create_messages.side_effect = VLLMValidationError(
+            "Invalid value for stop", parameter="stop"
+        )
+
+        app = self._make_api_app(handler)
+        with TestClient(app, raise_server_exceptions=False) as client:
+            response = client.post("/v1/messages", json=self._request_body())
+
+        assert response.status_code == HTTPStatus.BAD_REQUEST
+        assert response.json()["error"]["type"] == "BadRequestError"
+
+    def test_generic_error_still_returns_internal_server_error(self):
+        """Non-client errors keep the existing 500 behaviour."""
+        handler = MagicMock(spec=AnthropicServingMessages)
+        handler.create_messages.side_effect = RuntimeError("boom")
+
+        app = self._make_api_app(handler)
+        with TestClient(app, raise_server_exceptions=False) as client:
+            response = client.post("/v1/messages", json=self._request_body())
+
+        assert response.status_code == HTTPStatus.INTERNAL_SERVER_ERROR
+        assert response.json()["error"]["type"] == "InternalServerError"
+
+    def test_count_tokens_validation_error_returns_bad_request(self):
+        """The count_tokens route maps conversion errors to 400 as well."""
+        handler = MagicMock(spec=AnthropicServingMessages)
+        handler.count_tokens.side_effect = self._conversion_error()
+
+        app = self._make_api_app(handler)
+        with TestClient(app, raise_server_exceptions=False) as client:
+            response = client.post(
+                "/v1/messages/count_tokens", json=self._request_body()
+            )
+
+        assert response.status_code == HTTPStatus.BAD_REQUEST
+        assert response.json()["error"]["type"] == "BadRequestError"

tests/entrypoints/serve/exception_handling/test_error_sanitization.py

@@ -116,7 +116,6 @@ class TestAffectedModulesUseSanitize:
     @pytest.mark.parametrize(
         "module",
         [
-            "vllm.entrypoints.anthropic.api_router",
             "vllm.entrypoints.anthropic.serving",
             "vllm.entrypoints.speech_to_text.realtime.connection",
         ],

vllm/entrypoints/anthropic/api_router.py

@@ -17,7 +17,9 @@ from vllm.entrypoints.anthropic.protocol import (
 )
 from vllm.entrypoints.anthropic.serving import AnthropicServingMessages
 from vllm.entrypoints.openai.engine.protocol import ErrorResponse
-from vllm.entrypoints.serve.exception_handling.utils import sanitize_message
+from vllm.entrypoints.serve.exception_handling.error_response import (
+    create_error_response,
+)
 from vllm.entrypoints.serve.utils.api_utils import (
     load_aware_call,
     validate_json_request,
@@ -71,15 +73,7 @@ async def create_messages(request: AnthropicMessagesRequest, raw_request: Reques
         generator = await handler.create_messages(request, raw_request)
     except Exception as e:
         logger.exception("Error in create_messages: %s", e)
-        return JSONResponse(
-            status_code=HTTPStatus.INTERNAL_SERVER_ERROR.value,
-            content=AnthropicErrorResponse(
-                error=AnthropicError(
-                    type="internal_error",
-                    message=sanitize_message(str(e)),
-                )
-            ).model_dump(),
-        )
+        return translate_error_response(create_error_response(e))
 
     if isinstance(generator, ErrorResponse):
         return translate_error_response(generator)
@@ -117,15 +111,7 @@ async def count_tokens(request: AnthropicCountTokensRequest, raw_request: Reques
         response = await handler.count_tokens(request, raw_request)
     except Exception as e:
         logger.exception("Error in count_tokens: %s", e)
-        return JSONResponse(
-            status_code=HTTPStatus.INTERNAL_SERVER_ERROR.value,
-            content=AnthropicErrorResponse(
-                error=AnthropicError(
-                    type="internal_error",
-                    message=sanitize_message(str(e)),
-                )
-            ).model_dump(),
-        )
+        return translate_error_response(create_error_response(e))
 
     if isinstance(response, ErrorResponse):
         return translate_error_response(response)