[Bugfix] Fall back to Pydantic loc for param in validation errors (#46038)

Signed-off-by: professorsab <135441198+professorsab@users.noreply.github.com>
Co-authored-by: Mahad Durrani <114791389+mahadrehmann@users.noreply.github.com>

tests/entrypoints/serve/utils/test_server_utils.py

@@ -0,0 +1,65 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests that validation_exception_handler populates the `param` field
+in its error response using the Pydantic error's `loc`, even when no
+custom VLLMValidationError context is present.
+
+Previously, `param` was only populated for errors carrying a custom
+VLLMValidationError in their Pydantic `ctx`. Plain validation failures
+(missing fields, wrong types) left `param` as None, even though the
+field name was readily available from `error['loc']`.
+"""
+
+import json
+from types import SimpleNamespace
+
+import pytest
+from fastapi.exceptions import RequestValidationError
+
+from vllm.entrypoints.serve.utils.server_utils import validation_exception_handler
+
+
+def _fake_request(log_error_stack: bool = False) -> SimpleNamespace:
+    """Minimal stand-in for a FastAPI Request - just enough for the
+    handler to read req.app.state.args.log_error_stack."""
+    return SimpleNamespace(
+        app=SimpleNamespace(
+            state=SimpleNamespace(args=SimpleNamespace(log_error_stack=log_error_stack))
+        ),
+        state=SimpleNamespace(),  # no request_metadata -> hasattr(...) is False
+    )
+
+
+class TestValidationErrorParamFallback:
+    """Ensure `param` falls back to the Pydantic error's `loc` when no
+    custom VLLMValidationError context is present."""
+
+    @pytest.mark.parametrize(
+        ("error_type", "msg"),
+        [
+            ("missing", "Field required"),
+            ("list_type", "Input should be a valid list"),
+        ],
+        ids=["missing-field", "wrong-type"],
+    )
+    @pytest.mark.asyncio
+    async def test_param_falls_back_to_loc(self, error_type: str, msg: str):
+        errors = [{"type": error_type, "loc": ("body", "messages"), "msg": msg}]
+        exc = RequestValidationError(errors)
+
+        response = await validation_exception_handler(_fake_request(), exc)
+        body = json.loads(response.body)
+
+        assert body["error"]["param"] == "body.messages"
+
+    @pytest.mark.asyncio
+    async def test_param_fallback_does_not_crash_on_non_dict_error(self):
+        """Schemathesis fuzzing found that errors[0] isn't always a dict.
+        The fallback must not crash in that case - it should just leave
+        param as None instead of raising."""
+        exc = RequestValidationError(["some unexpected non-dict error"])
+
+        response = await validation_exception_handler(_fake_request(), exc)
+        body = json.loads(response.body)
+
+        assert body["error"]["param"] is None

vllm/entrypoints/serve/utils/server_utils.py

@@ -427,6 +427,12 @@ async def validation_exception_handler(req: Request, exc: RequestValidationError
                 param = ctx_error.parameter
                 break
 
+    if param is None and errors:
+        first_error = errors[0]
+        loc = first_error.get("loc") if isinstance(first_error, dict) else None
+        if loc:
+            param = ".".join(str(part) for part in loc)
+
     exc_str = str(exc)
     errors_str = str(errors)