diff --git a/tests/dfx/reliability/invalid_param_test/test_invalid_omni_chat.py b/tests/dfx/reliability/invalid_param_test/test_invalid_omni_chat.py
index 835d546e485..a6d5fd342db 100644
--- a/tests/dfx/reliability/invalid_param_test/test_invalid_omni_chat.py
+++ b/tests/dfx/reliability/invalid_param_test/test_invalid_omni_chat.py
@@ -15,8 +15,6 @@
 
 pytestmark = [pytest.mark.slow, pytest.mark.omni]
 
-_SKIP_ISSUE_3649 = pytest.mark.skip(reason="https://github.com/vllm-project/vllm-omni/issues/3649")
-
 
 def _minimal_chat_json(omni_server: OmniServer) -> dict[str, object]:
     """Minimal valid chat body; individual tests override one offending field."""
@@ -121,9 +119,8 @@ def _chat_completions_request_without_expectations(omni_server: OmniServer, case
         pytest.param(
             "modalities_list_bad",
             400,
-            ("modalities", "value_error", ""),
+            ("modalities", "list of strings"),
             id="modalities_list_bad_element",
-            marks=_SKIP_ISSUE_3649,
         ),
         pytest.param(
             "response_format_json_schema_incomplete",
@@ -131,7 +128,7 @@ def _chat_completions_request_without_expectations(omni_server: OmniServer, case
             ("response_format", "BadRequestError", "json_schema"),
             id="invalid_response_format_json_schema",
         ),
-        pytest.param("logprobs_wrong_type", 400, "logprobs", id="logprobs_wrong_type", marks=_SKIP_ISSUE_3649),
+        pytest.param("logprobs_wrong_type", 400, ("logprobs", "boolean"), id="logprobs_wrong_type"),
         pytest.param(
             "logprobs_top_without_enabled",
             400,
diff --git a/tests/entrypoints/openai_api/test_api_server_guards.py b/tests/entrypoints/openai_api/test_api_server_guards.py
index 43d88d6925c..58700b55207 100644
--- a/tests/entrypoints/openai_api/test_api_server_guards.py
+++ b/tests/entrypoints/openai_api/test_api_server_guards.py
@@ -821,6 +821,46 @@ def test_speech_without_handler_preserves_not_found_http_error() -> None:
     assert exc_info.value.detail == "The model does not support Speech API"
 
 
+@pytest.mark.parametrize(
+    ("field", "value", "detail"),
+    [
+        ("modalities", [123], "modalities must be a list of strings"),
+        ("logprobs", "yes", "logprobs must be a boolean"),
+    ],
+)
+def test_chat_completion_raw_body_guards_reject_lax_types(field, value, detail) -> None:
+    """The HTTP boundary rejects values that upstream Pydantic would coerce."""
+    app = FastAPI()
+    app.state.openai_serving_chat = None
+    app.state.serving_tokenization = None
+    app.add_api_route("/v1/chat/completions", api_server.create_chat_completion, methods=["POST"])
+    client = TestClient(app)
+
+    payload = {
+        "model": "demo-model",
+        "messages": [{"role": "user", "content": "hello"}],
+        "stream": False,
+        field: value,
+    }
+    response = client.post("/v1/chat/completions", json=payload)
+
+    assert response.status_code == 400
+    assert detail in response.json()["detail"]
+
+
+@pytest.mark.parametrize(
+    "raw_body",
+    [
+        {"modalities": None},
+        {"logprobs": None},
+        {"modalities": None, "logprobs": None},
+    ],
+)
+def test_chat_completion_raw_body_guards_allow_null_defaults(raw_body) -> None:
+    """Explicit JSON null keeps the upstream request model's default behavior."""
+    api_server._validate_chat_completion_raw_body(raw_body)
+
+
 @pytest.mark.asyncio
 async def test_multi_api_rejects_runtime_voice_upload() -> None:
     app = FastAPI()
diff --git a/vllm_omni/entrypoints/openai/api_server.py b/vllm_omni/entrypoints/openai/api_server.py
index 5f3a4bf182d..a1698e379b8 100644
--- a/vllm_omni/entrypoints/openai/api_server.py
+++ b/vllm_omni/entrypoints/openai/api_server.py
@@ -1198,6 +1198,22 @@ async def omni_init_app_state(
     state.server_load_metrics = 0
 
 
+def _validate_chat_completion_raw_body(raw_body: dict[str, Any]) -> None:
+    """Reject values that upstream ChatCompletionRequest coerces too broadly."""
+    if "modalities" in raw_body and raw_body["modalities"] is not None:
+        modalities = raw_body["modalities"]
+        if not isinstance(modalities, list) or not all(isinstance(m, str) for m in modalities):
+            raise HTTPException(
+                status_code=HTTPStatus.BAD_REQUEST.value,
+                detail='modalities must be a list of strings, e.g. ["text", "audio", "image"]',
+            )
+    if "logprobs" in raw_body and raw_body["logprobs"] is not None and not isinstance(raw_body["logprobs"], bool):
+        raise HTTPException(
+            status_code=HTTPStatus.BAD_REQUEST.value,
+            detail="logprobs must be a boolean (true or false)",
+        )
+
+
 @router.post(
     "/v1/chat/completions",
     dependencies=[Depends(validate_json_request)],
@@ -1211,6 +1227,8 @@ async def omni_init_app_state(
 @with_cancellation
 @load_aware_call
 async def create_chat_completion(request: ChatCompletionRequest, raw_request: Request):
+    raw_body = await raw_request.json()
+    _validate_chat_completion_raw_body(raw_body)
     metrics_header_format = raw_request.headers.get(ENDPOINT_LOAD_METRICS_FORMAT_HEADER_LABEL, "")
     handler = Omnichat(raw_request)
     if handler is None:
@@ -1287,6 +1305,8 @@ async def create_chat_completion(request: ChatCompletionRequest, raw_request: Re
 @with_cancellation
 @load_aware_call
 async def create_batch_chat_completion(request: BatchChatCompletionRequest, raw_request: Request):
+    raw_body = await raw_request.json()
+    _validate_chat_completion_raw_body(raw_body)
     handler = OmniBatchChat(raw_request)
     if handler is None:
         base_server = getattr(raw_request.app.state, "serving_tokenization", None)