[Bugfix] Prevent crash on empty grammar string (#28210)

Signed-off-by: tjandy98 <3953059+tjandy98@users.noreply.github.com>

tests/v1/entrypoints/openai/test_chat_completion.py

@@ -138,3 +138,23 @@ async def test_invalid_grammar(client: openai.AsyncOpenAI, model_name: str):
                 "structured_outputs": {"grammar": invalid_simplified_sql_grammar}
             },
         )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "model_name",
+    [MODEL_NAME],
+)
+async def test_empty_grammar(client: openai.AsyncOpenAI, model_name: str) -> None:
+    prompt = "Say hello"
+    with pytest.raises((openai.BadRequestError, openai.APIError)):
+        await client.chat.completions.create(
+            model=model_name,
+            messages=[
+                {
+                    "role": "user",
+                    "content": prompt,
+                }
+            ],
+            extra_body={"structured_outputs": {"grammar": ""}},
+        )

vllm/v1/engine/processor.py

@@ -270,6 +270,12 @@ class Processor:
             raise ValueError(
                 f"Choice '{params.structured_outputs.choice}' cannot be an empty list"  # noqa: E501
             )
+        # Reject empty string grammar early to avoid engine-side crashes
+        if (
+            isinstance(params.structured_outputs.grammar, str)
+            and params.structured_outputs.grammar.strip() == ""
+        ):
+            raise ValueError("structured_outputs.grammar cannot be an empty string")
 
         if backend.startswith("xgrammar"):
             # xgrammar with no fallback