[Frontend] Support setting logprobs to -1 (#25031)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

tests/entrypoints/openai/test_chat_echo.py

@@ -99,3 +99,26 @@ async def test_prompt_logprobs(client: openai.AsyncOpenAI):
 
     assert completion.prompt_logprobs is not None
     assert len(completion.prompt_logprobs) > 0
+
+
+@pytest.mark.asyncio
+async def test_top_logprobs(client: openai.AsyncOpenAI):
+    messages = [{
+        "role": "system",
+        "content": "You are a helpful assistant."
+    }, {
+        "role": "user",
+        "content": "Beijing is the capital of which country?"
+    }]
+
+    completion = await client.chat.completions.create(
+        model=MODEL_NAME,
+        messages=messages,
+        extra_body={
+            "top_logprobs": -1,
+            "logprobs": "true",
+        },
+    )
+    assert completion.choices[0].logprobs is not None
+    assert completion.choices[0].logprobs.content is not None
+    assert len(completion.choices[0].logprobs.content) > 0

vllm/entrypoints/openai/protocol.py

@@ -832,10 +832,12 @@ class ChatCompletionRequest(OpenAIBaseModel):
                 raise ValueError("`prompt_logprobs=-1` is only supported with "
                                  "vLLM engine V1.")
         if (top_logprobs := data.get("top_logprobs")) is not None:
-            if top_logprobs < 0:
+            if top_logprobs < 0 and top_logprobs != -1:
-                raise ValueError("`top_logprobs` must be a positive value.")
+                raise ValueError(
+                    "`top_logprobs` must be a positive value or -1.")
 
-            if top_logprobs > 0 and not data.get("logprobs"):
+            if (top_logprobs == -1
+                    or top_logprobs > 0) and not data.get("logprobs"):
                 raise ValueError(
                     "when using `top_logprobs`, `logprobs` must be set to true."
                 )