[Fix] Fix `logprobs=0` handling for `/inference/v1/generate` endpoint (#34010)

Signed-off-by: SumanthRH <sumanthrh99@gmail.com>

tests/entrypoints/openai/test_serving_tokens.py

@@ -87,6 +87,32 @@ async def test_generate_endpoint(client):
     assert "choices" in data
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("logprobs_value", [0, 1, 5])
+async def test_generate_logprobs(client, logprobs_value):
+    payload = {
+        "model": MODEL_NAME,
+        "token_ids": [1, 2, 3],
+        "sampling_params": {
+            "max_tokens": 5,
+            "temperature": 0.0,
+            "logprobs": logprobs_value,
+        },
+        "stream": False,
+    }
+    resp = await client.post(GEN_ENDPOINT, json=payload)
+    resp.raise_for_status()
+    data = resp.json()
+    choice = data["choices"][0]
+    assert choice["logprobs"] is not None
+    logprobs_content = choice["logprobs"]["content"]
+    assert len(logprobs_content) == len(choice["token_ids"])
+    for entry in logprobs_content:
+        assert "logprob" in entry
+        assert len(entry["top_logprobs"]) >= 1
+        assert len(entry["top_logprobs"]) == max(logprobs_value, 1)
+
+
 @pytest.mark.asyncio
 async def test_same_response_as_chat_completions(client, tokenizer, messages):
     token_ids = tokenizer.apply_chat_template(

vllm/entrypoints/serve/disagg/serving.py

@@ -184,7 +184,7 @@ class ServingTokens(OpenAIServing):
             out_logprobs = output.logprobs
 
             # This is top_logprobs in completions API
-            if sampling_params.logprobs:
+            if sampling_params.logprobs is not None:
                 assert out_logprobs is not None, "Did not output logprobs"
                 logprobs = self._create_tokens_logprobs(
                     token_ids=token_ids,
@@ -284,7 +284,8 @@ class ServingTokens(OpenAIServing):
                                 logprob=max(p[1].logprob, -9999.0),
                             )
                             for i, p in enumerate(step_top_logprobs.items())
-                            if num_output_top_logprobs and i < num_output_top_logprobs
+                            if num_output_top_logprobs is not None
+                            and i < max(num_output_top_logprobs, 1)
                         ],
                     )
                 )