[Frontend] Omit absent fields from /inference/v1/generate stream chunks (#57264)

Signed-off-by: aoshen02 <aoshen@inferact.ai>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

rust/src/server/src/routes/inference/generate.rs

@@ -679,6 +679,36 @@ mod tests {
         assert!(response.mm_placeholders.is_none());
     }
 
+    #[test]
+    fn stream_chunk_omits_absent_fields() {
+        // Contract shared with the Python frontend: absent optional fields
+        // are omitted from stream chunks, not serialized as null.
+        let chunk = GenerateStreamResponse {
+            request_id: "raw-1".to_string(),
+            choices: vec![GenerateResponseStreamChoice {
+                index: 0,
+                logprobs: None,
+                finish_reason: None,
+                token_ids: vec![1],
+            }],
+            usage: None,
+            prompt_token_ids: None,
+            mm_placeholders: None,
+        };
+
+        let json = serde_json::to_value(&chunk).expect("serialize chunk");
+
+        for key in ["usage", "prompt_token_ids", "mm_placeholders"] {
+            assert!(json.get(key).is_none(), "{key} should be omitted");
+        }
+        for key in ["logprobs", "finish_reason"] {
+            assert!(
+                json["choices"][0].get(key).is_none(),
+                "{key} should be omitted"
+            );
+        }
+    }
+
     #[test]
     fn extract_mm_placeholders_returns_none_without_features() {
         assert!(extract_mm_placeholders(None).is_none());

tests/entrypoints/scale_out/token_in_token_out/test_generate_stream.py

@@ -569,6 +569,40 @@ async def test_stream_omits_prompt_metadata_by_default():
         assert "mm_placeholders" not in chunk
 
 
+@pytest.mark.asyncio
+async def test_stream_chunks_omit_absent_fields():
+    """Absent optional fields are omitted from stream chunks, like Rust."""
+    engine = _mock_engine()
+
+    async def mock_generate(*args, **kwargs):
+        yield _make_request_output("req-1", token_ids=[10])
+        yield _make_request_output(
+            "req-1", token_ids=[20], finish_reason="stop", finished=True
+        )
+
+    engine.generate = MagicMock(side_effect=mock_generate)
+    serving = _build_serving_tokens(engine)
+    request = GenerateRequest(
+        token_ids=[1, 2, 3],
+        sampling_params=SamplingParams(max_tokens=2),
+        model=MODEL_NAME,
+        stream=True,
+    )
+
+    response = await serving.serve_tokens(request)
+    parsed = _parse_sse_chunks([chunk async for chunk in response])
+    data_chunks = [c for c in parsed if isinstance(c, dict) and c.get("choices")]
+
+    assert len(data_chunks) == 2
+    for chunk in data_chunks:
+        assert "usage" not in chunk
+        assert "prompt_token_ids" not in chunk
+        assert "mm_placeholders" not in chunk
+        assert "logprobs" not in chunk["choices"][0]
+    assert "finish_reason" not in data_chunks[0]["choices"][0]
+    assert data_chunks[1]["choices"][0]["finish_reason"] == "stop"
+
+
 @pytest.mark.asyncio
 async def test_stream_text_only_prompt_omits_mm_placeholders_key():
     engine = _mock_engine()

vllm/entrypoints/scale_out/token_in_token_out/serving.py

@@ -486,13 +486,9 @@ class ServingTokens(GenerateBaseServing):
                             total_tokens=(num_prompt_tokens + num_generated_tokens[i]),
                         )
 
-                    # Omit absent prompt metadata (Rust skips None fields).
-                    exclude = {
-                        name
-                        for name in ("prompt_token_ids", "mm_placeholders")
-                        if getattr(chunk, name) is None
-                    }
-                    yield f"data: {chunk.model_dump_json(exclude=exclude)}\n\n"
+                    # Omit absent fields, like the Rust frontend and the
+                    # final usage chunk below.
+                    yield f"data: {chunk.model_dump_json(exclude_none=True)}\n\n"
 
             total_completion_tokens = sum(num_generated_tokens)
             final_usage_info = UsageInfo(