[responsesAPI][ez] add a unit test for SimpleContext logprobs (#37126)

Signed-off-by: Andrew Xia <axia@meta.com>

tests/entrypoints/openai/responses/test_simple.py

@@ -137,6 +137,59 @@ async def test_streaming_output_consistency(client: OpenAI, model_name: str):
     )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_streaming_logprobs(client: OpenAI, model_name: str):
+    """Test that streaming with logprobs returns valid logprob data on
+    output_text.delta events and that top_logprobs has the requested count."""
+    response = await client.responses.create(
+        model=model_name,
+        input="Say hello.",
+        stream=True,
+        top_logprobs=3,
+        include=["message.output_text.logprobs"],
+    )
+
+    events = []
+    async for event in response:
+        events.append(event)
+
+    assert len(events) > 0
+
+    # Collect all output_text.delta events that carry logprobs
+    text_delta_events = [e for e in events if e.type == "response.output_text.delta"]
+    assert len(text_delta_events) > 0, "Expected at least one text delta event"
+
+    for delta_event in text_delta_events:
+        logprobs = delta_event.logprobs
+        assert logprobs is not None, "logprobs should be present on text delta events"
+        assert len(logprobs) > 0, "logprobs list should not be empty"
+        for lp in logprobs:
+            # Each logprob entry must have a token and a logprob value
+            assert lp.token is not None
+            assert isinstance(lp.logprob, float)
+            assert lp.logprob <= 0.0, f"logprob should be <= 0, got {lp.logprob}"
+            # top_logprobs should have up to 3 entries
+            assert lp.top_logprobs is not None
+            assert len(lp.top_logprobs) <= 3
+            for tl in lp.top_logprobs:
+                assert tl.token is not None
+                assert isinstance(tl.logprob, float)
+
+    # Verify that top_logprobs are actually populated, not always empty
+    all_top_logprobs = [
+        tl for e in text_delta_events for lp in e.logprobs for tl in lp.top_logprobs
+    ]
+    assert len(all_top_logprobs) > 0, (
+        "Expected at least one top_logprobs entry across all delta events"
+    )
+
+    # Verify the completed event still has valid output
+    completed = events[-1]
+    assert completed.type == "response.completed"
+    assert completed.response.status == "completed"
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])
 async def test_streaming_reasoning_tokens_e2e(client: OpenAI, model_name: str):