[responsesAPI] fix simpleContext streaming output_messages (#34188)

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

tests/entrypoints/test_context.py

@@ -8,6 +8,7 @@ from openai_harmony import Author, Message, Role, StreamState, TextContent
 
 from vllm.entrypoints.openai.responses.context import (
     HarmonyContext,
+    SimpleContext,
     StreamingHarmonyContext,
     TurnMetrics,
 )
@@ -597,3 +598,248 @@ def test_turn_metrics_copy_and_reset():
     assert copied_metrics.output_tokens == 20
     assert copied_metrics.cached_input_tokens == 5
     assert copied_metrics.tool_output_tokens == 3
+
+
+# ==================== SimpleContext Tests ====================
+
+
+def create_simple_context_output(
+    text="",
+    token_ids=None,
+    prompt="Test prompt",
+    prompt_token_ids=None,
+    num_cached_tokens=0,
+    logprobs=None,
+    finished=True,
+):
+    """Helper to create a RequestOutput with customizable text for
+    SimpleContext tests."""
+    if token_ids is None:
+        token_ids = []
+    return RequestOutput(
+        request_id="test-id",
+        prompt=prompt,
+        prompt_token_ids=prompt_token_ids,
+        prompt_logprobs=None,
+        outputs=[
+            CompletionOutput(
+                index=0,
+                text=text,
+                token_ids=token_ids,
+                cumulative_logprob=0.0,
+                logprobs=logprobs,
+                finish_reason=None,
+                stop_reason=None,
+            )
+        ],
+        finished=finished,
+        num_cached_tokens=num_cached_tokens,
+    )
+
+
+def test_simple_context_output_messages_empty():
+    """output_messages should be empty before any output is appended."""
+    context = SimpleContext()
+    assert context.output_messages == []
+
+
+def test_simple_context_output_messages_single_call():
+    """Non-streaming: single append_output produces a single output message."""
+    context = SimpleContext()
+    output = create_simple_context_output(
+        text="Hello world",
+        token_ids=[10, 20, 30],
+        prompt_token_ids=[1, 2, 3],
+    )
+    context.append_output(output)
+
+    messages = context.output_messages
+    assert len(messages) == 1
+    assert messages[0].message == "Hello world"
+    assert messages[0].tokens == [10, 20, 30]
+    assert messages[0].type == "raw_message_tokens"
+
+
+def test_simple_context_output_messages_streaming_consolidation():
+    """Streaming: multiple append_output calls consolidate into one message."""
+    context = SimpleContext()
+
+    # Simulate 3 streaming deltas
+    context.append_output(
+        create_simple_context_output(
+            text="Hello",
+            token_ids=[10],
+            prompt_token_ids=[1, 2, 3],
+        )
+    )
+    context.append_output(
+        create_simple_context_output(
+            text=" world",
+            token_ids=[20],
+            prompt_token_ids=[1, 2, 3],
+        )
+    )
+    context.append_output(
+        create_simple_context_output(
+            text="!",
+            token_ids=[30],
+            prompt_token_ids=[1, 2, 3],
+        )
+    )
+
+    messages = context.output_messages
+    assert len(messages) == 1
+    assert messages[0].message == "Hello world!"
+    assert messages[0].tokens == [10, 20, 30]
+
+
+def test_simple_context_output_messages_many_deltas():
+    """Streaming with many small deltas still produces a single message."""
+    context = SimpleContext()
+
+    words = ["The", " quick", " brown", " fox", " jumps"]
+    for i, word in enumerate(words):
+        context.append_output(
+            create_simple_context_output(
+                text=word,
+                token_ids=[100 + i],
+                prompt_token_ids=[1, 2],
+            )
+        )
+
+    messages = context.output_messages
+    assert len(messages) == 1
+    assert messages[0].message == "The quick brown fox jumps"
+    assert messages[0].tokens == [100, 101, 102, 103, 104]
+
+
+def test_simple_context_input_messages():
+    """input_messages is populated on the first append_output call."""
+    context = SimpleContext()
+    assert context.input_messages == []
+
+    context.append_output(
+        create_simple_context_output(
+            text="Hi",
+            token_ids=[10],
+            prompt="My prompt text",
+            prompt_token_ids=[1, 2, 3],
+        )
+    )
+
+    assert len(context.input_messages) == 1
+    assert context.input_messages[0].message == "My prompt text"
+    assert context.input_messages[0].tokens == [1, 2, 3]
+
+    # Second call should not add another input message
+    context.append_output(
+        create_simple_context_output(
+            text=" there",
+            token_ids=[20],
+            prompt="My prompt text",
+            prompt_token_ids=[1, 2, 3],
+        )
+    )
+
+    assert len(context.input_messages) == 1
+
+
+def test_simple_context_token_counting():
+    """Token counting accumulates across streaming deltas."""
+    context = SimpleContext()
+
+    context.append_output(
+        create_simple_context_output(
+            text="a",
+            token_ids=[10, 11],
+            prompt_token_ids=[1, 2, 3, 4, 5],
+            num_cached_tokens=2,
+        )
+    )
+    context.append_output(
+        create_simple_context_output(
+            text="b",
+            token_ids=[12],
+            prompt_token_ids=[1, 2, 3, 4, 5],
+            num_cached_tokens=2,
+        )
+    )
+
+    assert context.num_prompt_tokens == 5
+    assert context.num_output_tokens == 3  # 2 + 1
+    assert context.num_cached_tokens == 2
+
+
+def test_simple_context_final_output():
+    """final_output reconstructs accumulated text and token_ids."""
+    context = SimpleContext()
+
+    context.append_output(
+        create_simple_context_output(
+            text="foo",
+            token_ids=[1, 2],
+            prompt_token_ids=[10],
+        )
+    )
+    context.append_output(
+        create_simple_context_output(
+            text="bar",
+            token_ids=[3],
+            prompt_token_ids=[10],
+        )
+    )
+
+    final = context.final_output
+    assert final is not None
+    assert final.outputs[0].text == "foobar"
+    assert final.outputs[0].token_ids == (1, 2, 3)
+
+
+def test_simple_context_output_messages_empty_text_with_tokens():
+    """output_messages should be returned when tokens exist even if text is
+    empty (e.g. special tokens)."""
+    context = SimpleContext()
+    context.append_output(
+        create_simple_context_output(
+            text="",
+            token_ids=[99],
+            prompt_token_ids=[1],
+        )
+    )
+
+    messages = context.output_messages
+    assert len(messages) == 1
+    assert messages[0].message == ""
+    assert messages[0].tokens == [99]
+
+
+def test_simple_context_output_messages_no_mutation():
+    """Each call to output_messages returns a fresh list; callers can't
+    corrupt internal state."""
+    context = SimpleContext()
+    context.append_output(
+        create_simple_context_output(
+            text="hello",
+            token_ids=[1],
+            prompt_token_ids=[10],
+        )
+    )
+
+    msgs1 = context.output_messages
+    msgs2 = context.output_messages
+    assert msgs1 is not msgs2
+    assert msgs1[0].message == msgs2[0].message
+
+    # Appending more output updates the property
+    context.append_output(
+        create_simple_context_output(
+            text=" world",
+            token_ids=[2],
+            prompt_token_ids=[10],
+        )
+    )
+
+    msgs3 = context.output_messages
+    assert len(msgs3) == 1
+    assert msgs3[0].message == "hello world"
+    assert msgs3[0].tokens == [1, 2]