Start and finish streaming trace in impl metod (#540)

Closes #435 and closes #538.

Unit tests.

Author: rm-openai

.gitignore

@@ -141,4 +141,5 @@ cython_debug/
 .ruff_cache/
 
 # PyPI configuration file
-.pypirc
+.pypirc
+.aider*

pyproject.toml

@@ -61,6 +61,7 @@ dev = [
     "graphviz",
     "mkdocs-static-i18n>=1.3.0",
     "eval-type-backport>=0.2.2",
+    "fastapi >= 0.110.0, <1",
 ]
 
 [tool.uv.workspace]

src/agents/result.py

@@ -126,7 +126,7 @@ class RunResultStreaming(RunResultBase):
 
     _current_agent_output_schema: AgentOutputSchemaBase | None = field(repr=False)
 
-    _trace: Trace | None = field(repr=False)
+    trace: Trace | None = field(repr=False)
 
     is_complete: bool = False
     """Whether the agent has finished running."""
@@ -185,9 +185,6 @@ async def stream_events(self) -> AsyncIterator[StreamEvent]:
             yield item
             self._event_queue.task_done()
 
-        if self._trace:
-            self._trace.finish(reset_current=True)
-
         self._cleanup_tasks()
 
         if self._stored_exception:

src/agents/run.py

@@ -404,10 +404,6 @@ def run_streamed(
                 disabled=run_config.tracing_disabled,
             )
         )
-        # Need to start the trace here, because the current trace contextvar is captured at
-        # asyncio.create_task time
-        if new_trace:
-            new_trace.start(mark_as_current=True)
 
         output_schema = cls._get_output_schema(starting_agent)
         context_wrapper: RunContextWrapper[TContext] = RunContextWrapper(
@@ -426,7 +422,7 @@ def run_streamed(
             input_guardrail_results=[],
             output_guardrail_results=[],
             _current_agent_output_schema=output_schema,
-            _trace=new_trace,
+            trace=new_trace,
         )
 
         # Kick off the actual agent loop in the background and return the streamed result object.
@@ -499,6 +495,9 @@ async def _run_streamed_impl(
         run_config: RunConfig,
         previous_response_id: str | None,
     ):
+        if streamed_result.trace:
+            streamed_result.trace.start(mark_as_current=True)
+
         current_span: Span[AgentSpanData] | None = None
         current_agent = starting_agent
         current_turn = 0
@@ -625,6 +624,8 @@ async def _run_streamed_impl(
         finally:
             if current_span:
                 current_span.finish(reset_current=True)
+            if streamed_result.trace:
+                streamed_result.trace.finish(reset_current=True)
 
     @classmethod
     async def _run_single_turn_streamed(

tests/fastapi/__init__.py

@@ -0,0 +1,30 @@
+from collections.abc import AsyncIterator
+
+from fastapi import FastAPI
+from starlette.responses import StreamingResponse
+
+from agents import Agent, Runner, RunResultStreaming
+
+agent = Agent(
+    name="Assistant",
+    instructions="You are a helpful assistant.",
+)
+
+
+app = FastAPI()
+
+
+@app.post("/stream")
+async def stream():
+    result = Runner.run_streamed(agent, input="Tell me a joke")
+    stream_handler = StreamHandler(result)
+    return StreamingResponse(stream_handler.stream_events(), media_type="application/x-ndjson")
+
+
+class StreamHandler:
+    def __init__(self, result: RunResultStreaming):
+        self.result = result
+
+    async def stream_events(self) -> AsyncIterator[str]:
+        async for event in self.result.stream_events():
+            yield f"{event.type}\n\n"

tests/fastapi/streaming_app.py

@@ -0,0 +1,29 @@
+import pytest
+from httpx import ASGITransport, AsyncClient
+from inline_snapshot import snapshot
+
+from ..fake_model import FakeModel
+from ..test_responses import get_text_message
+from .streaming_app import agent, app
+
+
+@pytest.mark.asyncio
+async def test_streaming_context():
+    """This ensures that FastAPI streaming works. The context for this test is that the Runner
+    method was called in one async context, and the streaming was ended in another context,
+    leading to a tracing error because the context was closed in the wrong context. This test
+    ensures that this actually works.
+    """
+    model = FakeModel()
+    agent.model = model
+    model.set_next_output([get_text_message("done")])
+
+    transport = ASGITransport(app)
+    async with AsyncClient(transport=transport, base_url="http://test") as ac:
+        async with ac.stream("POST", "/stream") as r:
+            assert r.status_code == 200
+            body = (await r.aread()).decode("utf-8")
+            lines = [line for line in body.splitlines() if line]
+            assert lines == snapshot(
+                ["agent_updated_stream_event", "raw_response_event", "run_item_stream_event"]
+            )