fix(langgraph): return an awaited @task's value while streaming

With stream=True, arun_with_retry drains the task's astream() and returned
nothing, so a @task awaited inside a node got None whenever the node ran in
streaming mode: under astream_events (v1 and v2) and anything built on it.
Return the value the task wrote instead, on both stream paths.

Same idea as #9295 by AaronG18 and the lookup amishaguptaberk suggested.

libs/langgraph/langgraph/pregel/_retry.py

@@ -34,6 +34,7 @@ from langgraph._internal._constants import (
     CONFIG_KEY_THREAD_ID,
     CONFIG_KEY_TIMED_ATTEMPT_OBSERVER,
     NS_SEP,
+    RETURN,
 )
 from langgraph._internal._runnable import create_task_in_config_context
 from langgraph._internal._timeout import sync_timeout_unsupported
@@ -733,15 +734,14 @@ async def arun_with_retry(
                 if stream:
                     async for _ in task.proc.astream(task.input, config):
                         pass
-                    break
+                    return _return_value(task)
                 return await task.proc.ainvoke(task.input, config)
             result = await _arun_with_timeout(
                 task, config, resolved_timeout, attempt_ctx, stream=stream
             )
             _finish_timed_attempt(config, attempt_ctx)
             if stream:
-                # if successful, end
-                break
+                return _return_value(task)
             return result
         except ParentCommand as exc:
             ns: str = config[CONF][CONFIG_KEY_CHECKPOINT_NS]
@@ -831,6 +831,12 @@ async def arun_with_retry(
             config = patch_configurable(config, {CONFIG_KEY_RESUMING: True})
 
 
+def _return_value(task: PregelExecutableTask) -> Any:
+    # Streaming drains the task instead of returning its result, so a caller
+    # awaiting a `@task` needs the value the task wrote.
+    return next((value for channel, value in task.writes if channel == RETURN), None)
+
+
 def _should_retry_on(retry_policy: RetryPolicy, exc: Exception) -> bool:
     """Check if the given exception should be retried based on the retry policy."""
     if isinstance(retry_policy.retry_on, Sequence):

libs/langgraph/tests/test_pregel_async.py

@@ -7495,6 +7495,39 @@ async def test_entrypoint_from_async_generator() -> None:
             yield "b"
 
 
+@NEEDS_CONTEXTVARS
+@pytest.mark.parametrize("version", ["v1", "v2"])
+async def test_awaited_task_returns_its_value_under_astream_events(
+    version: Literal["v1", "v2"],
+) -> None:
+    class State(TypedDict):
+        doubled: int
+        zeroed: int
+
+    @task
+    async def double(x: int) -> int:
+        return x * 2
+
+    @task
+    async def zero() -> int:
+        return 0
+
+    async def node(state: State) -> State:
+        return {"doubled": await double(21), "zeroed": await zero()}
+
+    graph = StateGraph(State).add_node("node", node).add_edge(START, "node").compile()
+
+    outputs = [
+        event["data"]["output"]
+        async for event in graph.astream_events(
+            {"doubled": 0, "zeroed": 1}, version=version
+        )
+        if event["event"] == "on_chain_end" and event["name"] == "node"
+    ]
+
+    assert outputs == [{"doubled": 42, "zeroed": 0}]
+
+
 @NEEDS_CONTEXTVARS
 async def test_named_tasks_functional() -> None:
     class Foo: