fix: propagate ContextVars into async task threads

threading.Thread() does not inherit the parent's contextvars.Context,
causing ContextVar-based state (OpenTelemetry spans, Langfuse trace IDs,
and any other request-scoped vars) to be silently dropped in async tasks.

Fix by calling contextvars.copy_context() before spawning each thread and
using ctx.run() as the thread target, which runs the function inside the
captured context.

Affected locations:
- task.py: execute_async() — the primary async task execution path
- utilities/streaming.py: create_chunk_generator() — streaming execution path

Fixes: #4822
Related: #4168, #4286

Co-authored-by: Claude <noreply@anthropic.com>

lib/crewai/src/crewai/task.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import contextvars
 from concurrent.futures import Future
 from copy import copy as shallow_copy
 import datetime
@@ -524,10 +525,11 @@ class Task(BaseModel):
     ) -> Future[TaskOutput]:
         """Execute the task asynchronously."""
         future: Future[TaskOutput] = Future()
+        ctx = contextvars.copy_context()
         threading.Thread(
             daemon=True,
-            target=self._execute_task_async,
-            args=(agent, context, tools, future),
+            target=ctx.run,
+            args=(self._execute_task_async, agent, context, tools, future),
         ).start()
         return future
 

lib/crewai/src/crewai/utilities/streaming.py

@@ -2,6 +2,7 @@
 
 import asyncio
 from collections.abc import AsyncIterator, Callable, Iterator
+import contextvars
 import queue
 import threading
 from typing import Any, NamedTuple
@@ -240,7 +241,8 @@ def create_chunk_generator(
     Yields:
         StreamChunk objects as they arrive.
     """
-    thread = threading.Thread(target=run_func, daemon=True)
+    ctx = contextvars.copy_context()
+    thread = threading.Thread(target=ctx.run, args=(run_func,), daemon=True)
     thread.start()
 
     try: