[BUG] Task outputs lost when mixing sync and async tasks - _process_async_tasks replaces list instead of extending

[Rakshit-gen]

Description

When a Crew executes a mix of synchronous and asynchronous tasks, all sync task outputs that occur before async tasks are silently
lost. This is caused by _process_async_tasks and _aprocess_async_tasks returning a new list, which then replaces the existing
task_outputs list instead of extending it.

The bug affects:

• _execute_tasks() (sync execution path)
• _aexecute_tasks() (async execution path)
• _handle_conditional_task()
• _ahandle_conditional_task()

Steps to Reproduce

from crewai import Agent, Task, Crew

agent = Agent(
role="Researcher",
goal="Research topics",
backstory="Expert researcher",
llm="gpt-4o-mini"
)

Create mixed sync/async tasks

task1 = Task(
description="Task 1 - sync",
expected_output="Output 1",
agent=agent,
async_execution=False # Sync
)

task2 = Task(
description="Task 2 - async",
expected_output="Output 2",
agent=agent,
async_execution=True # Async
)

task3 = Task(
description="Task 3 - sync",
expected_output="Output 3",
agent=agent,
async_execution=False # Sync
)

crew = Crew(agents=[agent], tasks=[task1, task2, task3])
result = crew.kickoff()

BUG: result.tasks_output only contains 2 items (task2, task3)

task1's output is LOST

print(f"Expected 3 outputs, got: {len(result.tasks_output)}")

Expected behavior

result.tasks_output should contain outputs from all 3 tasks in order: [task1_output, task2_output, task3_output]

Screenshots/Code snippets

Root cause in crew.py:

Line 1155 - BUG: Replaces instead of extends

task_outputs = self._process_async_tasks(futures, was_replayed)

_process_async_tasks creates a NEW list:

def _process_async_tasks(self, futures, was_replayed=False):
task_outputs: list[TaskOutput] = [] # <-- NEW empty list
for future_task, future, task_index in futures:
task_output = future.result()
task_outputs.append(task_output)
return task_outputs # <-- Only contains async outputs

Execution trace showing data loss:

Step	Task	Action	task_outputs
1	Task1 (sync)	Execute & append	[output1]
2	Task2 (async)	Store future	[output1]
3	Task3 (sync)	Process futures	[output2] ← output1 LOST!
4	Task3 (sync)	Execute & append	[output2, output3]

Operating System

macOS Big Sur

Python Version

3.10

crewAI Version

Current main branch (and likely all versions with async task support)

crewAI Tools Version

N/A - core crewAI bug

Virtual Environment

Venv

Evidence

The bug is in lib/crewai/src/crewai/crew.py at multiple locations:

• Line 960-962 (_aexecute_tasks)
• Line 976 (_aexecute_tasks)
• Line 990 (_ahandle_conditional_task)
• Line 1155 (_execute_tasks)
• Line 1169 (_execute_tasks)
• Line 1182 (_handle_conditional_task)

All use the pattern task_outputs = ...process_async_tasks(...) which replaces instead of extends.

Possible Solution

Change all occurrences from assignment to extend():

Before (BUG):

task_outputs = self._process_async_tasks(futures, was_replayed)

After (FIX):

task_outputs.extend(self._process_async_tasks(futures, was_replayed))

This mutates the existing list in-place, preserving previously accumulated sync task outputs.

Additional context

• Severity: Critical - silent data loss with no error raised
  • Impact: Any crew mixing sync and async tasks loses outputs
  • Detection difficulty: Very high - outputs appear valid but are incomplete
  • Fix complexity: Low - single-character change at each location (= → .extend()