bug: Parallel stepping crashes entire simulation when a single agent's step fails

[yashhzd]

Bug Description

When using parallel agent stepping (step_agents_parallel, step_agents_multithreaded, or do_async), if any single agent's astep() or step() raises an exception, all other agent tasks are cancelled/abandoned and the entire simulation crashes.

This is particularly problematic for LLM-backed agents where transient failures are expected and common (rate limits, timeouts, malformed JSON responses, network errors, etc.). A single flaky LLM response should not kill a 100-agent simulation.

Environment

• mesa-llm: v0.3.0 (commit a8161c7)
• mesa: 3.5.0
• Python: 3.12

Root Cause

Three parallel execution paths in mesa_llm/parallel_stepping.py lack error isolation:

1. step_agents_parallel() (line 31) — asyncio.gather(*tasks) called without return_exceptions=True, so one failed coroutine cancels all others
2. step_agents_multithreaded() (lines 52–53) — bare future.result() in a loop with no try/except, so the first exception aborts remaining agents
3. _agentset_do_async() (line 124) — same asyncio.gather issue as Welcome to Mesa-LLM Discussions! #1

Reproduction

from mesa.model import Model
from mesa.agent import Agent
from mesa_llm.parallel_stepping import step_agents_parallel

class DummyModel(Model):
    def __init__(self):
        super().__init__(seed=42)

class FailingAgent(Agent):
    def __init__(self, model):
        super().__init__(model)

    async def astep(self):
        raise RuntimeError("LLM timeout")

class WorkingAgent(Agent):
    def __init__(self, model):
        super().__init__(model)
        self.counter = 0

    async def astep(self):
        self.counter += 1

import asyncio

async def main():
    m = DummyModel()
    failing = FailingAgent(m)
    working = WorkingAgent(m)
    await step_agents_parallel([failing, working])
    print(working.counter)  # Never reached

asyncio.run(main())

Actual Behavior

Traceback (most recent call last):
  File "repro.py", line 28, in main
    await step_agents_parallel([failing, working])
  File ".../mesa_llm/parallel_stepping.py", line 31, in step_agents_parallel
    await asyncio.gather(*tasks)
  File "repro.py", line 14, in astep
    raise RuntimeError("LLM timeout")
RuntimeError: LLM timeout

WorkingAgent.astep() is cancelled and never completes. The entire simulation crashes.

Expected Behavior

• Failed agents should be isolated — other agents complete normally
• Failures should be logged with agent ID and exception details
• Users should be able to inspect which agents failed and why (e.g., via a structured result object)
• An optional "raise" mode should be available for debugging, but the default should be resilient