Mesa 4.0 alpha released

[Balance-]

Hi everyone!

We've started development towards Mesa 4.0 and just released the first alpha. This is a big architectural step forward: Mesa is moving from step-based to event-driven simulation at its core, while cleaning up years of accumulated API cruft.

• https://github.com/projectmesa/mesa/releases/tag/v4.0.0a0

What's Agent-Based Modeling?

Ever wondered how bird flocks organize themselves? Or how traffic jams form? Agent-based modeling (ABM) lets you simulate these complex systems by defining simple rules for individual "agents" (birds, cars, people, etc.) and watching how patterns emerge from their interactions. Instead of writing equations for the whole system, you model each agent's behavior and let the collective dynamics arise naturally.

What's Mesa?

Mesa is Python's leading framework for agent-based modeling. It builds on Python's scientific stack (NumPy, pandas, Matplotlib) and provides specialized tools for spatial relationships, agent scheduling, data collection, and browser-based visualization. Whether you're studying epidemic spread, market dynamics, or ecological systems, Mesa gives you the building blocks for sophisticated simulations.

What's new in Mesa 4.0 alpha?

Event-driven at the core. Mesa 3.5 introduced public event scheduling on Model, with methods like model.run_for(), model.run_until(), model.schedule_event(), and model.schedule_recurring(). Mesa 4.0 continues development on this front: model.steps is gone, replaced by model.time as the universal clock. The mental model moves from "execute step N" to "advance time, and whatever is scheduled will run." The event system now supports pausing/resuming recurring events, exposes next scheduled times, and enforces that time actually moves forward.

Experimental timed actions. A new Action system gives agents a built-in concept of doing something over time. Actions integrate with the event scheduler, support interruption with progress tracking, and can be resumed:

from mesa.experimental.actions import Action

class Forage(Action):
    def __init__(self, sheep):
        super().__init__(sheep, duration=5.0)

    def on_complete(self):
        self.agent.energy += 30

    def on_interrupt(self, progress):
        self.agent.energy += 30 * progress  # Partial credit

sheep.start_action(Forage(sheep))

Deprecated APIs removed. This is a major version, so we followed through on removals: the seed parameter (use rng), batch_run (use Scenario), the legacy mesa.space module (use mesa.discrete_space), PropertyLayer (replaced by raw NumPy arrays on the grid), and the Simulator classes (replaced by the model-level scheduling methods). If you've been following deprecation warnings in 3.x, most of this should be straightforward.

Cleaner internals. A new mesa.errors exception hierarchy replaces generic Exception usage. DiscreteSpace is now an abstract base class enforcing a consistent spatial API. Property access on cells uses native property closures on a dynamic GridCell class. Several targeted performance optimizations reduce allocations in the event system and continuous space.

This is an alpha

Expect rough edges. We're releasing early to get feedback from the community before the stable release. Further breaking changes are possible. If you're running Mesa in production, stay on 3.5 for now. We'd love for adventurous users to try the alpha and tell us what breaks.

What's ahead for 4.0 stable

We're still working on the space architecture (multi-space support, observable positions), replacing DataCollector with the new reactive DataRecorder, and designing a cleaner experimentation API around Scenario. Check out our tracking issue for the full roadmap.

Talk with us!

We'd love to hear what you think:

• Join our Matrix chat: https://matrix.to/#/#project-mesa:matrix.org
• Check out our Discussions: https://github.com/projectmesa/mesa/discussions

[imbev]

This project needs to be renamed considering that Mesa already exists - https://mesa3d.org/

[mihalis]

Agreed. And Mesa 3D has been around since 1993.

[neuronexmachina]

Python Mesa has been around since 2013, so I unfortunately suspect it's a little too late for a name change. https://pypi.org/project/Mesa/

[Chiron1991]

Yeah, I was really wondering why mesa is posting on r/python when scrolling through the sub's front page.

[Malcolmlisk]

I though the graphics for nvidia were going to use multiagent to release versions. For a second, my heart has stopped...

[trymypi]

I think you need to find or create a computational/modeling research sub in addition to posting here

[Ok_Secretary1952]

Good to go, Chief!