Running Python code in a sandbox with MicroPython and WASM

https://simonwillison.net/2026/Jun/6/micropython-in-a-sandbox/

110 41 2 months ago

P.S. I was casually searching for "sandboxed Python" for an experiment I'm working on, and reached this article that was published "today". Very nice coincidence! Thanks.

tmaly• 2 months ago

I am trying to think of a use case for this.

I was thinking the client side WASM version would be useful as a platform for beginners to practice a subset of Python in.

I can't really think of any good WASI use cases.

theanonymousone2• 2 months ago

For me it is a tool I avail to an LLM so that it can provide correct answers to a certain category of questions, instead of hallucinating nonsense.

roywiggins• 2 months ago

The idea is to expose it as a tool to your LLM agent so it can run calculations on its own initiative.

andrewaylett2• 2 months ago

Running arbitrary untrusted code safely is pretty easy nowadays, so long as the code is written in Javascript and you want to run it in a browser. It's only a little harder if the code is written in another language but targets WASM and browser APIs, or if you want to run your WASM inside of NodeJS, and there's even good support for running Python in a browser or Node.

Once you get away from running in a JS environment or away from code that's written with the intention of running in a WASM sandbox, if you don't want to have to modify the code for your environment then you're going to start having problems. This looks like a good step for anyone wanting to run arbitrary Python outside of a browser environment.

simonw8• 2 months ago

I've actually found it pretty hard in a browser as well - if you want to run untrusted code without it breaking your app or stealing cookies etc.

I've been doing a bunch of work recently with iframe sandbox combined with CSP which appears to be a robust way to do this.

simonw8• 2 months ago

I've had lots of fun with WASM Python in the browser - a few of my experiments with that are:

- https://lite.datasette.io - my Datasette app in a browser

- https://simonw.github.io/research/pyodide-asgi-browser/datas... is a new, improved version of that using Service Workers that's still a little experimental - notes here: https://simonwillison.net/2026/May/30/pyodide-asgi-browser/

- https://tools.simonwillison.net/micropython runs a MicroPython playground in the browser via WebAssembly

My use-cases for server-side WASM Python are described here: https://simonwillison.net/2026/Jun/6/micropython-in-a-sandbo... - basically I want to offer end-user customization features that run custom code without buggy or malicious code crashing my app or leaking their data.

incognito1242• 2 months ago

If you're interested in not reinventing the sandbox for LLMs, consider Judge0: https://judge0.com/

I have absolutely no relation to the project except for the fact that I went to the same Uni as the creator.

era86• 2 months ago

I'm using judge0 for a Leetcode-clone I'm working on. Never thought of using it in the context of LLMs, though.

simonw8• 2 months ago

That one looks pretty good - it's been around since 2016, I'm surprise I haven't encountered it before.

It's not quite right for what I'm after because you can't just "pip install" it on multiple platforms.

christoff12• 2 months ago

This looks like something I can use for a project I'm working on. Thanks

dizhn• 2 months ago

Do you know if it has any kind of orchestration/queue support or if there's another project that does it embedding Judge0?

incognito1242• 2 months ago

No idea, I expect that it does since it's very performant. I would totally reach out to the support

hmokiguess• 2 months ago

Super tangential comment but glad to see I'm not the only one that send typos to sessions and still get good results.

Was reading your https://chatgpt.com/share/6a1e2a5c-58b8-8328-ba1c-0e6aadb0a0... and noticed the "my on Python tools" instead of "my own Python tools" (apologies for the grammar police)

This stuff always gets me anxious for no reason because of the underlying tokenizer and prediction stochastic parrot that runs stuff, makes me wonder if I should rerun the prompt correcting the typo or accept the token tax on some interpreter that spent translating the intention.

simonw8• 2 months ago

Yeah I'm very loose with my prompting now - I can usually tell from the reasoning traces if it correctly interpreted any typos.

If it looks like it didn't I hit "stop" and then edit and resubmit my prompt.

fzysingularity2• 2 months ago

What’s your experience with Monty? Been looking at it for one of our environments and it seems very promising.

simonw8• 2 months ago

I've tried it out a bit - it does look solid and it has a good team behind it.

It's a subset of Python though (much more so than MicroPython), which is fine for LLMs since they can easily work around any limitations but does mean you can't use a lot of existing Python code with it. I hope they implement classes soon!

I'm also a little bit nervous about the safety. It's a fresh implementation in Rust, which means plenty of possibilities for edge case security bugs. The thing I like about WebAssembly is that there's a robust, well tested sandbox already - better for defense in depth.

I certainly wouldn't bet against Monty though! It may well prove itself to be a great solution for this.

autogn0me• 2 months ago

Any thoughts on https://github.com/dylan-sutton-chavez/edge-python or http://edgepython.com/ ?

sprak• 2 months ago

Is this the place to look at if you want to play around with a live demo: https://tools.simonwillison.net/micropython ?

simonw8• 2 months ago

No that's a different demo - that one is running MicroPython WASM directly in the browser.

I have a live demo with datasette-agent-micropython running at https://agent.datasette.io - you need to sign in with GitHub to try it.

apignotti4• 2 months ago

We are working to solve the "sandoxing in Wasm" problem across multiple runtimes.

https://labs.leaningtech.com/blog/browserpod-deep-dive

Node.js is now fully supported, Python is in preview and Rust is coming soon.

For a glimpse of the possibilities, check our Claude Code running fully in the browser: https://browsercode.io/claude

binyu4• 2 months ago

Been working on something similar based on Webcontainers. How does your Node.js support compare with StackBlitz technology?

Are you running the version of Claude code that Anthropic distributes in the browser or did you have to adapt it to run on your stack?

Cheers

apignotti4• 2 months ago

Our technology is much more general that WebContainers, and it's based on a Linux-compatible WebAssembly kernel. It also supports real command line tools, including git, bash and the complete set of busybox utilities.

The version of Claude Code you see running is completely unmodified.

rdksu• 2 months ago

Google also released colab-cli a couple of days back and its pretty good at functioning as an isolated sandbox for running random python scriptS .

dvt• 2 months ago

Literally working on a product that does this, hah :) I really do think that AI + automation + carefully-designed guardrails will unleash a deluge of productivity for normies, and we've barely scratched the surface.

The state of AI apps is absolutely trash right now, it’s embarrassing that these companies that raised millions are releasing the shittiest slop around without any product ethos. Obviously we're seeing what sticks, but come on guys.

I'm using Brett Cannon's `https://github.com/brettcannon/cpython-wasi-build` running inside a WASI rust container with a carefully-designed host SDK (e.g. sandboxed Chromium access, diff, sandboxed filesystem, pandas subset, PDF reading, etc.). Essentially the AI sees a goal, a plan, and essentially treats the "task space" as a WASI-powered Python notebook.

Mainly focused on the user experience, and I think that local LLMs (secure/private) + standard Python + host functions + (some external stuff like screen reading & quarantined web access) is more than enough for 90% of actionable tasks.

Very exciting times ahead.

tuananh• 2 months ago

there's also monty by pydantic https://github.com/pydantic/monty

it's Rust so can be compile to wasm, example: https://github.com/hyper-mcp-rs/monty-plugin

nicolix• 2 months ago

On linux I devised this strategy for letting llm webuis or coding agent to securely run programs by burying their environment under multiple layers of locally arranged sandboxing.

Basically: run as another user -> run inside firejail sandbox -> run inside a stripped down alpine linux vm with smolvm.

See the whole procedure here: https://www.reddit.com/r/LocalLLaMA/comments/1tm93ng/how_i_d...

P.S. directories can be easily shared between the sandboxed guest and the host os

P.P.S. to stay a bit more on the safe side I also changed the name of the package manager for the guest os to something else so that when a coding agent would try to autonomously install external packages it will fail. I've then instructed it to (politely) ask for whatever it needs to be eventually manually installed by me

yaodub• 2 months ago

Interesting middle ground between full WASM lockdown and a bare environment. Did you end up needing to block anything else beyond the package manager?

binsquare2• 2 months ago

I build a lightweight vm here that fits your needs as well: https://github.com/smol-machines/smolvm

simonw8• 2 months ago

The thing that's missing is Windows support and the ability to get everything I need for it in a Python program by "pip install X" for something that includes relevant binaries as well as Python code.

binsquare2• 1 month ago

It actually supports wsl(windows) and has the ability to persist binaries.

tastyeffectco• 2 months ago

one question i have about the project? what is the main need to be scoped to python only ? it is for perf ? infrastructure stack ? or something else ?

Running Python code in a sandbox with MicroPython and WASM

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