Let's discuss sandbox isolation

I disagree with this section about WebAssembly:

> But the practical limitation is language support. You cannot run arbitrary Python scripts in WASM today without compiling the Python interpreter itself to WASM along with all its C extensions. For sandboxing arbitrary code in arbitrary languages, WASM is not yet viable.

There are several versions of the Python interpreter that are compiled to WASM already - Pyodide has one, and WASM is a "Tier 2" supported target for CPython: https://peps.python.org/pep-0011/#tier-2 - unofficial builds here: https://github.com/brettcannon/cpython-wasi-build/releases

Likewise I've experimented with running various JavaScript interpreters compiled to WASM, the most popular of those is probably QuickJS. Here's one of my many demos: https://tools.simonwillison.net/quickjs (I have one for MicroQuickJS too https://tools.simonwillison.net/microquickjs )

So don't rule out WASM as a target for running non-compiled languages, it can work pretty well!

Good write up. I was hoping to see V8 isolates (Cloudflare workers) as part of the comparison at I've always found that interesting.

OK, let’s survey how everybody is sandboxing their AI coding agents in early 2026.

What I’ve seen suggests the most common answers are (a) “containers” and (b) “YOLO!” (maybe adding, “Please play nice, agent.”).

One approach that I’m about to try is Sandvault [0] (macOS only), which uses the good old Unix user system together with some added precautions. Basically, give an agent its own unprivileged user account and interact with it via sudo, SSH, and shared directories.

0. https://github.com/webcoyote/sandvault

I appreciate the details in this, but I also notice it is very machine-focused. When a user wants to sandbox an AI agent, they don’t just want their local .ssh keys protected. They also want to be able to control access to a lot of off-machine resources - e.g. allowing the agent to read github issues and sometimes also make some kinds of changes.

QubesOS was built to give sandboxes kernel isolation via a hypervisor.

It’s not surprising that most people don’t know about it, because QubesOS as a daily driver can be painful. But with some improvements, I think it’s the right way to do it.

Its worth pointing out another boundary: speculative execution. If sensitive data is in process memory with a WASM VM it can be read even if the VM doesn't expose it. This is also true of multiple WASM VMs running for different parties. For WASM isolation to work the VM needs to be in a seperate process

Sandbox isolation is only slightly important, you don't need to make it fancy, just a plain old VM. The really important thing is how you control capabilities you give for the agent to act on your behalf.

> compute isolation means nothing if the sandbox can freely phone home.

Here's a project I've been working on to address the network risk. Uses nftables firewall allowing outbound traffic only to an explicit pinned domain allowlist (continuously refreshes DNS resolutions in the background).

https://github.com/noperator/cagent

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