# Iam 12 .I Accidentally Built a Secure JS Sandbox While Patching a Bug. Here is How KODA Runs AI Code.

> Source: <https://dev.to/koda2026/iam-12-i-accidentally-built-a-secure-js-sandbox-while-patching-a-bug-here-is-how-koda-runs-ai-45aj>
> Published: 2026-10-05 12:47:16+00:00

most ai wrappers just spit out a markdown block and say "good luck."

if the ai hallucinates and writes a malicious script or an infinite loop, your machine takes the hit.

yesterday, i was patching a truncated script error in koda v29. the file just... ended mid-sentence. while rebuilding the event listeners, i realized something: i didn't just want koda to *write* code. i wanted it to *prove* the code works.

so i accidentally built a local code execution sandbox. 

and it’s running entirely in a single 92kb html file. 

the trick is the html `sandbox` attribute. 

i render the ai’s code into a hidden `<iframe sandbox="allow-scripts">`. 

notice what is missing? `allow-same-origin`. 

by omitting that, the browser assigns the iframe a completely unique, opaque origin. it is trapped in a ghost dimension. if the ai hallucinates and tries to read my supabase tokens from `localStorage`, or tries to manipulate the parent dom, the browser just blocks it. 

it can execute javascript, but it can't touch the real world.

ais love writing infinite loops. 

`while(true) { console.log("oops") }`

if i just let the iframe run, it would freeze the user's browser tab. 

so i overrode `console.log` to pipe the output back to the ui via `postMessage`, and wrapped the execution in a hard 3-second `setTimeout`. 

if the code doesn't finish in 3 seconds, the parent process kills the iframe. no frozen tabs. no crashed phones.

right now, if you ask koda to run python or typescript, it just shows a "coming soon" toast.

people asked why i didn't just add python support immediately. 

the answer is **pyodide**. 

running python in the browser requires webassembly. pyodide adds 10mb+ to your bundle. 

koda’s entire frontend—chat, auth, streaming, ui, and now a code runner—is **92 kilobytes**. 

adding a 10mb python engine would destroy the core philosophy: an app that loads in under 1 second on a 3g mobile network. 

so for this test release, we are strict vanilla js. i'd rather have a blazing fast js sandbox than a bloated python one. 

koda v29 is live with the sandbox as a beta feature. 

go to koda-aicodementor.netlify.app. 

ask it to write a javascript fibonacci sequence. click the "Run" button. watch it execute locally. 

then, ask it to write an infinite loop. watch the 3-second bouncer kill it.

if you find a way to break out of the ghost dimension, tell me. i'll patch it.