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

A developer built a secure JavaScript execution sandbox for KODA, an AI coding assistant, by rendering AI-generated code into a hidden iframe with the sandbox="allow-scripts" attribute and omitting allow-same-origin, trapping the code in an opaque origin that cannot access localStorage or the parent DOM. Console output is piped back to the UI via postMessage and execution is capped with a hard 3-second timeout to kill infinite loops without freezing the browser tab. The sandbox ships as a beta feature in KODA v29, which the developer says keeps the entire frontend at 92KB rather than adding Pyodide's 10MB+ WebAssembly Python runtime.

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.