# Show HN: Building AI agents client-side JavaScript

> Source: <https://news.ycombinator.com/item?id=49557409>
> Published: 2026-09-03 21:34:31+00:00

Hey HN, most agent systems default to server-side Python inside containers and chain frameworks. I wanted to see how far we could push agent in the browser with vanilla JavaScript [https://buttercup.sh](https://buttercup.sh)

The reason this is interesting is because agent loops in the browser keeps infrastructure costs low. No need for proxy or API calls. And ollama/vLLM can be used for 100% offline. Also WebLLM for embedded. We need to consider CORS, API keys for remote models, access to visual state, and handling remote tool calls. I am working on a guide with references in vanilla JS. This is a short-lived guide starting mid-September with weekly topics.

Draft topics starting mid-September:

In-Browser Loops: Function calling, and deterministic multi-turn loops running purely in the browser runtime. Vision (Multimodal): Capturing viewport screenshots using browser APIs. Remote Agent Access & Transports: Connecting the in-browser agent to remote agents. Multi-Agent Coordination: Lightweight client-side agent, specialist delegation, and running concurrent sub-agents without locking the browser UI thread.

Everything is open source, zero-install, and runnable directly in the browser. Would love feedback from the HN community on agents running in browsers.

Comments URL: [https://news.ycombinator.com/item?id=49557409](https://news.ycombinator.com/item?id=49557409)

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