Show HN: Persona.js – a vanilla-JS agent UI library with native WebMCP (MIT)

Hey everyone. My cofounder and I are open sourcing Persona.js (https://www.persona-chat.dev/). It's a VanillaJS library that helps anyone build agentic experiences on the web, without a framework dependency, and full WebMCP support

So, why'd we do this?

1) We're super fans of the web and the browser can do a ton today

2) We've seen AI builds be way overly complex because the FE requires a large project within an existing app OR the site wasn't using a framework to begin with

If you've been a part of huge apps with multiple frameworks inside them, or work inside CMS / website builders / ecomm platforms... you know what we're talking about. A "simple" AI feature disrupts your life for months

If you have a singular React codebase and can't possibly imagine building an interface without JSX, nothing to see here! You already have a lot of great options that are really cool too. Check out Assistant UI / CopilotKit / AI Elements which are all MIT.

There's a demo video here: https://www.youtube.com/watch?v=68D80uNsfH0.

Some specifics:

Persona has a ~15 kB brotli to first paint (the full widget lazy-loads on first click) while being able to render most of the primary agent experiences you see on the web: from 'Fin' (pill launcher) to 'Claude' (fullscreen assistant) to 'Shopify Sidekick' (docked)

You can run it in Shadow DOM isolated mode so existing styles play nice

Everything has hooks and events, so you can add unique flavor (and easily share back!) in a few lines

We have a ton of demos on the library site, along with all the knobs to play with how each aspect renders. Tool and reasoning, custom loading animations, voice, approval UX, etc

We've also added a ton of examples across agent and frontend stacks

Check out the code @ https://github.com/runtypelabs/persona

Feedback and contributions welcome!

▲

becomevocal 2 hours ago | parent | next [-]

Appreciate the feedback and support so far!

Wanted to add, in terms of extensibility, we've really tried to make the key parts of the UX hookable. Most of what you see on this page like tool / reasoning calls, voice, loaders, approvals, etc are completely replaceable in a few lines of plain JS: https://www.persona-chat.dev/advanced.html

It means you can do interesting things like go straight to OpenAI or Elevenlabs voice for read-aloud while streaming the main chat from your framework.

I'd like to improve the shareability of what's created in the ecosystem, but we're not quite there yet.

If you have ideas for more hooks or extensions you'd like to see, we're very much in listening mode.

▲

bookernath 5 days ago | parent | prev | next [-]

Nathan here from Runtype - I worked closely with @becomevocal on this library. Here to answer questions!

My personal favorite demo is this JSPaint + WebMCP one https://www.persona-chat.dev/webmcp-paint.html

But more practical examples include: https://www.persona-chat.dev/webmcp-slides.html https://www.persona-chat.dev/webmcp-calendar.html

WebMCP's finally at a point where it makes sense to build against it (shipped in latest chrome + good polyfills available), and I've found it a pretty elegant way to extend existing FE apps by just hooking into your existing FE functions instead of having to build an AI assistant as a parallel thing that is ignorant of the user's session, what they're looking at, etc. This makes hybrid AI/app experiences feel much better.

▲

sirjonathan 5 days ago | parent | prev | next [-]

Love seeing more vanilla projects and tooling around standards. Long live the web.

Edit: I’ve been thinking about offering an overlay UI for a web based game and this could be a good fit for that. I’ll give it a spin later and let y’all know.

	▲	becomevocal 5 days ago \| parent [-]
		Thanks! Definitely interested in the game overlay use case. My gut says you'll need want to hook into custom render functions and theme config to map to your game styles, since it's going to look very different than the standard OOTB options. The best file to use as a starting reference is here: https://github.com/runtypelabs/persona/blob/main/packages/wi... If you run into anything, GH issues are welcome!

▲

aloopintolife 5 days ago | parent | prev | next [-]

Practically speaking, what would this mean for a designer? I’ve been building web apps and websites with Claude Code.

Not clear what this would do for me relative to all the other things out there.

▲

bookernath 5 days ago | parent [-]

If you're building apps that you want to surface as a "hybrid" UX - both conversational _and_ traditional UI - WebMCP gives you an elegant means of building for both agents and humans at the same time.

If you're building an AI app with a conversational front door, Persona helps you build your experience from a great starting point that already works, and we're constantly iterating on it to keep pace with frontier AI experiences. So you can just implement + configure/theme it instead of trying to start from scratch.

	▲	aloopintolife 4 days ago \| parent [-]
		Ah I think I get it. I am more using Claude Code to do frontend design as a replacement for Figma when I’m designing products. Not really working on agent ui. Sounds like this wouldn’t really be a match for what I need. Thanks though!

▲

miguelspizza 5 days ago | parent | prev | next [-]

Happy to see WebMCP adoption and a standards first framework.

The argument for react has always been dev velocity and ecosystem. But with AI the best web experiences will be as close to the “metal” as possible

	▲	becomevocal 5 days ago \| parent \| next [-]
		Appreciate it. And thank you for the work on WebMCP! For everyone passing by, Alex created https://docs.mcp-b.ai/ and we use two of his libraries to enable WebMCP more cleanly. We found MCP-B after we built our own It's interesting how experimental web tech vs. frameworks are. Kind of flipped once React and Next.js took off. Now we get to see what the browser can do again. Find the balance that works for each case Note we actually use React / Vite and Next with Persona across a few of our products. It's nice in our case because the agent specific state (including event debug stream if you turn that on) doesn't need to interact with the main app at all. Keeps updates to agent UX from blowing up other parts of the app
	▲	bookernath 5 days ago \| parent \| prev [-]
		Thanks, means a lot coming from you! I agree re: getting closer to the metal - it seems like HTML is having a big resurgence as agents are very productive at producing HTML artifacts which are often 'good enough' for many content use cases, and fortunately Persona works great as an embed in a static site. We spin up demos like that all the time.

▲

Xaena 5 days ago | parent | prev | next [-]

I haven’t seen much on WebMCP adoption yet, but I’ve been keen to use it for improving the AX of a web app at work. I’m curious how you all are seeing it be used.

	▲	bookernath 5 days ago \| parent [-]
		It's definitely early - it shipped in the very latest version of chrome just this week, and the spec may still evolve (as a fair warning to early adopters). You can turn it on for your site via the chrome origin trial, or just install the polyfill. Our use case with Persona is all about extending existing apps to have "AI assistance" that actually works well, and Persona being the AI assistant you can ship yourself in your site to enhance your app. There's a parallel camp who are interested in adding WebMCP tools to their sites so they can be used effectively by future browser-level agents (e.g. Gemini in Chrome), but that's not a thing today. Some AI agents can already do best-effort operation of existing websites via naive 'headless browser' approaches, or doing their best to interpret the existing nature of a page by reading the DOM or accessibility tree and trying to submit the forms, but that's flaky and token-inefficient. WebMCP is all about registering those things as explicit, designed tools built for agents. I'd draw a parallel between asking agents to just call existing REST APIs for an app vs intentionally designing MCP APIs for them; the design philosophy is different. For those interested in good MCP/WebMCP tool design principles, this article from Arcade.dev is great: https://www.arcade.dev/blog/mcp-tool-patterns/

▲

pulkas 5 days ago | parent | prev | next [-]

when will WASM integration become widely accepted? we need realtime canvas draw like oldschool desktop ui/ux.

	▲	bookernath 5 days ago \| parent \| next [-]
		Do you think the new WASI 0.3 stuff will help? I hope so. I'm always super impressed when I see an awesome WASM + WebGPU implementation that doesn't spin up my laptop fan. For canvas/WASM-style apps, I actually think explicit agent-facing APIs become even more important. If the UI is mostly pixels, an agent has less useful DOM/accessibility structure to infer from. WebMCP gives the app a way to expose semantic actions like “create shape”, “select layer”, “export”, “run simulation”, etc. regardless of whether the UI underneath is React, vanilla DOM, canvas, or WASM.
	▲	becomevocal 5 days ago \| parent \| prev [-]
		IMO there's a lot more we can do around hooking into the DOM directly, with more efficient data formats over the wire, before we absolutely need WASM We'd like to experiment more with this but haven't got their yet. Just some internal prototyping

▲

sfarshid 4 days ago | parent | prev [-]

this is awesome!

	▲	bookernath 4 days ago \| parent [-]
		Thanks! We're big fans of assistant-ui also :)