DevKit's AI Playground is a chat with a real language model that runs 100% in your browser. No backend, no API key, no cost per token, and — because there is no server at all — nothing you type can leave the page. It's a static HTML file on GitHub Pages, like the rest of DevKit.

How it works

• Transformers.js v3 does the inference, inside a Web Worker so generation never freezes the UI.
• The model is Google's Gemma 3 270M IT, 4-bit quantized (q4f16) — a 293 MB one-time download, cached by your browser for future visits. If Gemma can't start on your device (it needs real memory headroom), the page automatically falls back to the smaller SmolLM-135M (121 MB) in a fresh worker, so the chat still works.
• It tries WebGPU first (your graphics chip) and falls back to WASM on the CPU.
• Tokens stream in as they're generated, with a live tokens/sec readout — which is either a fun toy or a benchmark, depending on how serious your hardware is.

Click "Load the model", wait for the one-time download, and chat. That's the whole signup flow.

Honest limits

The models are small — 270M and 135M parameters — and live in your RAM. It writes plausible text, answers simple questions, does little creative tasks — and cheerfully makes things up. It is not GPT-class, and the page says so up front. The point isn't to replace an API; it's to show how far "the browser is the runtime" has come, and to give the LLM-curious a playground with literally zero setup and zero bill.

DevKit's other LLM tools are the grown-up counterparts: a cost calculator that prices 14 real API models with cached-read/cache-write splits, and a tokens/sec simulator. The playground is what those calculators look like when the model is small enough to be free.

Also new in this release

Four more client-side tools joined the kit: a JWT decoder (decode-only, and it says so — the signature is not verified), a cron expression explainer (plain English + next 5 run times), a color palette generator, and a Pomodoro timer whose countdown runs in the tab title. Same rules as everything on DevKit: no uploads, no accounts, no tracking, and every calculator's math is checked by automated tests (310 checks and counting).

Try it and tell me the tokens/sec number your machine gets — and which model you'd want to see in the playground next. SmolLM2-360M? Qwen? The download budget is the boss fight.

DevKit is a one-file calculator library with 69 tools, all running client-side in your browser. The tools are organised into tabbed pages, so you can find the one you need without scrolling through a long list.

What's new: an LLM cost calculator

The newest addition is for anyone working with language models. The LLM cost calculator helps you estimate what a request will cost, and the tokens/sec simulator shows how generation speed affects timing. It's the part I reach for most when I'm planning an LLM feature and want a quick sense of cost and speed before writing code.

Verified formulas

A calculator is only useful if you can trust its answers, so the formulas in DevKit are verified with tests. That gives me a way to catch mistakes when a formula changes, instead of hoping the numbers look right.

Guides

There's also a new guides hub with walkthroughs for the tools: https://astro-devkit.github.io/devkit/guides.html

Try it

Try DevKit here

What calculator should I add next? Let me know in the comments — I'd love to hear what you'd actually use.

Originally published on Dev.to.