Supercharge your web app with free AI that runs in your users' browser

There is a class of feature that used to be impossible to ship for free: anything that needed a language model. You wired up an API key, you ate the per-token bill, and every prompt your users typed went off to someone else's server. For a small public tool, that math usually killed the idea before it started. That changed. Recent versions of Chrome ship a language model, Gemini Nano, and expose it to any web page through the Prompt API . The model runs on the user's own machine. No API key. No inference bill. No data leaving the browser. We put this into a real, live tool, a free Mermaid diagram editor where you describe a diagram in plain English and the browser writes the Mermaid code for you. This post is the developer's version of that story: how the API actually works, the code that makes a small on-device model trustworthy, and an honest accounting of what you gain and what you give up. What "AI in the browser" means in 2026 The important word is built-in . This is not WebGPU plus a 4 GB model you download and run yourself. The model ships with Chrome, and you talk to it through a small standard-track JavaScript API. As of Chrome 148, the Prompt API is stable for web pages (it had been available to extensions since Chrome 138). It is the general-purpose member of a growing family of built-in APIs: Prompt API ( LanguageModel ): general natural-language prompting, now multimodal (text, plus image and audio input). Summarizer, Writer, Rewriter, Proofreader : task-specific, text-to-text. Translator and Language Detector : backed by expert models, desktop only. The Prompt API is the one you reach for when you need something the task APIs don't cover, like "turn this description into Mermaid source." So that is the one this post focuses on. The 15-line version Here is the whole happy path. Check availability, create a session, prompt it. // Feature-detect first. Old browsers won't have this at all. if ( ' LanguageModel ' in self ) { const status = await LanguageMod