Google ships an official polyfill for the Prompt API

Chrome's developer team has released an experimental polyfill that lets developers use the browser Prompt API even where there is no native implementation. By default it uses Transformers.js with the lightweight local model Gemma 3 1B, while also allowing a switch to cloud backends; developers can get started simply by importing prompt-api-polyfill.js. The official documentation was published on May 14, 2026, and the Chrome for Developers account announced it on June 2.

The Prompt API is a proposed Web API for sending natural-language prompts directly to a language model built into the browser (Gemini Nano in Chrome's case), offered experimentally via Origin Trial or flags. Touting privacy and low latency from on-device AI, it sits alongside built-in AI features such as the Writer and Summarizer APIs. This polyfill aims to let developers write API-compatible code even on browsers and platforms without native support, smoothing the eventual migration to a native implementation. Its hybrid design, backing both local (Transformers.js) and cloud paths, is a key trait; community-made polyfills also exist, but Google's official version differs in claiming to be spec-compliant.