Use desktop Google Chrome 148 or newer on Windows, macOS, Linux or ChromeOS. Other Chromium browsers may not include the API. Update Chrome at chrome://settings/help.
On Chrome 148 or newer the API is normally on without flags. If it is still missing, open chrome://flags, search for Prompt API and on device, enable any matching entries that exist, and relaunch. Flag names vary by version, and some builds have none.
Check the model status, and why it is unavailable, at chrome://on-device-internals. If that page is blocked, enable internal debug pages at chrome://chrome-urls first.
The API is present, but this device or profile can't run the model.
Common causes are too little free disk space, an unsupported GPU or CPU, or an enterprise policy.
chrome://on-device-internals shows the exact reason. If that page is blocked, enable internal debug pages at chrome://chrome-urls first.