No source available but the JS code for the extension should be readable on your extensions dir. I'm using MutationObserver to check for tweets appearing and then extract their texts and send them to the server.

Server side code is just some Go to handle calls to Gemini Flash, auth, tracking quota usage...

To be honest even the prompt used is quite simple, the fun part has been optimizing it using promptfoo for running the evals.

There's a surprising number of extensions like this (see https://github.com/thomasj02/AiFilter, https://unbaited.danielpetho.com/) although it seems most are Chrome-exclusive