  Bluesky is a new social network built upon the AT Protocol, a decentralized
foundation for public social media. It was launched in private beta in February
2023, and has grown to over 3 million registered users in the following year.
In this paper we introduce the architecture of Bluesky and the AT Protocol,
which is inspired by the web itself, but modernized to include streams of
real-time updates and cryptographic authentication. We explain how the
technical design of Bluesky is informed by our goals: to enable
decentralization by having multiple interoperable providers for every part of
the system; to make it easy for users to switch providers; to give users agency
over the content they see; and to provide a simple user experience that does
not burden users with complexity arising from the system's decentralized
nature. The system's openness allows anybody to contribute to content
moderation and community management, and we invite the research community to
use Bluesky as a dataset and testing ground for new approaches in social media
moderation.
