AT Protocol

The AT Protocol (Authenticated Transfer Protocol, pronounced "at protocol", commonly shortened to ATproto or "ATP") is a protocol and set of open standards for decentralized publishing and distribution of self-authenticating data within the social web. It serves as the technical foundation of the Bluesky social network, originally developed as a reference implementation for the protocol, as well as an ecosystem of interoperable social applications and services collectively referred to as the Atmosphere.

The AT Protocol aims to address perceived issues with earlier decentralized social networking protocols such as ActivityPub and Nostr; these include user experience, semantic interoperability, discoverability, network scalability, and portability of user data and social graphs. It employs a modular microservice architecture and a federated, server-agnostic user identity to enable seamless movement between network services, with the goal of providing an integrated online experience without dependence on any single privileged entity.

As of January 2026, the protocol's general architecture, user repository, and data synchronization specifications are in the process of standardization within the Internet Engineering Task Force (IETF). Further specifications—including data schemas, identity systems, OAuth implementation, and private/limited visibility data—are under development by Bluesky Social PBC. The company has said it may seek to standardize additional specifications through the IETF in the future, depending on the outcome of current efforts.