AT Protocol

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AT Protocol or Authenticated Transfer Protocol, previously known as Authenticated Data Experiment (ADX), is a federated social network protocol, designed and first implemented by the Bluesky project, subsequently supported by IndieWeb-friendly service micro.blog and libraries. The term "Atmosphere" can be used to describe the ecosystem around the AT Protocol.

IndieWeb Examples

Currently the only IndieWeb Examples are hardcoded to post directly to Bluesky rather than arbitrary AT Protocol supporting services.

See: Bluesky#IndieWeb_Examples

Service Support

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Bluesky
micro.blog per 2023-04-29 Manton Reece: Getting started with Bluesky XRPC
Blacksky
- 2025-08-24: How Blacksky grew to millions of users without spending a dollar
Bridgy Fed has added support for AT Protocol
- https://github.com/snarfed/bridgy-fed/issues/381
Bluecast audio streaming for discussions with up to 10 people
pckt.blog is a blogging platform (silo in that it is dependent on their domain?) that can store posts in the PDS and provides a styled view and RSS. See devlog.pckt.blog

In progress support

See also AT Protocol analysis here:
- https://fed.brid.gy/docs#translate

Criticism

Blocks Publicly Accessible via API

2026-01-18 Veronica Explains: My comments are littered with folks asking "why is it a problem if your blocklist is public on Bluesky?"
2026-01-17 Elena Rossini: Blocking on Bluesky is NOT PRIVATE
2023-12-09 [deleted]: Blocks being public is already leading to problems imo
2023-05-02 Mosquito Capital: Bluesky Is Not Ready

Your list of blocked users is public. Anyone can see it through the API. Someone harassing you? They, and all of their buddies, will know as soon as you block them. People can build profiles of who's ripe for harassment just by searching for large blocklists.

Official overview: Why are blocks on Bluesky public?
- The converse argument is that blocks on Mastodon are opaque and under the control of your instance host, not part of your account.

Heavy Architecture

2025-12-27 @mcc: backfill is still a problem for actual ATproto/Bluesky decentralization:

Follow up, 2025-12-27: Rudy here confirms the Blacksky appview is still being worked on (eg: blacksky uses bluesky's appview still) The sticking point, as he describes it, is "backfill". This alludes to the issue that makes me compare ATProto to blockchain: to get the features users expect, every node on the network must mirror the network's entire history. This is impractical, which is why bluesky is as of this moment a federated network with effectively only one node.

- Backfill is being worked on; there is an open source project Tap that will backfill selected parts of the full repository or firehose. code announcement

- Mastodon makes no provision for backfill at all, accordingly it always lacks 'the features users expect'

2025-10-02 Azhdarchid: Delusions of a Protocol

ATProto's complicated architecture more or less puts running an alternative app into the realm of well-funded nonprofits and startups, as opposed to Mastodon instances which can easily be run by hobbyists on their own domestic hardware.

- Mastodon has terrible scaling costs as each instance has to deal with traffic from other instances that follow it. You can probably run a single user instance OK, but any reasonable number of users makes the cost in admin_tax and hosting disproportionate. See https://web.archive.org/web/20230627133937/https://cheapskatesguide.org/articles/mastodon-servers.html

2024-11-22 Christine Lemmer-Webber: How decentralized is Bluesky really?

The likely answer to this is that there will always have to be a large corporation at the heart of Bluesky/ATProto, and the network will have to rely on that corporation to do the work of abuse mitigation, particularly in terms of illegal content and spam. This may be a good enough solution for Bluesky's purposes, but on the economics alone it's going to be a centralized system that relies on trusting centralized authorities.

2023-04-27 Michał Woźniak: BlueSky is cosplaying decentralization

How To

2025-08-27 bryan newbold: A Full-Network Relay for $34 a Month

But i'm still really psyched at how much simpler things have become, so I set up my own $34/month VPS demo instance, and this is the write-up.
The relay describe here is running at: https://relay-vps.demo.bsky.dev

History

"ADX" iteration of the protocol renamed to “Authenticated Transfer Protocol” — or the “AT Protocol.” https://atproto.com/

2022-10-18 The Bluesky Team: The AT Protocol (archived)