The decentralized social web runs on protocols, and the two most significant ones in 2026 are ActivityPub (powering Mastodon and the fediverse) and AT Protocol (powering Bluesky). Understanding how these protocols differ is essential for anyone building on, administering, or making informed choices about decentralized social platforms.
What You’ll Know by the End
- The core design philosophy behind each protocol
- How identity, data, and federation work differently in each
- Technical trade-offs that affect everyday users
- Where the protocols overlap and where they diverge
- Whether interoperability between them is realistic
ActivityPub: The Fediverse Standard
ActivityPub is a W3C standard published in 2018. It defines how servers communicate to create federated social networks.
Core Architecture
Server-to-server federation: Servers (instances) send messages to each other. When you post on Mastodon, your server sends the post to every server that hosts a follower of yours.
Actor model: Every user (and some other entities) is an “actor” with an inbox and outbox. Federation is essentially actors sending messages to each other’s inboxes.
Instance-based identity: Your identity is tied to your instance (@user@instance.example). If your instance disappears, your identity goes with it (unless you’ve migrated).
Decentralized by default: There is no central authority. Any server running ActivityPub-compatible software can join the network.
Strengths
- Broad ecosystem: Mastodon, Pixelfed, Lemmy, PeerTube, and many others use ActivityPub
- W3C standard: Formal specification with community governance
- Proven at scale: The largest decentralized social network runs on it
- Simplicity: The protocol is relatively straightforward to implement
- True federation: No central infrastructure required
Limitations
- Identity portability: Moving accounts between instances loses post history and requires manual steps
- Data model flexibility: ActivityPub does not prescribe data schemas, leading to implementation differences
- Discovery: No built-in mechanism for cross-instance search or discovery
- Consistency: Different implementations handle edge cases differently
AT Protocol: Bluesky’s Foundation
AT Protocol (Authenticated Transfer Protocol) was developed by Bluesky as a new approach to decentralized social networking.
Core Architecture
Personal Data Servers (PDS): Your data lives in your own PDS. You can move it between providers without losing anything.
DID-based identity: Your identity is a Decentralized Identifier (DID), independent of any server. You can change providers while keeping your identity.
Relay and AppView architecture: Relays aggregate data from PDSes. AppViews (like the Bluesky app) consume relay data to build the user experience. This creates a layered architecture.
Lexicon schemas: AT Protocol uses formal schemas (Lexicons) to define data structures. This ensures consistency across implementations.
Strengths
- True data portability: Your data is in your PDS; moving providers moves your data
- Persistent identity: DIDs survive server changes
- Custom algorithms: Anyone can build and publish feed algorithms
- Formal data schemas: Lexicons enforce consistency
- Account recovery: Key rotation and recovery mechanisms are built in
Limitations
- Centralization risk: In practice, most users use Bluesky’s infrastructure (relay, AppView)
- Complexity: More moving parts than ActivityPub; harder to self-host completely
- Single ecosystem: Currently only Bluesky uses AT Protocol meaningfully
- Governance: Bluesky PBC controls protocol development more tightly than ActivityPub’s W3C process
Side-by-Side Comparison
| Aspect | ActivityPub | AT Protocol |
|---|---|---|
| Standard body | W3C | Bluesky PBC |
| Identity | Instance-based handle | DID (portable) |
| Data location | On your instance | In your PDS |
| Data portability | Account migration (partial) | Full data portability |
| Feed model | Chronological (server-decided) | Custom algorithms (user-chosen) |
| Federation model | Peer-to-peer between servers | Hub-and-spoke (relay + AppView) |
| Ecosystem breadth | Many platforms (Mastodon, Pixelfed, Lemmy, PeerTube) | Primarily Bluesky |
| Self-hosting complexity | Moderate | Higher |
| Discovery | Hashtags, relays | Global search, custom feeds |
| Moderation | Instance-level | Labeler services |
Design Philosophy Differences
The protocols reflect different priorities:
ActivityPub prioritizes: Simplicity, instance autonomy, community governance, and a broad federation model. It trusts instance admins to make decisions for their communities.
AT Protocol prioritizes: Individual data ownership, algorithmic choice, portable identity, and consistent data schemas. It aims to prevent lock-in at any layer.
Neither approach is objectively better. They optimize for different values and use cases.
Interoperability: Can They Talk to Each Other?
As of 2026, ActivityPub and AT Protocol do not interoperate natively. They are fundamentally different protocols with different data models.
Bridges: Third-party bridge services can translate between protocols, allowing some cross-platform following and posting. These bridges are imperfect — not all features translate, and the experience is degraded compared to native use.
Long-term prospects: True interoperability would require either protocol adaptation or standardized bridge specifications. This is an area of active discussion but not imminent resolution.
For more on cross-protocol tools, see our articles hub.
What This Means for Developers
If you are building social applications:
- ActivityPub is the right choice if you want to join the existing fediverse ecosystem and benefit from its breadth
- AT Protocol is worth exploring if data portability and custom algorithms are core to your product
- Supporting both is possible but significantly increases implementation complexity
- Watch the standards process: Both protocols are evolving; stay current with their development
Our developer notes cover practical considerations for building on these protocols.
What This Means for Users
As a user, the protocol choice matters primarily through:
- Which communities you can reach: ActivityPub gives you the broader fediverse. AT Protocol gives you the Bluesky ecosystem.
- How your data is handled: AT Protocol offers stronger portability guarantees.
- What your timeline looks like: AT Protocol offers algorithmic choice; ActivityPub defaults to chronological.
- Where you can migrate: ActivityPub migration is instance-to-instance. AT Protocol migration is PDS-to-PDS.
Common Mistakes
- Assuming one protocol will win: Both serve real needs and will likely coexist for years
- Treating AT Protocol as “better ActivityPub”: They make different design trade-offs, not better/worse ones
- Ignoring the ecosystem around the protocol: A great protocol with no users is less useful than a good protocol with a large community
- Confusing the protocol with the platform: Mastodon is one ActivityPub implementation; Bluesky is one AT Protocol implementation. Neither defines the protocol completely
- Expecting seamless interoperability soon: Bridge quality will improve but native cross-protocol federation is not on the near-term horizon
Frequently Asked Questions
Can I use both Mastodon and Bluesky? Yes. Many people maintain accounts on both. They are independent platforms with independent accounts.
Will Mastodon adopt AT Protocol? Unlikely. Mastodon is built on ActivityPub and its community values align with that protocol’s design. Some interoperability through bridges is more realistic.
Which protocol is more “decentralized”? ActivityPub’s peer-to-peer model is more decentralized in architecture. AT Protocol’s data portability is more decentralized in terms of user empowerment. The answer depends on what you mean by decentralization.
Can I build an app that supports both? Yes, but it requires implementing both protocols and handling the differences in data models, identity, and federation. Some projects are attempting this. See our tools page for cross-protocol tools.
Is one protocol more private than the other? Both protocols make public content public. Privacy depends more on implementation (how the platform handles data) than on the protocol itself. Our best tools guide covers privacy-focused options.