Comparison of ActivityPub, AT Protocol, and Matrix Protocol (mainly a reminder for myself) “

Feature	ActivityPub	AT Protocol	Matrix Protocol
Primary Use Case	Social networking (microblogging, content sharing)	Social networking with algorithmic control	Real-time communication (chat, VoIP)
Commands/Verbs	Create, Update, Delete, Follow, Like, Announce (Repost)	Create, Update, Delete, Follow, Like, Repost, Algorithmic Choice	Send, Receive, Join Room, Invite, Leave
Data Model	Activity-Object Model in JSON-LD	JSON with user-defined schemas	JSON (events model for real-time updates)
Transport Protocol	HTTPS, with JSON-LD	HTTPS, with JSON	HTTP (REST) and WebSocket, with support for end-to-end encryption (E2EE)
Identity Management	Tied to server domain (e.g., @user@domain.com), uses WebFinger for discovery	Portable DIDs for decentralized identity	Tied to server domain but portable; user ID format is @user:domain.com
Federation	Federated, allowing instances to share content and social connections across domains	Federated with content and algorithm control	Federated, with real-time, synchronized state across servers
Interoperability	Widely interoperable with other ActivityPub-compliant platforms in the Fediverse	Designed for custom app experiences, interoperability is in development	Supports interoperability with other Matrix clients; bridges to other protocols (e.g., Slack, IRC)
End-to-End Encryption	Not native to protocol but possible with extensions	Not natively specified	Built-in and widely supported, particularly in 1:1 and group chats
Moderation	Instance-based moderation policies, customizable filters and blocks	User-level and instance-level moderation, customizable algorithms	Room-level moderation, with granular permissions for room admins
Popular Platforms	Mastodon, PeerTube, Pixelfed, WriteFreely	Bluesky Social, upcoming decentralized apps	Element (main Matrix client), Synapse (server), bridges for Slack, Discord, Telegram, etc.

Summary of Key Differences

• ActivityPub is best suited for federated social networking, particularly for applications that prioritize openness and content sharing across platforms in the Fediverse. It uses an Activity-Object model with JSON-LD and supports instance-based identity.
• AT Protocol focuses on user control over content algorithms and portable identities using DIDs, with a vision for interoperability in custom social applications. It is also designed for federated social networks but with more control over data portability and algorithmic transparency.
• Matrix Protocol excels in real-time, federated communication, supporting secure, encrypted messaging with granular moderation capabilities. It’s heavily used for chat, VoIP, and collaborative tools, emphasizing interoperability with other platforms through bridges.