ActivityPub connects about 13.8 million registered accounts (roughly 1.37 million active monthly) across Mastodon, Misskey, Pixelfed, PeerTube, Lemmy and many other servers, according to FediDB in October 2026. This guide is the 2026 update of our 2025 guide: what you need to build, what changed this year, and which tools to use.
A dated roundup of the changes since June 2026 that affect people building ActivityPub software, and what we updated on SocialDocs as a result.
:::info Updated edition available This post is from December 2025 and parts of it are out of date (for example, it recommends libraries that are no longer maintained). Read The Complete Guide to ActivityPub Development in 2026 instead. :::
:::danger Security note (October 2026) The code in this post is illustrative and not safe for production as written. See issue #37 and use a maintained library for request verification. :::
ActivityPub has become the backbone of the decentralized social web. With over 13 million users across Mastodon, Lemmy, PeerTube, Pixelfed, and dozens of other platforms, understanding ActivityPub is essential for developers building the next generation of social applications.
This guide provides everything you need to start building with ActivityPub — from core concepts to production deployment.