Mastodon is free, open-source social network software run on thousands of independent servers that connect to each other through the ActivityPub protocol.
How Mastodon works
Eugen Rochko released Mastodon in 2016. Instead of one company running one site, anyone can run a Mastodon server, often called an instance. Each server has its own rules and moderators. Servers talk to each other through ActivityPub, so a user on one server can follow and reply to users on another. Together they form a large part of the fediverse.
Your address includes your server, like an email address: @name@server.example.
Things that differ from centralized networks:
- Chronological feeds, with no ranking algorithm on the home timeline.
- Boosts instead of quote posts by default, though quote posts are being added.
- Content warnings that hide text behind a label.
- Strong norms around alt text and not spamming hashtags.
Posting limits
- 500 characters by default; each server can set a different limit.
- Up to 4 media attachments.
- Links count as 23 characters.
An example
A nonprofit posts a weekly update on its own Mastodon server. Followers on dozens of other servers see it in their home feeds.
Why it matters for AI-agent publishing
Each server has its own API base URL, limits, and moderation rules, so an agent needs to know which server it is posting to. Automated accounts are usually expected to set the bot flag, and some servers restrict them.
SocialHandoff publishes Mastodon threads and up to 4 attachments, and uses idempotency keys so retries never double-post. See connect Mastodon and scheduling for Mastodon.