When you follow a user on a remote Mastodon instance for the first time, you may notice that their older posts do not immediately appear in your Home timeline. This is not a bug or a missing feature—it is a deliberate design choice to control server load and bandwidth usage. The mechanism that controls how many past posts a new follower receives is called backfill, and its behavior varies across different Mastodon versions and server configurations. This article explains exactly what posts sync when you first follow someone, why the system works this way, and what limits you can expect on both the sending and receiving ends.
Understanding backfill rules helps you avoid confusion when a followed account appears to have an empty timeline. It also helps instance administrators decide whether to enable third-party backfill features for their users. This article covers the default Mastodon behavior, the role of the Mastodon API, and the differences introduced by server-side settings and third-party tools like FediFetcher.