From Meta, a Wikimedia project coordination wiki

The toolserver is a Wikimedia server that hosts tools and extra functionality that benefit other Wikimedia servers. Although the toolserver is located at, tools located there can generate reports for most Wikimedia wikis. The original and most well-known tool on the toolserver is Kate's Tool. There are also many other toolserver developers who have written a variety of other tools.

The toolserver is not part of the main Wikimedia server cluster, and does not have a redundant fallback, so in some cases it may be down when Wikipedia is not. The toolserver's database is also separate from the main database cluster, and instead is constantly updated by replication. Occasionally replication will lag minutes, hours or even more behind real-time, and toolserver tools will appear to reflect an older state of Wikipedia — changes made on Wikipedia won't be seen immediately on the toolserver. (You can view the current toolserver replication lag[dead link], en wiki and the other wikipedias have different replags for technical reasons. You can also see graphs of the replag[dead link] showing how it is getting better/worse as time goes by..) This is not a serious problem, because in most cases replication will automatically slowly move back towards real-time replication, though in the meantime the older data can cause some confusion.

The toolserver's database is read-only — tools can't directly update Wikimedia pages — so toolserver tools tend to be report-oriented. Also, while most of the database data is available on the toolserver, developers there don't have access to private information such as account preferences or watchlists.