In software, a side effect changes something outside a function's returned value. Examples include writing a file, updating a database, sending a request, creating a message or changing an account setting.
Agent tools need accurate side-effect descriptions because labels can be misleading. An interface presented as read-only may still cause writes if its underlying endpoint accepts state-changing requests.
ELI5
A side effect is an extra change that happens while software performs an action. Looking up information should only read it, while saving a note changes something other people or later runs can see.
For example, a tool may appear to open a page for reading but also send a request that updates the page. That update is a side effect.
