What is a Quarkus BuildItem?

A BuildItem is the fundamental unit of data passed between build steps in Quarkus's build-time augmentation pipeline — an immutable object representing one discrete piece of information, such as "this class needs to be registered for reflection" or "this bean was discovered as a CDI candidate."

Extension authors write @BuildStep methods that consume some BuildItems as input and produce new BuildItems as output, and Quarkus's build engine automatically figures out the correct execution order for all these steps across every active extension based on those input/output dependencies, similar to how a task graph works in a build system.

This design is what lets dozens of independently-developed extensions cooperate correctly during a single build: an extension doesn't need to know anything about another extension directly, only which BuildItem types it needs and which ones it produces, and the framework wires the overall pipeline together.