ABCI++ is a major evolution of ABCI (Application Blockchain Interface). Like its predecessor, ABCI++ is the interface between CometBFT (a state-machine replication engine) and the actual state machine being replicated (i.e., the Application). The API consists of a set of methods, each with a corresponding Request and Response message type.

The methods are always initiated by CometBFT. The Application implements its logic for handling all ABCI++ methods. Thus, CometBFT always sends the Request* messages and receives the Response* messages in return.

All ABCI++ messages and methods are defined in protocol buffers. This allows CometBFT to run with applications written in many programming languages.

This specification is split as follows:

• Overview and basic concepts - interface's overview and concepts needed to understand other parts of this specification.
• Methods - complete details on all ABCI++ methods and message types.
• Requirements for the Application - formal requirements on the Application's logic to ensure CometBFT properties such as liveness. These requirements define what CometBFT expects from the Application; second part on managing ABCI application state and related topics.
• CometBFT's expected behavior - specification of how the different ABCI++ methods may be called by CometBFT. This explains what the Application is to expect from CometBFT.
• Example scenarios - specific scenarios showing why the Application needs to account for any CometBFT's behaviour prescribed by the specification.
• Client and Server - for those looking to implement their own ABCI application servers.