arXiv:2502.13267v2 Announce Type: replace Abstract: BeforeIT is an open-source software for building and simulating state-of-the-art macroeconomic agent-based models (macro ABMs) based on the recently introduced macro ABM developed in [1] and here referred to as the base model. Written in Julia, it combines extraordinary computational efficiency with user-friendliness and extensibility. We present the main structure of the software, demonstrate its ease of use with illustrative examples, and benchmark its performance. Our benchmarks demonstrate that the base model built with BeforeIT is orders of magnitude faster than its original Matlab implementation, while achieving performance comparable to a highly optimised HPC implementation. Furthermore, we show how BeforeIT supports an easy construction of model variants such as the 'CANVAS' model of [2], how these variants can be evaluated at scale via an automated multicountry forecasting pipeline, and how the entire simulation can be automatically differentiated. BeforeIT is designed to facilitate reproducibility, extensibility, and experimentation. As the first open-source, industry-grade software to build macro ABMs, BeforeIT can significantly foster collaboration and innovation in the field of agent-based macroeconomic modelling. The package, along with its documentation, is freely available at https://github.com/bancaditalia/BeforeIT.jl under the Apache 2.0 license.