Tax-Calculator is a mature, open-source microsimulation library for static (and simple behavioral) analysis of the US federal individual income and payroll tax system. It provides a Python API and a CLI tool (`tc`) that allow users to define policy reforms via JSON parameter files, apply them to microdata samples (CPS, PUF, or TMD), and produce distributional tables, revenue estimates, marginal tax rates, and welfare measures. The core architecture separates policy parameters (`Policy`), taxpayer records (`Records`), and economic adjustments (`GrowFactors`, `GrowDiff`, `Consumption`) from the calculation engine (`Calculator`), which uses Numba JIT compilation for performance. While primarily an application/library with formal releases and CI/CD, it also bundles research data (CPS microdata, grow factors) and includes extensive cross-validation against NBER's TAXSIM-35 model. The codebase is well-structured, heavily tested, and actively maintained, with a clear modular design that supports both CLI and programmatic use.