The .bzl file extension is used for Starlark files, which are configuration files used by the Bazel build system. Bazel is an open-source build and test tool developed by Google that is designed to handle large-scale software projects with multiple languages and platforms. Starlark is a dialect of Python that is intentionally restricted to be deterministic and hermetic, ensuring that build results are reproducible regardless of the environment. Within a Bazel project, .bzl files are used to define custom build rules, macros, and constants that extend the core functionality of the build system. By using these files, developers can encapsulate complex build logic, manage dependencies, and define how source code should be compiled, tested, and packaged. Because Starlark lacks certain Python features like arbitrary recursion or mutable global state, it provides a safe and efficient way to describe build processes in a declarative manner, making it a critical component for continuous integration and large-scale software engineering workflows.