F-ASTMut is an open-source mutation testing research tool for the C language family based on manipulation of the abstract syntax tree. The tool is designed for detailed measurements, analysis, and tuning of optimisation techniques. The goal of F-ASTMut is to analyse the speedups of optimisation techniques to ultimately enable mutation testing in industrial settings. Currently, F-ASTMut features four optimisation techniques; an exclusion scheme for invalid mutants, a test-suit-scope reduction to only cover relevant mutants, mutant schemata, and split-stream mutation testing. The implementation relies on the Clang front-end, allowing future work to extend or build on top of our solution. © 2023 The Author(s)