Worked on enhancing solver reliability in the pmgbergen/porepy repository by developing comprehensive testing features focused on nonlinear solver behavior. Implemented Python-based unit tests to ensure that, in the event of solver failures, the iterate reliably resets to the last known good state, thereby preventing NaN propagation during production runs. Incorporated assertions to monitor and validate the number of linear system assembly and solve calls, improving traceability and debugging clarity. Emphasized robust test coverage and long-term maintainability, utilizing numerical methods and pytest-style testing to strengthen solver state management and provide explicit commit traceability for future development and troubleshooting.