The paper significantly updates the open-source software package Modules for Experiments in Stellar Astrophysics (MESA) and its one-dimensional stellar evolution module, MESA star. Key improvements include: 1. **Giant Planets and Low-Mass Stars**: MESA now models giant planets down to masses as low as one-tenth that of Jupiter, and low-mass stars up to 3–8 M⊙, including the evolution through core He burning, onset of He thermal pulses, and arrival on the white dwarf cooling sequence. 2. **Asteroseismology**: The ADIPLS adiabatic pulsation code is fully coupled with MESA star, enabling more accurate and efficient calculations of stellar mode frequencies and eigenfunctions. This is particularly useful for space-based missions like Kepler and CoRoT, which have provided extensive asteroseismic data. 3. **Rotation and Magnetic Fields**: MESA now incorporates stellar rotation and magnetic fields in radiative regions, allowing for the exploration of rotational effects on stellar evolution and magnetic field dynamics. 4. **Massive Stars**: New capabilities for evolving rotating massive stars to core collapse are introduced, enabling the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. 5. **Physics Modules**: Updates to input physics modules, including equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions, are provided. 6. **Software Development Kit (SDK)**: The MESA SDK is described, providing a unified, maintained, and well-validated build environment for MESA. 7. **Community Tools**: Several community-developed tools for rapid visualization of MESA star results are highlighted. The paper also includes detailed sections on the implementation of these features, along with comparisons and tests to demonstrate their effectiveness.The paper significantly updates the open-source software package Modules for Experiments in Stellar Astrophysics (MESA) and its one-dimensional stellar evolution module, MESA star. Key improvements include: 1. **Giant Planets and Low-Mass Stars**: MESA now models giant planets down to masses as low as one-tenth that of Jupiter, and low-mass stars up to 3–8 M⊙, including the evolution through core He burning, onset of He thermal pulses, and arrival on the white dwarf cooling sequence. 2. **Asteroseismology**: The ADIPLS adiabatic pulsation code is fully coupled with MESA star, enabling more accurate and efficient calculations of stellar mode frequencies and eigenfunctions. This is particularly useful for space-based missions like Kepler and CoRoT, which have provided extensive asteroseismic data. 3. **Rotation and Magnetic Fields**: MESA now incorporates stellar rotation and magnetic fields in radiative regions, allowing for the exploration of rotational effects on stellar evolution and magnetic field dynamics. 4. **Massive Stars**: New capabilities for evolving rotating massive stars to core collapse are introduced, enabling the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. 5. **Physics Modules**: Updates to input physics modules, including equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions, are provided. 6. **Software Development Kit (SDK)**: The MESA SDK is described, providing a unified, maintained, and well-validated build environment for MESA. 7. **Community Tools**: Several community-developed tools for rapid visualization of MESA star results are highlighted. The paper also includes detailed sections on the implementation of these features, along with comparisons and tests to demonstrate their effectiveness.