LongRunMIP is an ongoing model intercomparison to study century and millennia time scales of atmosphere-ocean general circulation model (AOGCM) equilibration and the linearity assumptions around feedback analysis. As an ensemble of opportunity, there is no previously agreed upon protocol and the only condition to participate is a coupled model simulation of any stabilizing scenario simulating more than 1,000 years. LongRunMIP currently contains fifty simulations of fifteen different models originating from ten modelling centers, each between 1,000 and 6,000 years long.