This BIS Working Paper examines the interaction between monetary and fiscal policy in a heterogeneous-agent New Keynesian model with a fiscal block. The authors find that the stock of public debt affects the natural interest rate, which in turn requires the central bank to adjust its monetary policy rule to maintain inflation at its target. However, there is a minimum level of debt below which the steady-state inflation deviates from its target due to the zero lower bound on nominal rates. The paper provides insights into the challenges faced by policymakers in navigating monetary-fiscal interactions and maintaining inflation stability in the face of fiscal shocks.