This paper studies how the prevailing level of public debt shapes the transmission of fiscal and monetary policy shocks in a tractable heterogeneous three-agent New Keynesian model. When households rely on the liquidity services of government bonds to self-insure against idiosyncratic risk, higher public indebtedness amplifies the deterioration in debt sustainability after expansionary government spending shocks. In such economies, fiscal expansions weaken precautionary bond demand, requiring the central bank to keep real interest rates higher for longer and thereby raising debt servicing costs and narrowing fiscal space. […]