Free ammonia (FA) is a major inhibitor in anaerobic digestion (AD), limiting the potential of high-rate operation, particularly when digesting nitrogen-rich feedstocks. Many full-scale AD facilities operate conservatively at low organic loading rates (OLRs) due to the risk of FA inhibition, undermining energy recovery and process efficiency especially in systems treating thermally pre-treated feedstocks. This study explores the potential of osmoprotectant (OP) supplementation to mitigate FA inhibition and enable more resilient methane production. Batch experiments were conducted using conventional AD sludge, THP sludge, and food waste under elevated FA concentrations ranging from 44 to 1200 mg/L (pH 7.5, 40 °C). At baseline, FA concentrations were 44 ± 2.9 mg/L for conventional sludge, 126.5 ± 6.2 mg/L for THP sludge, and 242.9 ± 6.2 mg/L for food waste. Results showed that methane production declined with increasing FA in all systems. However, the extent of inhibition varied significantly. Conventional sludge systems were strongly inhibited at FA 250 mg/L, showing substantial VFA accumulation, while THP and food waste systems showed delayed inhibition and maintained methane yields at FA >250 mg/L. Correspondingly, a delayed onset of VFA accumulation was recorded and was more dominant at FA ≥700 mg/l. Notably, addition of 60 mg/l of OP (MgCl2) alleviated the inhibitory effects of FA, particularly in THP sludge and food waste systems, enhancing methane yield and moderating VFA build-up at intermediate FA concentrations. THP sludge exhibited complete recovery at 250 mg/L and 77% recovery at 400 mg/L, while food waste achieved 90% recovery at 400 mg/L. These findings show that AD performance under stress is controlled by the physicochemical matrix of the digesting broth rather than the feedstock type. This insight reframes how stability is managed in high-rate ADs and offers a path toward unlocking the full capacity of systems limited by ammonia toxicity.