The heavy GPT thinking model is automatically routed. I can’t use the GPT-5.5 ‘Thinking Heavy’ model. Make sure to check if you also have this bug. Repro seems specific to ChatGPT web on desktop when saved memory or project memory/context is enabled. The same account/model works normally on mobile and in private/no-memory projects, but desktop web appears to ignore or downgrade the selected Heavy/Extra High reasoning effort.