This one cost me a real mistake before I understood what was happening.
Models have a context window, a limit on how much of the conversation they can see. Here is the part nobody tells you: when a chat outgrows it, nothing fails. No error, no warning. The oldest messages just fall out of view, and ChatGPT keeps answering confidently based on whatever still fits. It does not know what it forgot, so it cannot tell you.
The symptoms, in the order I usually hit them:
It re-asks something you answered near the start. It contradicts a decision you made early on, a name, a budget, a constraint, and defends the new version. You ask for a summary and the first third of the conversation is just gone from it. If any of this feels familiar from your long chats, this is what was happening.
The mistake it cost me: a pricing decision made in hour one of a planning chat, silently dropped in hour three, and the final plan it wrote used different numbers. I did not catch it until someone else did.
The free fix takes two minutes. When a chat gets long, send this:
Summarize everything we decided in this conversation as a briefing for a new chat that has no memory of it. Include the goal, every decision made, the constraints, anything still open, and the exact wording of any final text we agreed on.
Open a fresh chat, paste the briefing, continue there. You lose nothing that matters and the model is sharp again. Do it when a chat starts feeling long, not when the symptoms show, because by then early details are already gone.
For knowing when: I use a small gauge that shows how full the conversation's window is, part of AI Toolbox, a browser extension I built, and honestly that gauge exists because of the pricing incident above. But the briefing habit is the actual fix and it needs nothing installed.
Does anyone else hit this? The people I talk to either recognized every symptom instantly or had no idea this was a thing, with nothing in between, and I am starting to think the difference is just who runs week-long chats and who does not.