“You've hit the Free plan limit for GPT-5. Responses will use another model until your limit resets.”

What it means

Free users get a small number of flagship messages per window; ChatGPT then silently falls back to a smaller model.

When it resets

Rolling 5-hour window.

What to do now

1. Wait for the reset time in the banner.
2. Upgrade if you need the flagship model consistently.