One limit for two tools
OpenAI says Plus, Pro, and Business plans will share one usage allowance across ChatGPT and Codex starting in December. In other words, using one tool can leave less available for the other until the limit resets. Instant chat will not count toward this shared allowance. [1]
Usage will be measured in tokens: the small pieces of text the AI processes or produces. More advanced models and more complex work can use more. OpenAI has not given an exact December start date or numerical allowance on the help page we checked November 1. [1]
Where to see what you have left
OpenAI says Settings → Usage will show your combined remaining balance and reset times. Its Business pricing page also confirms the December change. We have not tested the future limits. [1][2]
Our take
If you use ChatGPT for everyday writing and Codex for coding, pay attention to what happens after a busy coding session once the change arrives. Watch a normal week's use before paying for a different plan. The useful question is whether you can finish your work without an inconvenient wait.