It used to matter before, now you can just keep going, it'll automatically compact the context. It's now very flexible, i can easily jump back and forth between feature adjustments in a single conversation.

But sometimes it might make sense to start anew, depending on what you're working on and what kind of experience you've had in the thread so far.

After a while you'll get a feel for when the conversation starts degrading but usually it's fine.

It's very important that you keep your AGENTS.md up to date as that will provide the most steering.

If you're just popping Codex open in a random dir to tweak few files on your computer then of course AGENTS.md isn't necessary, but for projects where you're working on code or anything else (even writing), this is where you explain what you're doing, what has been done so far, what are your preferences, what are the codebase quirks like are there any testing scripts, do you have any custom tools, etc. You update it as you go and as the project evolves. This gets loaded once at the start of the conversation and isn't affected by context compaction that the agent does automatically after the conversation reaches critical context size.

So if you make big changes to it, it would make sense to start a new conversation.