The article describes it.

Both Codex and Claude Code have it, but they work slightly differently.

Claude Code uses Haiku to read through the transcript and decide if the goal has been completed. If not, Haiku injects a prompt back to the main model to indicate what still needs to be done.

In Codex, instead it's a tool available to the main model, plus some part of the surrounding harness that will re-prompt it if the tool calls haven't yet indicated that the goal is complete.

The issue that they are trying to solve is that sometimes models will stop before they have actually fully completed whatever task they were given; attention isn't perfect, and someitmes they'll complete part of it but not the whole task. Rather than making the user come back and re-prompt to keep going, they add a way to automatically do a bit more nudging to try to get the model to finish the task.