While prompts allow users to control Codex by describing tasks and expected results, they are sometimes insufficient for deep customization. To run custom logic at specific stages of a session, developers can utilize Codex hooks. These hooks attach user-defined scripts to events emitted during the agentic loop's lifecycle.

The agentic loop emits events such as Session Start, Pre Tool Use, Post Tool Use, Stop, and Session End. A hook configuration requires three components: an event (lifecycle point), a matcher (conditions for execution), and a handler (the action). For instance, a Pre Tool Use hook can inspect commands before they run, filtering for specific actions like shell executions.

A case study demonstrates adding a quality gate to a deep research workflow. Codex is tasked with identifying trends from a specified date window and returning a structured brief. A hook verifies that the final output contains sufficient sources from varied domains. If the check fails, the hook returns errors to Codex, forcing it to continue researching within the same run until the criteria are met.