MODULE 3.1
AGENTS, Skills and memory
Create project instructions and a reusable capability with a review criterion.
What it is
AGENTS.md is the instruction file that Codex discovers in the applicable scope. It describes how to work: initial files, verification commands, limits and delivery format. No need to explain the whole organization history; prefer short rules that change a real decision.
Why learn
Objective instructions avoid repeating the same details in each conversation. The file should help the agent choose a concrete action, such as verifying the report before considering it finished.
Key concepts
Operational instruction; scope; observable rule; conciseness.
What it is
By default, ~/.codex/AGENTS.md stores global instructions. In the project, AGENTS.md adds specific rules; files in closer folders can prevail in the corresponding scope. AGENTS.override.md has priority over AGENTS.md at the same level. This does not override system instructions, permissions or the explicit request of the session.
Why learn
Path matters. A local rule may not apply to another folder, and a forgotten override file can explain unexpected behavior. Keep the global small and leave local details in the project.
Key concepts
Discovery; hierarchy; directory scope; override.
What it is
A Skill gathers instructions for a recurring activity, with name and description at the beginning of SKILL.md. It may include supporting resources and programs. Personal skills reside in ~/.agents/skills; project skills can be in .agents/skills within the repository. The ~/.codex folder remains Codex configuration.
Why learn
A rule says what to respect; a Skill teaches a procedure that can be triggered when needed. Separating these roles avoids loading the entire manual into every task.
Key concepts
Name; trigger description; procedure; input and output; validation.
What it is
The course's operational memory is a set of queryable files, not a change to the model weights. It works when the agent reads the relevant information and when someone keeps that information up to date. Store stable facts, decisions and causes of failures; do not preserve every sentence of every conversation.
Why learn
Copying the entire history increases volume and can reintroduce old instructions. A small, dated and reviewed memory helps more than a massive file that no one can validate.
Key concepts
External memory; explicit query; summary; validity; source.
What it is
Record the symptom, the observed cause, the minimal fix and how to verify. Differentiate failure of request, such as ambiguous objective, from infrastructure failure, such as a terminated process. The record only generates operational learning when it changes the next procedure.
Why learn
Redoing the whole project can mask a simple problem. A small protection, like checking for a file's existence before reading, is usually easier to test and maintain.
Key concepts
Symptom is not cause; minimal correction; prevention; evidence.
What it is
Test the Skill with a normal input, another incomplete one, and one out of scope. Observe whether the result preserves data, signals uncertainty, and stops when it should. The test should measure behavior, not just look for the Skill name in the response.
Why learn
A procedure that works once may be relying on accidental context. Varying inputs helps discover what needs to be made explicit in the instructions.
Key concepts
Normal case; incomplete case; scope limit; acceptance criterion.