Heidelberg AICurriculum

OpenAI Codex: The Complete Beginner's Tutorial (2026)

Leon van Zyl ·2026-07-02 ·2 min read

Summary written by us from the video's transcript. The video, and everything in it, is Leon van Zyl's work.

Learn how to set up OpenAI Codex, manage projects and sessions, add plugins and skills, and guide the agent through planning, design, and implementation of an app.

Takeaways

  • You must have a ChatGPT account and may need a paid plan to avoid strict usage limits.
  • Codex’s visual app lets you run multiple sessions per project, jump between projects, and control a built‑in browser for self‑testing.
  • Plugins and skills (e.g., image‑gen, grilme) extend Codex’s capabilities without code changes.
  • Plan mode allows safe planning; splitting plans into feature files prevents context overload.
  • The /goal command lets the agent autonomously implement and test all features until completion.

Getting Started with Codex

You need a ChatGPT account; the free tier is limited, so upgrade for meaningful coding. Download the Codex app from openai.com/codex for Windows or macOS (Linux uses only the CLI). Opening the app shows either existing projects or an empty workspace.

The sidebar lists all workspaces; you can run multiple sessions per project, something that requires manual juggling in the CLI.

Configuring the App

In Settings you can change the theme, select a pet avatar, and adjust keyboard shortcuts. The Usage & Billing tab shows five‑hour and weekly limits similar to other platforms; exceeding them pauses access until reset.

Under Integrations you can enable the built‑in browser (letting Codex test its own code), control Chrome or other apps, and manage MCP servers.

Adding Plugins and Skills

Plugins let Codex create files like Word docs, PDFs, or integrate with tools such as Canva or Figma. Skills are listed under the Skills tab; you can enable existing ones (e.g., image‑gen) or create new ones via a skill creator.

The tutorial installs the popular “grilme” skill from skills.sh, which forces the agent to ask many clarifying questions before generating an implementation plan.

Creating and Planning a Project

Start a new project (e.g., MyFitnessGuru); Codex creates a folder automatically. Choose approval mode, model (GPT 5.5), reasoning effort (extra high) and speed (standard or fast). Use Shift+Tab to enter Plan mode where the agent only suggests changes.

In Plan mode you can ask the agent to generate mock‑ups with image‑gen, add design concepts, and store plans in a /plans folder without executing them.

Structuring Work with Feature Files

Split a large implementation plan into individual feature files inside a subfolder; this keeps the context window low (13 % used) and avoids overwhelming the agent.

Each feature file contains clear instructions and acceptance criteria, making it easier for Codex to work incrementally.

Driving Implementation with Goals

Use the /goal command (or click the plus button → Goal) to tell Codex to implement all features in the folder until they are built and tested, using the integrated browser for testing.

The pet avatar shows real‑time status; you can still work on other tasks while Codex runs. Sessions can be archived from the workspace when finished.