Introducing Codex: OpenAI’s AI Coding Agent That Can Multitask Like a Pro
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On May 16, OpenAI unveiled Codex, a powerful new AI agent that’s set to redefine how developers approach software engineering. Codex can write new features, fix bugs, run tests, answer questions about your codebase, and even generate pull requests—all in parallel, and all in the cloud.
What is Codex?
Codex is OpenAI’s latest AI-driven, cloud-based coding assistant. It’s available in the sidebar of the ChatGPT web app and is powered by codex-1, a new variant of the o3 model, specially trained to handle real-world programming tasks.
Codex-1 goes beyond simple code completion—it's been fine-tuned with reinforcement learning so it can run tests iteratively until it gets the desired result. When tested against OpenAI’s SWE-bench and SWE-bench Verified benchmarks, it outperformed previous models in both accuracy and output quality.
How It Works
Codex operates within a secure cloud sandbox preloaded with your codebase. It understands the project structure and can execute testing tools, linters, and type checkers to complete tasks within 1 to 30 minutes, depending on complexity.
For even better results, developers can add an AGENTS.md file to guide Codex through their repositories—similar to a README, but for AI agents.
What’s even more impressive is Codex’s step-by-step transparency. Every action is documented with citations to logs and test outputs, offering a verifiable audit trail for every code change.
Key Use Cases
Codex isn’t just smart—it’s practical. Here’s how developers and teams are already using it:
Scaffolding and implementing new features
Refactoring and renaming code
Writing and executing unit/integration tests
Debugging and fixing failed builds
Generating documentation
Reviewing code and creating GitHub PRs
Answering questions about unfamiliar parts of the codebase
Whether you're a startup founder speeding up MVP development or a large engineering team streamlining your CI/CD pipeline, Codex can help offload repetitive and time-consuming tasks.
How to Use Codex
There are two main modes:
Code: To generate or refactor code
Ask: To get insights, answers, or suggestions about your codebase
To get started, simply load your repo in ChatGPT, open the Codex sidebar, and enter a prompt.
Codex vs. Codex CLI
OpenAI also introduced Codex CLI, a command-line version of the tool that runs locally (macOS and Linux only for now). Codex CLI is open-source and integrates OpenAI’s models directly into your terminal for fast, secure, and local execution.
Codex CLI now features a smaller, faster model variant (codex-mini-latest) and has simplified login via ChatGPT accounts. Plus and Pro users can claim $5–$50 in free API credits just by signing in.
Responsible Coding with AI
Despite its capabilities, OpenAI stresses that Codex is not a replacement for human developers. Manual review and validation of its output is still essential. Codex is a powerful assistant—not a fully autonomous developer.
Final Thoughts
Codex represents a significant leap forward in how AI can assist with software development. It's not just about writing code—it's about accelerating workflows, improving quality, and enabling teams to focus on what really matters: building great products.
If you're a ChatGPT Pro, Enterprise, or Team user, you already have early access. ChatGPT Plus and Edu customers will get access soon.
Explore Codex today and see how AI can transform your development process.
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