Codex

Codex combines GPT-4's reasoning capabilities with an autonomous execution loop. When you assign a task, it first analyzes your repository—understanding the file structure, dependencies, coding patterns, and existing conventions. It reads your README, package.json, existing tests, and other files that reveal how the project works. Then it plans an approach, breaking the task into concrete steps: which files to create, which to modify, what to test. It executes these steps in a sandboxed environment—a fresh cloud container with your repo cloned in. This isolation is crucial for security and reproducibility. It can run npm install, execute tests, spin up databases, and even start servers without affecting your local machine or other users. As it works, it monitors the results. If tests fail, it reads the error messages, traces the issue, and fixes the code. If a command fails, it adjusts and retries. This iterative loop continues until the task is complete or it genuinely gets stuck. The key insight is that Codex doesn't just generate code—it validates its own work by running it. When satisfied, it packages everything into a clean PR with descriptive commits. You see the final output: a branch with changes, a PR description explaining what was done, and passing CI (ideally). This is why good test coverage in your repo helps Codex produce better results—it has a way to verify correctness.