Codex harness revealed: how the App Server was built

Codex is a coding agent that works across various environments, including the web app, CLI, IDE extensions, and the macOS app. All of these services are built on the same Codex harness (agent loop and logic), and the key element connecting them is the Codex App Server.

The Codex App Server is a client-friendly, bidirectional JSON-RPC API that helps various product lines deliver the same agent experience. It initially started as a way to reuse logic between the CLI and the VS Code extension, but has now evolved into a stable platform that partners such as JetBrains and Xcode can use to integrate agent capabilities.

The Codex harness provides the following core functions:

• Thread lifecycle and persistence: creates, resumes, branches, and archives threads while maintaining conversation history.
• Config and auth: handles configuration management and authentication flows such as 'Sign in with ChatGPT'.
• Tool execution and extensions: executes shell/file tools within a sandbox, and supports MCP server and skill integrations.

All of this agent logic is implemented in a library and runtime called Codex core.