Codex is no longer just a model that answers requests: it's an engine that lives behind several surfaces — the web, the CLI, IDE extensions and the new macOS app — and now it has a stable, integration-friendly gateway: the Codex App Server.

Interested in adding an agent that reviews code, runs tests, or acts as an SRE inside your product? Here I explain, step by step and without unnecessary technical jargon, how this layer works and why it's worth using.

What is the Codex App Server and why it matters

The App Server is two things at once: a bidirectional protocol based on JSON-RPC and a long-running process that hosts the Codex engine sessions (the "harness"). That lets different clients — from VS Code to a web app or a CLI — talk to the same core without duplicating logic.

Why does that matter? Because a single user request can trigger a sequence of steps: explore the workspace, run tools, ask for approvals, emit diffs and return a final message. The App Server turns all that into simple, stable events a UI can render in real time.