Codex App Server

Overview

The Codex App Server is the long-lived protocol boundary between Codex core and its client surfaces. It matters because OpenAI is no longer describing Codex as merely a terminal tool; the App Server is what lets the same runtime power the CLI, IDE, web, and app experiences without each client inventing its own harness.

Core protocol model

The central decomposition is threads, turns, and items. That sounds dry, but it is the real architectural move: sessions become durable containers with explicit work units, resumable history, and UI-visible lifecycle events rather than one giant transcript blob. See codex-cli, agent-harness-anatomy, and context-engineering.

Reviews and approvals

The App Server surface includes approval requests and review flows, which means permission handling and code review are first-class protocol concerns. This places Codex closer to a controllable operating substrate than to a thin text-generation shell. Read with evaluation-and-review-loops and safety-and-permissions.

Why it matters

The App Server is the cleanest statement of OpenAI’s harness philosophy in this corpus: separate the agent core from the client surfaces, make session state durable, and express operational control through a stable interface. It is one of the main reasons codex-cli is useful as a reference architecture rather than only as a product.

Relationships

This page belongs with codex-cli, harness-engineering, harness-architecture-comparison, and new-harness-design-notes.