tech
February 4, 2026
Unlocking the Codex harness: how we built the App Server
OpenAI’s coding agent Codex exists across many different surfaces: the web app(opens in a new window), the CLI(opens in a new window), the IDE extension(opens in a new window), and the new Codex macOS app. Under the hood, they’re all powered by the same Codex harness—the agent loop and logic that underlies all Codex experiences. The critical link between them? The Codex App Server(opens in a new window), a client-friendly, bidirectional JSON-RPC1 API.
TL;DR
- OpenAI's Codex coding agent is accessible via web app, CLI, IDE extension, and a macOS app.
- All Codex experiences are powered by the same Codex harness (agent loop and logic).
- The Codex App Server serves as the critical link between these different surfaces.
- The Codex App Server exposes a client-friendly, bidirectional JSON-RPC API.
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