# Unlocking the Codex harness: how we built the App Server

DevFeed: [Unlocking the Codex harness: how we built the App Server](<https://devfeed.tech/articles/unlocking-the-codex-harness-how-we-built-the-app-server-6702.md>)

Original publisher: [Read original article](<https://openai.com/index/unlocking-the-codex-harness>)

Published: 2026-02-04T13:00:00Z

Content type: article

Language: en

Sources: [OpenAI News](<https://devfeed.tech/sources/openai-news.md>)

Topics: [codex](<https://devfeed.tech/topics/codex.md>), [API](<https://devfeed.tech/topics/api.md>), [Remote Procedure Call (RPC)](<https://devfeed.tech/topics/rpc.md>), [JSON](<https://devfeed.tech/topics/json.md>), [ide](<https://devfeed.tech/topics/ide.md>), [Command-line interface](<https://devfeed.tech/topics/cli.md>), [VS Code Extension](<https://devfeed.tech/topics/vscode-extension.md>), [vs-code](<https://devfeed.tech/topics/vs-code.md>), [Xcode](<https://devfeed.tech/topics/xcode.md>), [MCP](<https://devfeed.tech/topics/mcp.md>), [MCP Server](<https://devfeed.tech/topics/mcp-server.md>), [site-reliability-engineering](<https://devfeed.tech/topics/site-reliability-engineering.md>)

Tags: [api](<https://devfeed.tech/tags/api.md>), [cli](<https://devfeed.tech/tags/cli.md>), [codex](<https://devfeed.tech/tags/codex.md>), [engineering](<https://devfeed.tech/tags/engineering.md>), [ide](<https://devfeed.tech/tags/ide.md>), [json](<https://devfeed.tech/tags/json.md>), [mcp](<https://devfeed.tech/tags/mcp.md>), [openai](<https://devfeed.tech/tags/openai.md>), [rpc](<https://devfeed.tech/tags/rpc.md>), [software-development](<https://devfeed.tech/tags/software-development.md>), [vs-code](<https://devfeed.tech/tags/vs-code.md>), [xcode](<https://devfeed.tech/tags/xcode.md>)

## AI overview

OpenAI explains how the Codex App Server evolved into a bidirectional JSON-RPC API for embedding the shared Codex harness across products and partner integrations. The article covers its architecture, protocol, streaming progress, tool use, approvals, diffs, and integration with Codex surfaces such as the CLI, IDE extension, web app, and macOS app.

## Source excerpt

Learn how to embed the Codex agent using the Codex App Server, a bidirectional JSON-RPC API powering streaming progress, tool use, approvals, and diffs.