OpenAI details Codex App Server for agent-based developer tools
JSON-RPC platform brings Codex harness to CLI, IDEs and web apps with open-source support
OpenAI has released technical details of its Codex App Server, a JSON-RPC platform designed to expose the Codex harness to developer-facing tools including command line interfaces, integrated development environments and desktop applications.
In a blog post, the company said the server evolved from early infrastructure used by the terminal interface and Visual Studio Code extension, and has since been reworked into a stable, backwards-compatible runtime to support growing internal and partner adoption.
The App Server functions as both a long-running process and a protocol for bidirectional messaging. It manages Codex threads, sessions, and messaging via a structure of items and turns, with each thread hosting a single Codex session. Clients initiate a connection through a JSON-RPC handshake over standard input and output, and communication follows a model of started, delta and completed events. Turns are paused for approvals and resumed based on allow or deny responses from the client.
OpenAI said bindings are available in multiple programming languages including Go, Python, TypeScript, Swift and Kotlin. Developers can generate TypeScript interfaces using the codex app-server generate-ts command or output JSON Schema bundles using generate-json-schema.
Web applications typically run the App Server in container workers, while local tools pin known platform binaries. A planned refactor of the Codex terminal UI will align it with other clients by launching the App Server and speaking JSON-RPC natively.
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The post also outlined alternative methods for embedding Codex capabilities, including through MCP servers, Codex Exec and the Codex software development kit.
All source code for the App Server is included in the open-source Codex CLI repository. OpenAI credited contributors to the project in the blog.
The Recap
- OpenAI released the Codex App Server exposing the harness.
- The protocol uses bidirectional JSON‑RPC over stdio channels.
- Source code and integrations are available in the Codex CLI repo.
