OpenAI has recently published a detailed architecture description of the Codex App Server, a bidirectional protocol that decouples the Codex coding agent's core logic from its various client surfaces.

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OpenAI published the architecture of Codex App Server, a bidirectional protocol that unifies all Codex coding agent experiences (CLI, VS Code, web, desktop, JetBrains, Xcode) through a single API. The protocol uses three conversation primitives (Item, Turn, Thread) to model agent interactions beyond simple request/response patterns, communicates via JSON-RPC over stdio, and supports server-initiated requests for approval flows. OpenAI tried and rejected the Model Context Protocol (MCP) for this use case, finding it insufficient for rich IDE interactions. The architecture parallels industry efforts like the Agent Client Protocol (ACP), which aims to standardize agent-editor communication similar to how Language Server Protocol standardized language tooling.

OpenAI Publishes Codex App Server Architecture for Unifying AI Agent Surfaces