OpenAI Agents API Public Beta in September 2026: A Practical Guide to the Managed Codex Harness
OpenAI Agents API Public Beta in September 2026: A Practical Guide to the Managed Codex Harness OpenAI’s Agents API public beta is aimed at teams that have moved beyond a single model call but do not want to maintain every part of an agent runtime themselves. The managed Codex harness provides durable sessions, sandbox execution, MCP tool access, delegated subagents, context compaction, and streamed events behind an API surface. The important distinction is architectural: you still define the agent’s instructions, tools, permissions, and business logic, but OpenAI manages much of the loop that decides when to call a tool, continue reasoning, compact history, or hand work to another agent. Image: mikemacmarketing / photo on flickr via Wikimedia Commons (CC BY 2.0) Image: Midjourney; prompt suggested by Grok via Wikimedia Commons (Public domain) What the managed harness actually provides A conventional agent is often a loop around a model: Send the convers...