OpenAI exposes the Codex harness as the Agents API

Sources: primary, discussion
Summary: OpenAI documented an Agents API that offers the harness behind Codex as a service OpenAI runs, reached through a beta client surface at client.beta.agents.sessions. The documented surface covers durable agent sessions, sandboxes, MCP connections, context compaction and subagents. The documentation states usage bills at standard model, tool and container rates.
Why it matters: Teams building agents no longer own the harness they were writing themselves, and the state that harness holds moves to a service OpenAI runs and meters.