OpenAI’s DevDay last week was mostly pitched as an agents event. Persistent agents, cloud Codex, a cheaper model. Fine. But if you’re learning to build and design APIs, two of the smaller releases are worth more of your attention than the headline model, because they say a lot about where API contracts are going.

Here’s what shipped, then what to take from it.

What OpenAI released

The Agents API is now in public beta. It runs agents on OpenAI’s own infrastructure, with memory, tool calling, tool search, context compaction and multi-agent support carried over from Codex. The new piece is computer use: an agent can now drive software through its graphical interface, clicking and typing the way a person would.

Every few weeks someone asks whether MCP replaces REST. It does not. The question is still worth answering carefully, because the line between the two moved in July, and most of the explanations written before then are now describing a protocol that no longer exists.

An HTTP API is a contract between two pieces of software written by people who read documentation. A developer opens the reference, learns that POST /v1/invoices wants a customer ID and an array of line items, writes the call, and ships it. The knowledge lived in a human head at build time and then got frozen into source code. When the endpoint changes, something breaks in production and a person goes and fixes it.