OpenAI's third DevDay: AgentKit, Apps SDK and Codex general availability

OpenAI’s third annual DevDay, held at Fort Mason in San Francisco, centred on turning ChatGPT into a platform other companies could build on, rather than only a product OpenAI shipped itself. The headline release was the Apps SDK, which let third-party developers build applications that run inside ChatGPT’s chat interface, discoverable and usable without leaving the conversation. The SDK was built on the Model Context Protocol, the open standard for connecting models to external tools and data that Anthropic had introduced roughly a year earlier — OpenAI’s adoption of a rival lab’s protocol was itself notable, treating MCP as a shared standard rather than building a competing one.

Alongside it, OpenAI launched AgentKit, a bundle of tools for building, deploying and evaluating AI agents: Agent Builder, a visual drag-and-drop canvas for designing multi-agent workflows; ChatKit, an embeddable, brandable chat interface; a Connector Registry for giving agents managed access to internal and third-party systems; and an evaluations tool for grading agent traces and optimising prompts. OpenAI said Codex, its coding agent, had built roughly 80% of Agent Builder itself in under six weeks — offered as a demonstration of the agent’s own capability as much as a product statistic.

Codex reached general availability at the event, moving from preview to a production-supported tool, with a new Slack integration and a software development kit for embedding it into other workflows. OpenAI also introduced GPT-5 Pro and GPT-5-Codex to the API, and made Sora 2 available there as well.

During the keynote, Altman disclosed that ChatGPT had reached more than 800 million weekly active users, a figure that framed the day’s developer announcements as reaching an unusually large existing audience. Collectively, the releases marked OpenAI’s clearest move yet toward positioning ChatGPT as an operating layer for other companies’ software, competing directly with the app-store and platform ambitions of Google, Microsoft and Anthropic’s own emerging agent ecosystem.