If you build an agent against OpenAI's Responses API then the simplest way to get output to the user is streaming over HTTP/SSE. If the user refreshes the page, loses connection, switches devices, or needs to approve a tool call, then there's nothing in the API to help you. AI Transport is Ably's session layer for agent-to-user conversations. An agent built on it gets resumable streams, multi-device sessions, and approval gates that wait for a human user, without deploying additional infrastructure.