Agent product spinners tell the truth badly.
There’s this common pattern in agent UX: a person clicks a button and after a few moments a spinner shows up in the center of the page and the product locks up. The user has no idea what tool is running, what changes are happening in the system, what subagents or agents are involved. Later on the team can go back and read through a long transcript to figure out what happened. It’s better than nothing, but ultimately useless for a human doing real work with real deadlines.
Token streaming solves the model-call UX problem and makes the answer feel more alive as the model writes out the answer. But agent products have a wider problem: the work of an agent to produce output happens across tools, subagents, checkpoints in a workflow, patches of state, approval gates, background jobs, reconnectable sessions, and different applications that a person is staring at to see the results of the agent’s work.
This problem has been named by LangChain in its post "From Token Streams to Agent Streams". Generative UI cannot be a pretty wrapper around a transcript. The UI/runtime boundary needs a contract.
The stream stopped being a text pipe. It became the UI contract.
