Chat is good at working out what you want. It's a poor place to watch work happen. Once an agent is triaging fifty issues, the transcript stops being a workspace and becomes a wall of text you have to re-read before you can trust any of it.

Canvas extensions in the GitHub Copilot app change that shape. A canvas is a live surface that opens beside the session: the agent updates it while it works, you edit that same surface, and it continues from your change. People act through UI controls, agents through callable actions, both against one shared state.

We'll take one apart: see what's inside, and how it actually runs.