Formally define how to transport A2UI over MCP #558
Description
We want to make it clear how to use A2UI over MCP in a standardized way.
- How are multiple messages aggregated (see make a list of a2ui messages a top level construct #549)?
- What MCP concept do we use to store A2UI? Resources etc?
- Can UIs be updated after they are defined?
- How does catalog negotiation work?