ACP almost certainly stands for the Agent Client Protocol — an open protocol (originally from Zed) that lets code editors and IDEs communicate with external AI coding agents like Claude Code.

So the exchange means: the feature (a model selector or a hint) is available when you run the agent through the ACP integration (e.g., inside Zed or another editor that connects via Agent Client Protocol), but it's not available in the web interface.

In other words: "You can get it through the editor/agent-protocol integration — the web client just doesn't have it yet."