I think the fact that MCP is two ways, and tool call is asynchronous (since it's over network), means that it should be possible to code an approval flow with a prompt and a slack approval tool?

Yeah it still takes a bit of work, but it's more extendable (can swap slack to email) and more versatile (human can message agent to interrupt or ask for clarification).

I haven't used langchain (beyond RAG) or crewai, not sure what was capable before.