Listing enables a CATALOG entry instead — the id from GET /v1/tools/catalog. The endpoint is the listing's own streamable-http remote, so a listing that only ships a stdio package is refused: there is nothing to reach yet.
URL is the server's JSON-RPC endpoint. It must be an http(s) URL naming a PUBLIC host: loopback, link-local, private and cloud-metadata addresses are refused here and again when the dialer connects.