MCP sign-in stops early with a clear message when the server URL is bad

MCP servers are outside tools and data sources that Claude Code can connect to, and some ask you to sign in with OAuth, a standard sign-in flow. Before starting that sign-in, Claude Code now checks that the server's URL is valid. If it is missing or not a valid URL, sign-in does not start. You see the server's own configuration error if it has one, or otherwise a message saying the URL is missing or invalid and that you should fix it in the server's MCP config, or set the environment variable it uses, and try again. Before, sign-in went ahead with the bad URL.