The clients for mcp can drop a url and http in the mcp.json and get access to the application. Can the client do that for every rest api?

Ya, if you just use OpenAPI. That's why I'm saying MCP adds nothing. It's just another standard for documenting APIs. There are many that have been around for a long time and that are better integrated with existing ecosystems. There's also gRPC reflection. I'm sure there are others. LLMs can use them all equally effectively.