Theres transport and context management. A cross agent status cache with semaphore over a testing harness driving a browser that can rewind and retry is much easier for agents to drive from mcp than selenium or whatever is fashionable these days. Didn't replace unit testing per se but to rca and fix it's a much better tool.
Exactly! Swagger descriptions+GUI[1], HATEOAS[2], and JSONAPI[3] all coming together in jubilous harmony if MCP pulled its head out of the sand, but instead we get the XKCD#927[4] situation.