If you build on MCP servers you don't control, you've probably hit this. A tool's schema changes, your agent quietly starts failing, and nothing anywhere told you.
mcpi is a native MCP inspector that remembers what a server's contract used to look like. Connect to stdio or remote servers, browse their tools, resources and prompts, call them from forms generated off their JSON schemas, replay anything out of history. Then diff the server against any point in its past, with every change classified as breaking, compatible or cosmetic.
That classification is where the work went. Putting two JSON blobs side by side is easy, and two terminal windows already do it. Deciding whether an edit breaks your callers is harder, and the rules have to run in opposite directions for input and output schemas, because one is data you send and the other is data you get back.
Rust top to bottom. No Node, no Electron. The CLI drops into CI with no toolchain and exits 1 on a breaking change.
Connecting, browsing, calling and replaying stay free. €39 one-time unlocks diffing and collections, with a signed key verified offline: no activation server, no phone-home. mcpi.app is also a public directory of remote MCP servers, re-probed every six hours and free to browse.
— Hauke