Originally published on AI Tech Connect.
What "correct" means when the caller is a model You are testing two contracts, not one. The protocol contract — does the server speak MCP properly, validate what it claims to validate, return the errors it advertises. And the model-facing contract — does an agent doing a realistic task make the right calls in the right order, and stop when it should. The tool description is a prompt. It is the text a model reads when deciding whether to call your tool. Change it and you have changed behaviour, whether or not any implementation moved — so it belongs in the fingerprint you snapshot, alongside the schema. An error is an instruction. A structured error with a stable code and a next step teaches an agent to recover. A stack trace or a polite paragraph of prose teaches it to retry until the…
