A valid output can still break a consumer

A server upgrade can change its output schema from an integer amount to a formatted string. The new response is valid against the new schema, yet existing consumers expecting a number can fail.

Before: {"total": 25}
After:  {"total": "25.00 USD"}

A comparison needs three separate checks: whether the original input remains accepted, whether the new output satisfies the new schema, and whether that output still satisfies the consumer's prior contract or explicit assertions.

Try this in Callstead

Save the previous schema and recorded output. Add the new schema and new recorded output. An equals assertion on a stable field or a baseline output schema captures the expectation. The report identifies the specific case that failed without executing the service.

Recorded examples have limits. Ten examples passing cannot prove compatibility for every valid input. Schema changes remain review items even when every supplied case passes.

The MCP specification defines output schemas and structured results. Callstead adds a comparison with the prior consumer contract; this is a product check, not a requirement imposed by the protocol.