What counts as breaking
Additive OK; removals, type narrows, required adds fail
Additive OK
New optional fields, new endpoints, and new enum values do not fail the Check.
Removals
Removing a path, operation, or property is breaking.
Type narrows
Narrowing a type (string → integer, looser → stricter format) fails the Check.
Required adds fail
Adding a required property or required request field fails the Check. Existing clients did not send it.