"format" is an annotation

Author: handrews

versions/3.1.0.md

@@ -153,6 +153,8 @@ However, to support documentation needs, the `format` property is an open `strin
 Additional formats MAY be used even though undefined by either JSON Schema or this specification.
 Types that are not accompanied by a `format` property follow the type definition in the JSON Schema. Tools that do not recognize a specific `format` MAY default back to the `type` alone, as if the `format` is not specified.
 
+Note that by default, JSON Schema validators MUST NOT attempt to validate the `format` keyword.  It is primarily treated as an annotation that informs applications of the intended nature of the data.  For historical reasons, it is possible to configure JSON Schema validators to validate "format", but this is not consistently implemented and SHOULD NOT be relied upon in any environment where an unknown validator or unknown validator configuration might be used.
+
 The formats defined by the OAS are:
 
 [`type`](#dataTypes) | [`format`](#dataTypeFormat) | Comments