Today I learned that in the latest GraphQL specification (October 2021 Edition), when defining a custom scalar type, the GraphQL service should provide a URL of a human-readable specification. It can be provided via either the new built-in @specifiedBy directive or the specifiedByURL introspection field. The @specifiedBy directive takes the URL as its argument like this:

scalar DateTime @specifiedBy(url: "https://tools.ietf.org/html/rfc3339")

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