How do you implement Custom Schema Directives?

Custom directives allow you to augment schema definitions with custom behaviors, modifying execution or schema generation at startup.

You define the directive in SDL (e.g., directive @upper on FIELD_DEFINITION). Using tools like @graphql-tools/schema, you implement a SchemaDirectiveVisitor (or mapSchema in modern versions) that intercepts field resolvers and wraps them with new logic, such as formatting data or enforcing auth.