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The idea of using a schema document together with the custom_features text seems to be very powerful. For a demostration of how such a schema would work, please visit any of the JSON schema validators available online, for example, https://json-schema-validator.herokuapp.com/. Just copy and paste over there the above JSON schema and the value of the custom_feature field and check on the validation results:
TOSCA 1.2 allows using a JSON schema as a TOSCA constraint in parameter definition and node_filter clauses.
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