Abstract: Methods for automatically generating schemata of input and output data of application programming interfaces are described. Providers of an API make requests to an API. A computer program collects the data of the requests and responses from the API. Another computer program extracts schemata of the data. JSON schema, XSD and YAML schema are used in extracting schemata of JSON-, XML- and YAML-formatted data respectively. If extracted schemata from a series of data is different element by element, a schema is created in such a way that its validation results for all elements are true.