An API generation system can be used to generate complete (or almost complete) APIs in response to received requests for new or modified APIs. Received API generation requests can include API descriptive data which is augmented to generate a set of data requirements defining one or more inputs and outputs for the API. The API functions can be mapped to one of a set of reference data models defining data types as used by the backend system, and the mappings used to automatically generate an API design defining input and output parameters for each API function of the requested API. The API generation system then assembles a code foundation for the requested API based on a set of software components implementing a portion of the API function and generates API source code based on the code foundation to complete the requested API.