What is an SOAP API?

SOAP predates REST as a standard for web services and remains common in enterprise, banking, healthcare (HL7-adjacent systems), and government integrations where formal contracts and built-in extensions matter. Every SOAP message is wrapped in an envelope containing a header (metadata like authentication tokens or transaction IDs) and a body (the actual payload), and the WSDL file acts as a machine-readable contract specifying exactly which operations exist, their inputs, and their outputs — letting client code be generated automatically from the spec. Compared to REST/JSON APIs, SOAP is more verbose and rigid but offers stronger built-in standards for security (WS-Security), transactions, and reliable messaging, which is why many legacy enterprise systems never migrated off it. Integrating with a SOAP API generally means parsing the WSDL, constructing XML envelopes correctly, and handling SOAP faults (its structured error format) rather than simple HTTP status codes.