An application programming interface compatibility layer maps requests, responses and errors from one protocol to another. For model APIs it may convert message roles, reasoning blocks, tool definitions, tool results and token-usage fields while preserving the client's expected workflow.
Reliable compatibility requires semantic translation, not only renaming fields. Providers can interpret tools, streaming, stop conditions and context differently, so unsupported features must fail clearly instead of appearing to work while changing behavior.
ELI5
An application programming interface compatibility layer translates between two different API formats so a client built for one interface can use another service. It changes the shape of requests and replies while trying to preserve their meaning.
For example, a model client may send tool definitions in one format while another provider expects different fields. The compatibility layer maps them and translates the result back. If a feature has no true equivalent, it should report that clearly instead of pretending the behavior is the same.
