@ApiResponse messages inheritance

this is not supported atm as the container annotation ApiRepsonses is overriden in full, and there are no short term plans to change this behaviour; however you can possibly achieve what you need, more as a side effect, by including your ApiResponse annotations of "base interface" (first and second enpoint) within `@Operation` annotation, and keeping controller as is; as responses within Operation are resolved first, and responses defined on method are then merged with these, this match your desired behaviour.

e.g.

public interface FirstEndpoint<C> {

    @POST
    @Path("/")
    @Operation(responses = {
            @ApiResponse(responseCode = "201", description = "Created"),
            @ApiResponse(responseCode = "400", description = "Bad Request"),
            @ApiResponse(responseCode = "403", description = "Forbidden"),
            @ApiResponse(responseCode = "404", description = "Not Found")})
    default String firstEndpoint(C param) {
        return "firstEndpoint-ok";
    }
}

public interface SecondEndpoint<C> {

    @GET
    @Path("/{id}")
    @Operation(responses = {
            @ApiResponse(responseCode = "200", description = "OK"),
            @ApiResponse(responseCode = "400", description = "Bad Request"),
            @ApiResponse(responseCode = "403", description = "Forbidden"),
            @ApiResponse(responseCode = "404", description = "Not Found")})
    default String secondEnpoint(C param) {
        return "secondEnpoint-ok";
    }