No validation error for additional fields in request payload using goa

Question

current situation

for our service we have the current working implementation (same behaviour for all other endpoints):

var _ = goa.Service("serviceDemo", func() { 
    goa.Method("secondEndpoint", func() {
        goa.Payload(SecondEndpointRequestPayload)
        goa.Result(SecondEndpointResponsePayload)
        goa.HTTP(func() {
            goa.Headers(func() {
                goa.Header("first-header", goa.String)
                goa.Header("second-header", goa.String)
            })
            goa.POST("secondEndpoint")
            goa.Response(func() {
                goa.Code(goa.StatusOK)
                goa.ContentType(HTTPContentTypeApplicationJSONCharsetUTF8)
            })
        })
    })

    goa.Error(ErrorNameUnexpected, UnexpectedErrorResponsePayload)
    goa.Error(ErrorNameInvalidPayload, InvalidPayloadErrorResponsePayload)
    goa.Error(ErrorNameNotFound, NotFoundErrorResponsePayload)
    goa.Error(ErrorNameForbidden, ForbiddenErrorResponsePayload)
    goa.HTTP(func() {
        goa.Path("/v1/serviceDemo")
        goa.Response(ErrorNameUnexpected, func() {
            goa.Code(goa.StatusInternalServerError)
            goa.ContentType(HTTPContentTypeApplicationJSONCharsetUTF8)
        })
        goa.Response(ErrorNameInvalidPayload, func() {
            goa.Code(goa.StatusBadRequest)
            goa.ContentType(HTTPContentTypeApplicationJSONCharsetUTF8)
        })
        goa.Response(ErrorNameNotFound, func() {
            goa.Code(goa.StatusNotFound)
            goa.ContentType(HTTPContentTypeApplicationJSONCharsetUTF8)
        })
        goa.Response(ErrorNameForbidden, func() {
            goa.Code(goa.StatusForbidden)
            goa.ContentType(HTTPContentTypeApplicationJSONCharsetUTF8)
        })
    })
})
...
var SecondEndpointRequestPayload = goa.Type("SecondEndpointRequestPayload", func() {
    goa.TypeName("SecondEndpointRequestPayload")

    goa.Attribute("first-header", goa.String)
    goa.Attribute("second-header", goa.String)

    goa.Attribute("field1", goa.String)

    goa.Required(
        "field1",
    )
})

Expected

validation error when sending payload with extra field Example:

{
"field1": "value1",
"field2": "value2"
}

received:

200 statusCode and only field1 is passed to the service implementation

Note: i went through goa documentation i can not find how to add validation to forbid additionnal fields , as far as i understood they don't get mapped at all.