Consider this rest api call.

app.MapGet("/getorder/{id}", 
    async (Guid id, [FromServices] TTService service) => await service.GetOrder(fileid, ct));

GetOrder has this interface, returning an Order object.

Order GetOrder(Guid id)

Then we have a client that calls the API

Order GetOrder(Guid id)
{
  return await _client.Request("getorder", id).GetJsonAsync<Order>();
}

So we can simply call

Order o = client.GetOrder(new Guid("..."));

A unit test that tests that client around that.

[Theory]
[MemberData(nameof(GetOrderSearchData))]
public void Test_OrderSearch_OrderFound(string[] inputs)
{
    var order = new OrderFaker().UseSeed(1).Generate();

    _api.GetOrder(order.ID).Returns(order);
    _page.SetOrderSearchValue(input);
    _page.OrderInfo.OrderNumber.Should().Be(order.OrderNumber);
}

So far so good.

Now somewhere down the line the service interface was changed that it did not return a single Order anymore but an IEnumerable.

IEnumerable<Order> GetOrder(Guid id)

The service was updated but not the client, that still expected a single Order.

Code committed, selenium tests succeeded and deployed.
But in testing the call failed with could not serialize json error as the Order collection could not be serialized to a single Order of course.

So how can we write unit tests that check that the interface of a service matches that of a client?