diff --git a/EXERCISE.md b/EXERCISE.md
index 1889365..19ba25a 100644
--- a/EXERCISE.md
+++ b/EXERCISE.md
@@ -22,6 +22,11 @@ explicit — and that is where this exercise lives.
   percentage *actually applied* (0 when the total does not strictly exceed the
   barrier), and the discounted total. Rounding is half-up, in integer math.
 - Unit tests for all of the above already pass.
+- `ValidOrder()` / `ValidClient()` test data builders in the test project
+  (`tests/BeforeAfter.Tests/TestData/`) — construct domain objects in tests
+  through them, so tests survive domain evolution. Import their static
+  factories with `using static BeforeAfter.Tests.TestData.OrderBuilder;`
+  (and `...ClientBuilder;`).
 
 ## The challenge
 
@@ -61,6 +66,8 @@ instead of assuming there is only one order around.
 
 ```csharp
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -74,9 +81,8 @@ public class DiscountedOrderMappingTests
     public void A_discounted_order_can_be_correlated_onto_its_order()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
@@ -98,9 +104,8 @@ public class DiscountedOrderMappingTests
     public void Saving_a_discounted_order_keeps_the_stored_total_untouched()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "big order", TotalInCents = 150_00 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("big order").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
diff --git a/tests/BeforeAfter.Tests/AfterPersistenceTests.cs b/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
index eb859a3..08db56b 100644
--- a/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
+++ b/tests/BeforeAfter.Tests/AfterPersistenceTests.cs
@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -15,9 +17,8 @@ public class AfterPersistenceTests
     public void Save_maps_domain_state_into_data_records()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one", TotalInCents = 15000 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
 
         db.Save(client);
         db.Save(order);
@@ -29,7 +30,7 @@ public class AfterPersistenceTests
         var orderRow = Assert.Single(db.Orders);
         Assert.Equal(order.Id, orderRow.Id);
         Assert.Equal("one", orderRow.Description);
-        Assert.Equal(15000, orderRow.TotalInCents);
+        Assert.Equal(150_00, orderRow.TotalInCents);
         Assert.Equal(client.Id, orderRow.ClientId);   // the graph is flattened to a key
     }
 
@@ -37,7 +38,7 @@ public class AfterPersistenceTests
     public void An_order_without_client_stores_an_empty_client_id()
     {
         var db = new DbContext();
-        var order = new Order { Description = "orphan" };
+        var order = ValidOrder().WithDescription("orphan").Build();
 
         db.Save(order);
 
@@ -48,14 +49,13 @@ public class AfterPersistenceTests
     public void Saving_again_refreshes_the_row_instead_of_duplicating_it()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
         order.Description = "one (edited)";
-        order.TotalInCents = 9900;
+        order.TotalInCents = 99_00;
         client.Name = "Acme GmbH";
         db.Save(order);
         db.Save(client);
@@ -64,7 +64,7 @@ public class AfterPersistenceTests
         var orderRow = Assert.Single(db.Orders);
         Assert.Equal(order.Id, orderRow.Id);
         Assert.Equal("one (edited)", orderRow.Description);
-        Assert.Equal(9900, orderRow.TotalInCents);
+        Assert.Equal(99_00, orderRow.TotalInCents);
         Assert.Equal(client.Id, orderRow.ClientId);
 
         var clientRow = Assert.Single(db.Clients);
@@ -75,9 +75,8 @@ public class AfterPersistenceTests
     public void Rows_are_reachable_through_Set_and_Find()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
@@ -98,9 +97,8 @@ public class AfterPersistenceTests
     public void Loading_an_order_restores_its_state_and_its_client()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one", TotalInCents = 150_00 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
         db.Save(client);
         db.Save(order);
 
@@ -122,12 +120,12 @@ public class AfterPersistenceTests
     public void Loading_a_client_restores_it_with_all_of_its_orders()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        client.Orders.Add(new Order { Description = "one", TotalInCents = 150_00 });
-        client.Orders.Add(new Order { Description = "two", TotalInCents = 250_00 });
+        var first = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
+        var second = ValidOrder().WithDescription("two").WithTotal(250_00).Build();
+        var client = ValidClient().WithName("Acme").WithOrder(first).WithOrder(second).Build();
         db.Save(client);
-        db.Save(client.Orders[0]);
-        db.Save(client.Orders[1]);
+        db.Save(first);
+        db.Save(second);
 
         var loaded = db.LoadClient(client.Id);
 
diff --git a/tests/BeforeAfter.Tests/AfterTests.cs b/tests/BeforeAfter.Tests/AfterTests.cs
index 5e78e81..b703456 100644
--- a/tests/BeforeAfter.Tests/AfterTests.cs
+++ b/tests/BeforeAfter.Tests/AfterTests.cs
@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 using System.Reflection;
 
 namespace BeforeAfter.Tests;
@@ -7,6 +9,11 @@ namespace BeforeAfter.Tests;
 /// Tests for the AFTER situation (Yak 02): plain domain objects (no
 /// <see cref="DbBase"/>), DTOs (records) carrying the data including the
 /// relations, hand-written mappers, and a WebApp that consumes only DTOs.
+///
+/// Domain objects in these tests are constructed through the TestDataBuilders
+/// (<see cref="OrderBuilder.ValidOrder"/> / <see cref="ClientBuilder.ValidClient"/>)
+/// so the tests survive domain evolution; DTO initializers stay literal —
+/// DTOs are the transport shapes under test.
 /// </summary>
 public class AfterTests
 {
@@ -42,7 +49,7 @@ public class AfterTests
     public void ToClientDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var client = new Client { Id = id, Name = "Acme" };
+        var client = ValidClient().WithId(id).WithName("Acme").Build();
 
         var dto = Mappers.ToClientDto(client);
 
@@ -54,13 +61,13 @@ public class AfterTests
     public void ToOrderDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var order = new Order { Id = id, Description = "one", TotalInCents = 15000 };
+        var order = ValidOrder().WithId(id).WithDescription("one").WithTotal(150_00).Build();
 
         var dto = Mappers.ToOrderDto(order);
 
         Assert.Equal(id, dto.Id);
         Assert.Equal("one", dto.Description);
-        Assert.Equal(15000, dto.TotalInCents);
+        Assert.Equal(150_00, dto.TotalInCents);
     }
 
     [Fact]
@@ -79,13 +86,13 @@ public class AfterTests
     public void FromOrderDto_maps_all_scalar_fields()
     {
         var id = Guid.NewGuid();
-        var dto = new OrderDto { Id = id, Description = "one", TotalInCents = 15000 };
+        var dto = new OrderDto { Id = id, Description = "one", TotalInCents = 150_00 };
 
         var order = Mappers.FromOrderDto(dto);
 
         Assert.Equal(id, order.Id);
         Assert.Equal("one", order.Description);
-        Assert.Equal(15000, order.TotalInCents);
+        Assert.Equal(150_00, order.TotalInCents);
     }
 
     // ---------------------------------------------------------------------
@@ -95,9 +102,8 @@ public class AfterTests
     [Fact]
     public void ToClientDto_maps_orders_and_keeps_the_back_reference_consistent()
     {
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
 
         var dto = Mappers.ToClientDto(client);
         var orderDto = Assert.Single(dto.Orders);
@@ -130,9 +136,8 @@ public class AfterTests
     [Fact]
     public void Domain_round_trip_preserves_all_state()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
-        var order = new Order { Id = Guid.NewGuid(), Description = "one", TotalInCents = 15000 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
 
         var roundTrip = Mappers.FromClientDto(Mappers.ToClientDto(client));
 
@@ -149,9 +154,8 @@ public class AfterTests
     [Fact]
     public void Dto_round_trip_preserves_all_state()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
-        var order = new Order { Id = Guid.NewGuid(), Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").WithTotal(150_00).Build();
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
         var dto = Mappers.ToClientDto(client);
         var dtoOrder = dto.Orders[0];
 
@@ -172,9 +176,8 @@ public class AfterTests
     [Fact]
     public void ToClientDto_does_not_mutate_the_domain()
     {
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
-        var order = new Order { Id = Guid.NewGuid(), Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("one").Build();
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
         var ordersCount = client.Orders.Count;
 
         Mappers.ToClientDto(client);
@@ -247,9 +250,8 @@ public class AfterTests
     public void After_WebApp_consumes_dtos()
     {
         var app = new WebApp();
-        var client = new Client { Id = Guid.NewGuid(), Name = "Acme" };
-        var order = new Order { Id = Guid.NewGuid(), Description = "order one", TotalInCents = 15000 };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithId(Guid.NewGuid()).WithDescription("order one").WithTotal(150_00).Build();
+        var client = ValidClient().WithId(Guid.NewGuid()).WithName("Acme").WithOrder(order).Build();
 
         var view = app.ShowClient(Mappers.ToClientDto(client));
 
@@ -266,9 +268,8 @@ public class AfterTests
     public void DbContext_saves_plain_domain_objects_and_tracks_them()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
-        var order = new Order { Description = "one" };
-        client.Orders.Add(order);
+        var order = ValidOrder().WithDescription("one").Build();
+        var client = ValidClient().WithName("Acme").WithOrder(order).Build();
 
         Assert.Equal(Guid.Empty, client.Id);
         db.Save(client);
@@ -286,7 +287,7 @@ public class AfterTests
     public void Saving_twice_does_not_reassign_the_id()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var client = ValidClient().WithName("Acme").Build();
         db.Save(client);
         var id = client.Id;
 
@@ -300,7 +301,7 @@ public class AfterTests
     public void A_saved_domain_object_keeps_no_reference_to_the_context()
     {
         var db = new DbContext();
-        var client = new Client { Name = "Acme" };
+        var client = ValidClient().WithName("Acme").Build();
         db.Save(client);
 
         // Saved (the id was assigned by the context) ...
@@ -313,4 +314,4 @@ public class AfterTests
         var dto = Mappers.ToClientDto(client);
         Assert.Equal(client.Id, dto.Id);   // the id flows on into the DTO
     }
-}
\ No newline at end of file
+}
diff --git a/tests/BeforeAfter.Tests/DiscountCalculatorTests.cs b/tests/BeforeAfter.Tests/DiscountCalculatorTests.cs
index aa3b2e3..0a6beee 100644
--- a/tests/BeforeAfter.Tests/DiscountCalculatorTests.cs
+++ b/tests/BeforeAfter.Tests/DiscountCalculatorTests.cs
@@ -1,4 +1,6 @@
 using After;
+using static BeforeAfter.Tests.TestData.ClientBuilder;
+using static BeforeAfter.Tests.TestData.OrderBuilder;
 
 namespace BeforeAfter.Tests;
 
@@ -16,7 +18,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_below_the_barrier_gets_no_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), Description = "small", TotalInCents = 50_00 };
+        var order = ValidOrder().WithTotal(50_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -28,7 +30,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_exactly_at_the_barrier_gets_no_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 100_00 };
+        var order = ValidOrder().WithTotal(100_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -42,7 +44,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_total_above_the_barrier_gets_the_percentage_discount()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -54,7 +56,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_zero_percentage_discounts_nothing_even_above_the_barrier()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 0);
 
@@ -65,7 +67,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void A_hundred_percent_discount_leaves_nothing()
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = 150_00 };
+        var order = ValidOrder().WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 100);
 
@@ -83,7 +85,7 @@ public class DiscountCalculatorTests
     [InlineData(151, 15, 136)]   // 15.1 rounds down to 15
     public void The_discount_amount_is_rounded_half_up(int totalInCents, int expectedDiscountCents, int expectedDiscountedTotal)
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = totalInCents };
+        var order = ValidOrder().WithTotal(totalInCents).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100, discountPercentage: 10);
 
@@ -98,7 +100,7 @@ public class DiscountCalculatorTests
     public void The_result_is_correlated_to_the_order_by_id()
     {
         var id = Guid.NewGuid();
-        var order = new Order { Id = id, TotalInCents = 150_00 };
+        var order = ValidOrder().WithId(id).WithTotal(150_00).Build();
 
         var discounted = DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -108,7 +110,7 @@ public class DiscountCalculatorTests
     [Fact]
     public void Calculating_does_not_mutate_the_order()
     {
-        var order = new Order { Id = Guid.NewGuid(), Description = "one", TotalInCents = 150_00 };
+        var order = ValidOrder().WithDescription("one").WithTotal(150_00).Build();
 
         DiscountCalculator.CalculateDiscount(order, discountBarrierCents: 100_00, discountPercentage: 10);
 
@@ -120,13 +122,13 @@ public class DiscountCalculatorTests
     // Input validation: invalid arguments throw (loudly, not silently).
     // ---------------------------------------------------------------------
     [Theory]
-    [InlineData(-1, 100_00, 10)]    // negative total
-    [InlineData(150_00, -1, 10)]    // negative barrier
-    [InlineData(150_00, 100_00, -1)]// percentage below 0
+    [InlineData(-1, 100_00, 10)]     // negative total
+    [InlineData(150_00, -1, 10)]     // negative barrier
+    [InlineData(150_00, 100_00, -1)] // percentage below 0
     [InlineData(150_00, 100_00, 101)]// percentage above 100
     public void Invalid_arguments_throw(int totalInCents, int discountBarrierCents, int discountPercentage)
     {
-        var order = new Order { Id = Guid.NewGuid(), TotalInCents = totalInCents };
+        var order = ValidOrder().WithTotal(totalInCents).Build();
 
         Assert.Throws<ArgumentOutOfRangeException>(
             () => DiscountCalculator.CalculateDiscount(order, discountBarrierCents, discountPercentage));
diff --git a/tests/BeforeAfter.Tests/TestData/ClientBuilder.cs b/tests/BeforeAfter.Tests/TestData/ClientBuilder.cs
new file mode 100644
index 0000000..be16bc1
--- /dev/null
+++ b/tests/BeforeAfter.Tests/TestData/ClientBuilder.cs
@@ -0,0 +1,40 @@
+using After;
+
+namespace BeforeAfter.Tests.TestData;
+
+/// <summary>
+/// Test data builder for a <see cref="Client"/> with its orders
+/// (TestDataBuilder pattern). <see cref="ValidClient"/> starts from a
+/// <em>fresh</em> client, exactly what <c>new Client()</c> gives.
+/// </summary>
+public sealed class ClientBuilder
+{
+    private readonly Client _client = new();
+
+    /// <summary>A valid, fresh client — the neutral starting point for tests.</summary>
+    public static ClientBuilder ValidClient() => new();
+
+    public ClientBuilder WithId(Guid id)
+    {
+        _client.Id = id;
+        return this;
+    }
+
+    public ClientBuilder WithName(string name)
+    {
+        _client.Name = name;
+        return this;
+    }
+
+    /// <summary>Adds an order to the client (navigation fix-up wires the back-reference).</summary>
+    public ClientBuilder WithOrder(Order order)
+    {
+        _client.Orders.Add(order);
+        return this;
+    }
+
+    /// <summary>Builds the given builder and adds its order — chaining sugar for <c>.WithOrder(builder.Build())</c>.</summary>
+    public ClientBuilder WithOrder(OrderBuilder order) => WithOrder(order.Build());
+
+    public Client Build() => _client;
+}
diff --git a/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs b/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs
new file mode 100644
index 0000000..d9a33a6
--- /dev/null
+++ b/tests/BeforeAfter.Tests/TestData/OrderBuilder.cs
@@ -0,0 +1,47 @@
+using After;
+
+namespace BeforeAfter.Tests.TestData;
+
+/// <summary>
+/// Test data builder for an <see cref="Order"/> (TestDataBuilder pattern).
+/// <see cref="ValidOrder"/> starts from a <em>fresh</em> order —
+/// <see cref="Guid.Empty"/> id, empty description, zero total, exactly what
+/// <c>new Order()</c> gives — and every <c>With...</c> returns the builder for
+/// chaining; <see cref="Build"/> hands back the order.
+///
+/// This class is the only place tests touch an order's *construction*: when
+/// the domain evolves (e.g. OrderLines making the total computed), only these
+/// internals change and the tests keep reading the same.
+/// </summary>
+public sealed class OrderBuilder
+{
+    private readonly Order _order = new();
+
+    /// <summary>A valid, fresh order — the neutral starting point for tests.</summary>
+    public static OrderBuilder ValidOrder() => new();
+
+    public OrderBuilder WithId(Guid id)
+    {
+        _order.Id = id;
+        return this;
+    }
+
+    public OrderBuilder WithDescription(string description)
+    {
+        _order.Description = description;
+        return this;
+    }
+
+    /// <summary>
+    /// Sets the total in cents (<c>150_00</c> = €150.00). How the requested
+    /// total is realised inside the order is an implementation detail that may
+    /// change as the domain evolves; this API stays.
+    /// </summary>
+    public OrderBuilder WithTotal(int totalInCents)
+    {
+        _order.TotalInCents = totalInCents;
+        return this;
+    }
+
+    public Order Build() => _order;
+}