diff --git a/tests/DesignPatterns.SourceGenerators.Tests/Generators/IncrementalCacheTests.cs b/tests/DesignPatterns.SourceGenerators.Tests/Generators/IncrementalCacheTests.cs
index a30d2a2..1a03b5e 100644
--- a/tests/DesignPatterns.SourceGenerators.Tests/Generators/IncrementalCacheTests.cs
+++ b/tests/DesignPatterns.SourceGenerators.Tests/Generators/IncrementalCacheTests.cs
@@ -1,4 +1,5 @@
 using DesignPatterns.SourceGenerators.Generators;
+using Microsoft.CodeAnalysis;
 
 namespace DesignPatterns.SourceGenerators.Tests.Generators;
 
@@ -176,4 +177,253 @@ public static partial class OrderStatusMachine;
         SourceGeneratorTestContext.AssertCacheHit<StateTransitionGenerator>(
             ("OrderMachine.cs", source));
     }
+
+    // ───────────────────────────────────────────────────────────────────
+    //  Incremental edit tests — verify that editing one file only
+    //  regenerates the contracts in that file, while other contracts
+    //  remain cached. This is the core incremental value of source
+    //  generators in large projects.
+    // ───────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Two strategy contracts in separate files. After adding a new
+    /// strategy to one file, the transform for the modified file should
+    /// be Modified/New, and the Collect/Combine stages should be Modified
+    /// (because one input changed).
+    /// <para>
+    /// Note: the unmodified file's transform is also Modified because
+    /// RegisterStrategy's Transform returns List&lt;KeyedRegistration&gt;
+    /// (reference equality). Once LocationInfo + EquatableArray are applied
+    /// (PR #216), the unmodified file's transform will be Unchanged. Until
+    /// then, this test verifies that the Combine stage correctly reflects
+    /// the edit.
+    /// </para>
+    /// </summary>
+    [Fact]
+    public void RegisterStrategy_EditOneFile_CombineReflectsChange()
+    {
+        var shippingSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public interface IShippingStrategy
+            {
+                decimal Calculate(decimal weight);
+            }
+
+            [RegisterStrategy("express", typeof(IShippingStrategy))]
+            public partial class ExpressShipping : IShippingStrategy
+            {
+                public decimal Calculate(decimal weight) => weight * 2m;
+            }
+            """;
+
+        var paymentSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public interface IPaymentStrategy
+            {
+                void Pay(decimal amount);
+            }
+
+            [RegisterStrategy("credit", typeof(IPaymentStrategy))]
+            public partial class CreditPayment : IPaymentStrategy
+            {
+                public void Pay(decimal amount) { }
+            }
+            """;
+
+        // Modified shipping source: add a second strategy.
+        var modifiedShippingSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public interface IShippingStrategy
+            {
+                decimal Calculate(decimal weight);
+            }
+
+            [RegisterStrategy("express", typeof(IShippingStrategy))]
+            public partial class ExpressShipping : IShippingStrategy
+            {
+                public decimal Calculate(decimal weight) => weight * 2m;
+            }
+
+            [RegisterStrategy("standard", typeof(IShippingStrategy))]
+            public partial class StandardShipping : IShippingStrategy
+            {
+                public decimal Calculate(decimal weight) => weight * 1m;
+            }
+            """;
+
+        var secondResult = SourceGeneratorTestContext.RunIncrementalEdit<RegisterStrategyGenerator>(
+            new[] { ("Shipping.cs", shippingSource), ("Payment.cs", paymentSource) },
+            "Shipping.cs",
+            modifiedShippingSource);
+
+        // The Combine stage should be Modified (one input changed).
+        // StrategyCollect is not tracked (no WithTrackingName on Collect),
+        // but StrategyCombine covers the Collect+Combine aggregation.
+        var combineReasons = SourceGeneratorTestContext.GetTrackedStepReasons(
+            secondResult, TrackingNames.StrategyCombine);
+        Assert.NotEmpty(combineReasons);
+        Assert.All(combineReasons, reason =>
+            Assert.True(
+                reason is IncrementalStepRunReason.Modified or IncrementalStepRunReason.New,
+                $"StrategyCombine expected Modified or New, but was {reason}."));
+
+        // The transform stage should reflect the edit: at least one output
+        // should be Modified or New (the added StandardShipping strategy).
+        var transformReasons = SourceGeneratorTestContext.GetTrackedStepReasons(
+            secondResult, TrackingNames.StrategyNonGenericTransform);
+        Assert.NotEmpty(transformReasons);
+        Assert.Contains(transformReasons, reason =>
+            reason is IncrementalStepRunReason.Modified or IncrementalStepRunReason.New);
+    }
+
+    /// <summary>
+    /// Two singleton targets in separate files. After modifying one file
+    /// (adding a second singleton), the transform stage should reflect
+    /// the change — at least one output should be Modified or Added.
+    /// GenerateSingleton has no Collect stage (per-item output), so we
+    /// check the transform tracking name directly.
+    /// </summary>
+    [Fact]
+    public void GenerateSingleton_EditOneFile_TransformReflectsChange()
+    {
+        var serviceSource = """
+            using DesignPatterns.Creational;
+
+            namespace TestAssembly;
+
+            [GenerateSingleton]
+            public partial class MyService;
+            """;
+
+        var repoSource = """
+            using DesignPatterns.Creational;
+
+            namespace TestAssembly;
+
+            [GenerateSingleton]
+            public partial class MyRepository;
+            """;
+
+        // Modified service source: add a second singleton.
+        var modifiedServiceSource = """
+            using DesignPatterns.Creational;
+
+            namespace TestAssembly;
+
+            [GenerateSingleton]
+            public partial class MyService;
+
+            [GenerateSingleton]
+            public partial class MyCache;
+            """;
+
+        var secondResult = SourceGeneratorTestContext.RunIncrementalEdit<GenerateSingletonGenerator>(
+            new[] { ("Service.cs", serviceSource), ("Repo.cs", repoSource) },
+            "Service.cs",
+            modifiedServiceSource);
+
+        // The transform stage should have at least one Modified or Added
+        // output (the new MyCache singleton). The existing MyService and
+        // MyRepository may be Cached if their syntax nodes are unchanged.
+        var transformReasons = SourceGeneratorTestContext.GetTrackedStepReasons(
+            secondResult, TrackingNames.SingletonTransform);
+        Assert.NotEmpty(transformReasons);
+        Assert.Contains(transformReasons, reason =>
+            reason is IncrementalStepRunReason.Modified or IncrementalStepRunReason.New);
+
+        // The unmodified file's transform should be Cached or Unchanged —
+        // Repo.cs was not edited, so MyRepository's transform output should
+        // be unchanged. (Cached = not re-executed; Unchanged = re-executed
+        // but produced identical output.)
+        Assert.Contains(transformReasons, reason =>
+            reason is IncrementalStepRunReason.Cached or IncrementalStepRunReason.Unchanged);
+    }
+
+    /// <summary>
+    /// Two state machines in separate files. After modifying one file
+    /// (adding a new transition), the Collect stage should be Modified.
+    /// </summary>
+    [Fact]
+    public void StateTransition_EditOneFile_CollectIsModified()
+    {
+        var orderSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public enum OrderStatus { Draft, Submitted }
+            public enum OrderTrigger { Submit }
+
+            [StateMachine(typeof(OrderStatus), typeof(OrderTrigger), Initial = OrderStatus.Draft)]
+            [Transition(OrderStatus.Draft, OrderTrigger.Submit, OrderStatus.Submitted)]
+            public static partial class OrderStatusMachine;
+            """;
+
+        var paymentSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public enum PaymentStatus { Pending, Paid }
+            public enum PaymentTrigger { Confirm }
+
+            [StateMachine(typeof(PaymentStatus), typeof(PaymentTrigger), Initial = PaymentStatus.Pending)]
+            [Transition(PaymentStatus.Pending, PaymentTrigger.Confirm, PaymentStatus.Paid)]
+            public static partial class PaymentStatusMachine;
+            """;
+
+        // Modified order source: add a new transition.
+        var modifiedOrderSource = """
+            using DesignPatterns.Behavioral;
+
+            namespace TestAssembly;
+
+            public enum OrderStatus { Draft, Submitted, Cancelled }
+            public enum OrderTrigger { Submit, Cancel }
+
+            [StateMachine(typeof(OrderStatus), typeof(OrderTrigger), Initial = OrderStatus.Draft)]
+            [Transition(OrderStatus.Draft, OrderTrigger.Submit, OrderStatus.Submitted)]
+            [Transition(OrderStatus.Draft, OrderTrigger.Cancel, OrderStatus.Cancelled)]
+            public static partial class OrderStatusMachine;
+            """;
+
+        var secondResult = SourceGeneratorTestContext.RunIncrementalEdit<StateTransitionGenerator>(
+            new[] { ("OrderMachine.cs", orderSource), ("PaymentMachine.cs", paymentSource) },
+            "OrderMachine.cs",
+            modifiedOrderSource);
+
+        // The Combine stage should be Modified (one input changed).
+        // StateMachineCollect is not tracked (no WithTrackingName on Collect),
+        // but StateMachineCombine covers the Collect+Combine aggregation.
+        var combineReasons = SourceGeneratorTestContext.GetTrackedStepReasons(
+            secondResult, TrackingNames.StateMachineCombine);
+        Assert.NotEmpty(combineReasons);
+        Assert.All(combineReasons, reason =>
+            Assert.True(
+                reason is IncrementalStepRunReason.Modified or IncrementalStepRunReason.New,
+                $"StateMachineCombine expected Modified or New, but was {reason}."));
+
+        // The transform stage should reflect the edit.
+        var transformReasons = SourceGeneratorTestContext.GetTrackedStepReasons(
+            secondResult, TrackingNames.StateMachineTransform);
+        Assert.NotEmpty(transformReasons);
+        Assert.Contains(transformReasons, reason =>
+            reason is IncrementalStepRunReason.Modified or IncrementalStepRunReason.New);
+
+        // The unmodified file's transform should be Cached or Unchanged —
+        // PaymentMachine.cs was not edited, so PaymentStatusMachine's
+        // transform should be unchanged. (Cached = not re-executed;
+        // Unchanged = re-executed but produced identical output.)
+        Assert.Contains(transformReasons, reason =>
+            reason is IncrementalStepRunReason.Cached or IncrementalStepRunReason.Unchanged);
+    }
 }
diff --git a/tests/DesignPatterns.SourceGenerators.Tests/SourceGeneratorTestContext.cs b/tests/DesignPatterns.SourceGenerators.Tests/SourceGeneratorTestContext.cs
index 5e5a033..c6a433a 100644
--- a/tests/DesignPatterns.SourceGenerators.Tests/SourceGeneratorTestContext.cs
+++ b/tests/DesignPatterns.SourceGenerators.Tests/SourceGeneratorTestContext.cs
@@ -118,6 +118,85 @@ internal static GeneratorDriverRunResult RunWithReferencedAssembly<TGenerator>(
         return driver.GetRunResult();
     }
 
+    /// <summary>
+    /// Runs the generator on an initial compilation, then re-runs it after replacing
+    /// one syntax tree with a modified version. Returns the second run's result so
+    /// the caller can inspect tracked step reasons (Cached vs Modified).
+    /// <para>
+    /// This is the core incremental edit test: it proves that editing one file
+    /// only regenerates the contracts in that file, while other contracts remain
+    /// cached.
+    /// </para>
+    /// </summary>
+    /// <param name="initialSources">All source files for the first run.</param>
+    /// <param name="modifiedPath">The path of the file to replace.</param>
+    /// <param name="modifiedSource">The new content for the replaced file.</param>
+    /// <returns>The <see cref="GeneratorDriverRunResult"/> for the second (post-edit) run.</returns>
+    internal static GeneratorDriverRunResult RunIncrementalEdit<TGenerator>(
+        (string Path, string Source)[] initialSources,
+        string modifiedPath,
+        string modifiedSource)
+        where TGenerator : IIncrementalGenerator, new()
+    {
+        var parseOptions = new CSharpParseOptions(LanguageVersion.Latest);
+        var driverOptions = new GeneratorDriverOptions(
+            disabledOutputs: IncrementalGeneratorOutputKind.None,
+            trackIncrementalGeneratorSteps: true);
+
+        var initialTrees = initialSources
+            .Select(s => CSharpSyntaxTree.ParseText(s.Source, parseOptions, path: s.Path))
+            .ToArray();
+
+        var compilation = CSharpCompilation.Create(
+            assemblyName: "DesignPatterns.SourceGenerators.Tests.IncrementalEdit",
+            syntaxTrees: initialTrees,
+            references: References,
+            options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));
+
+        GeneratorDriver driver = CSharpGeneratorDriver.Create(
+            new[] { new TGenerator().AsSourceGenerator() },
+            driverOptions: driverOptions,
+            parseOptions: parseOptions);
+
+        // First run — populates the cache.
+        driver = driver.RunGeneratorsAndUpdateCompilation(compilation, out _, out _);
+
+        // Replace one syntax tree with the modified version.
+        var oldTree = initialTrees.First(t => t.FilePath == modifiedPath);
+        var modifiedTree = CSharpSyntaxTree.ParseText(modifiedSource, parseOptions, path: modifiedPath);
+        var updatedCompilation = compilation.ReplaceSyntaxTree(oldTree, modifiedTree);
+
+        // Second run — should only re-execute stages for the modified file.
+        var secondResult = driver.RunGenerators(updatedCompilation).GetRunResult();
+
+        return secondResult;
+    }
+
+    /// <summary>
+    /// Gets the tracked step reasons for a specific tracking name from a run result.
+    /// Returns the list of (reason) for each output of the tracked step.
+    /// </summary>
+    internal static IReadOnlyList<IncrementalStepRunReason> GetTrackedStepReasons(
+        GeneratorDriverRunResult runResult,
+        string trackingName)
+    {
+        var reasons = new List<IncrementalStepRunReason>();
+        foreach (var generatorResult in runResult.Results)
+        {
+            if (generatorResult.TrackedSteps.TryGetValue(trackingName, out var steps))
+            {
+                foreach (var step in steps)
+                {
+                    foreach (var output in step.Outputs)
+                    {
+                        reasons.Add(output.Reason);
+                    }
+                }
+            }
+        }
+        return reasons;
+    }
+
     internal static IReadOnlyDictionary<string, string> GetGeneratedSources(GeneratorDriverRunResult runResult) =>
         runResult
             .Results