diff --git a/tests/Trax.Scheduler.Tests.Integration/Trax.Scheduler.Tests.Integration.csproj b/tests/Trax.Scheduler.Tests.Integration/Trax.Scheduler.Tests.Integration.csproj
index c3f6b53..36e7a89 100644
--- a/tests/Trax.Scheduler.Tests.Integration/Trax.Scheduler.Tests.Integration.csproj
+++ b/tests/Trax.Scheduler.Tests.Integration/Trax.Scheduler.Tests.Integration.csproj
@@ -16,6 +16,7 @@
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="10.0.3" />
     <PackageReference Include="Microsoft.Extensions.Logging.Console" Version="10.0.3" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
+    <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="NUnit" Version="4.4.0" />
     <PackageReference Include="NUnit.Analyzers" Version="4.11.2" />
     <PackageReference Include="NUnit3TestAdapter" Version="6.1.0" />
diff --git a/tests/Trax.Scheduler.Tests.Integration/UnitTests/SchedulerBuilderLambdaInvocationTests.cs b/tests/Trax.Scheduler.Tests.Integration/UnitTests/SchedulerBuilderLambdaInvocationTests.cs
new file mode 100644
index 0000000..b280328
--- /dev/null
+++ b/tests/Trax.Scheduler.Tests.Integration/UnitTests/SchedulerBuilderLambdaInvocationTests.cs
@@ -0,0 +1,163 @@
+using FluentAssertions;
+using LanguageExt;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+using NUnit.Framework;
+using Trax.Effect.Data.InMemory.Extensions;
+using Trax.Effect.Extensions;
+using Trax.Effect.Models.Manifest;
+using Trax.Mediator.Extensions;
+using Trax.Scheduler.Configuration;
+using Trax.Scheduler.Extensions;
+using Trax.Scheduler.Services.Scheduling;
+using Trax.Scheduler.Services.TraxScheduler;
+using Trax.Scheduler.Tests.Integration.Fakes.Trains;
+
+namespace Trax.Scheduler.Tests.Integration.UnitTests;
+
+/// <summary>
+/// Exercises the ScheduleFunc lambda bodies inside SchedulerConfigurationBuilder by
+/// invoking each pending manifest's closure against a substitute scheduler and
+/// verifying it routes to the expected ITraxScheduler method.
+/// </summary>
+[TestFixture]
+public class SchedulerBuilderLambdaInvocationTests
+{
+    private static SchedulerConfiguration ResolveConfiguration(
+        Action<SchedulerConfigurationBuilder> configure
+    )
+    {
+        var services = new ServiceCollection();
+        services.AddLogging();
+        services.AddTrax(trax =>
+            trax.AddEffects(effects => effects.UseInMemory())
+                .AddMediator(typeof(AssemblyMarker).Assembly)
+                .AddScheduler(scheduler =>
+                {
+                    scheduler.UseInMemoryWorkers();
+                    configure(scheduler);
+                    return scheduler;
+                })
+        );
+        using var provider = services.BuildServiceProvider();
+        return provider.GetRequiredService<SchedulerConfiguration>();
+    }
+
+    [Test]
+    public async Task Schedule_LambdaInvocation_RoutesToScheduleAsync()
+    {
+        var config = ResolveConfiguration(s =>
+            s.Schedule<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                "ext-sched",
+                new SchedulerTestInput(),
+                Every.Minutes(5)
+            )
+        );
+
+        var scheduler = Substitute.For<ITraxScheduler>();
+        var dummyManifest = (Manifest)
+            System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(typeof(Manifest));
+        scheduler
+            .ScheduleAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                Arg.Any<string>(),
+                Arg.Any<SchedulerTestInput>(),
+                Arg.Any<Trax.Scheduler.Services.Scheduling.Schedule>(),
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            )
+            .Returns(dummyManifest);
+
+        var pending = config.PendingManifests.Single(m => m.ExternalId == "ext-sched");
+        await pending.ScheduleFunc(scheduler, CancellationToken.None);
+
+        await scheduler
+            .Received(1)
+            .ScheduleAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                "ext-sched",
+                Arg.Any<SchedulerTestInput>(),
+                Arg.Any<Trax.Scheduler.Services.Scheduling.Schedule>(),
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            );
+    }
+
+    [Test]
+    public async Task ScheduleOnce_LambdaInvocation_RoutesToScheduleOnceAsync()
+    {
+        var config = ResolveConfiguration(s =>
+            s.ScheduleOnce<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                "once-ext",
+                new SchedulerTestInput(),
+                TimeSpan.FromMinutes(2)
+            )
+        );
+
+        var scheduler = Substitute.For<ITraxScheduler>();
+        var dummyManifest = (Manifest)
+            System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(typeof(Manifest));
+        scheduler
+            .ScheduleOnceAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                Arg.Any<string>(),
+                Arg.Any<SchedulerTestInput>(),
+                Arg.Any<TimeSpan>(),
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            )
+            .Returns(dummyManifest);
+
+        var pending = config.PendingManifests.Single(m => m.ExternalId == "once-ext");
+        await pending.ScheduleFunc(scheduler, CancellationToken.None);
+
+        await scheduler
+            .Received(1)
+            .ScheduleOnceAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                "once-ext",
+                Arg.Any<SchedulerTestInput>(),
+                TimeSpan.FromMinutes(2),
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            );
+    }
+
+    [Test]
+    public async Task Include_LambdaInvocation_RoutesToScheduleDependentAsync()
+    {
+        var config = ResolveConfiguration(s =>
+            s.Schedule<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                    "root-x",
+                    new SchedulerTestInput(),
+                    Every.Minutes(5)
+                )
+                .Include<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                    "branch-x",
+                    new SchedulerTestInput()
+                )
+        );
+
+        var scheduler = Substitute.For<ITraxScheduler>();
+        var dummyManifest = (Manifest)
+            System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(typeof(Manifest));
+        scheduler
+            .ScheduleDependentAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                Arg.Any<string>(),
+                Arg.Any<SchedulerTestInput>(),
+                Arg.Any<string>(),
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            )
+            .Returns(dummyManifest);
+
+        var pending = config.PendingManifests.Single(m => m.ExternalId == "branch-x");
+        await pending.ScheduleFunc(scheduler, CancellationToken.None);
+
+        await scheduler
+            .Received(1)
+            .ScheduleDependentAsync<SchedulerTestTrain, SchedulerTestInput, Unit>(
+                "branch-x",
+                Arg.Any<SchedulerTestInput>(),
+                "root-x",
+                Arg.Any<Action<ScheduleOptions>?>(),
+                Arg.Any<CancellationToken>()
+            );
+    }
+}