diff --git a/src/tests/TeamsISO.Engine.IntegrationTests/IntegrationTestsScaffold.cs b/src/tests/TeamsISO.Engine.IntegrationTests/IntegrationTestsScaffold.cs
index 6d33330..05ebdf6 100644
--- a/src/tests/TeamsISO.Engine.IntegrationTests/IntegrationTestsScaffold.cs
+++ b/src/tests/TeamsISO.Engine.IntegrationTests/IntegrationTestsScaffold.cs
@@ -1,11 +1,124 @@
+using System.Runtime.Versioning;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using TeamsISO.Engine.Interop;
+using TeamsISO.Engine.NdiInterop;
+
 namespace TeamsISO.Engine.IntegrationTests;
 
-public class IntegrationTestsScaffold
+/// <summary>
+/// Integration tests that talk to a real NDI runtime — gated behind
+/// <c>requires=ndi</c> so the default CI run skips them. Run locally with:
+///   dotnet test --filter requires=ndi
+///
+/// Pre-conditions:
+///   - Windows host
+///   - NDI 6 Runtime installed (NDI_RUNTIME_DIR_V6 set)
+///   - Network discovery permitted on the loopback / local subnet
+/// </summary>
+[SupportedOSPlatform("windows")]
+public class NdiInteropIntegrationTests
 {
-    [Fact(Skip = "Phase A: integration tests require NDI runtime — added in Phase B.")]
+    private static NdiInteropPInvoke NewInterop() =>
+        new(NullLogger<NdiInteropPInvoke>.Instance);
+
+    [Fact]
     [Trait("requires", "ndi")]
-    public void ScaffoldFactSkipsCleanly()
+    public void NdiRuntime_LoadsAndReportsVersion()
     {
-        Assert.True(true);
+        using var interop = NewInterop();
+
+        var version = interop.GetRuntimeVersion();
+
+        version.Should().NotBeNullOrEmpty();
+        version.Should().StartWith(NdiVersion.ExpectedRuntimeVersionPrefix,
+            because: "the engine probe asserts this prefix; if it ever drifts CI must catch it");
+    }
+
+    [Fact]
+    [Trait("requires", "ndi")]
+    public void Finder_DefaultGroups_CreatesAndDisposesCleanly()
+    {
+        using var interop = NewInterop();
+
+        using (var finder = interop.CreateFinder())
+        {
+            finder.Should().NotBeNull(because: "default-group finder must construct successfully");
+            // Snapshot any visible sources — exercises the path; we don't assert on count
+            // because the test environment's NDI sources are unknowable.
+            _ = interop.GetCurrentSources(finder);
+        }
+    }
+
+    [Theory]
+    [Trait("requires", "ndi")]
+    [InlineData("teamsiso-test-input")]
+    [InlineData("teamsiso-test-input,production")]
+    [InlineData(" teamsiso-test-input ")]
+    public void Finder_CustomGroups_DoesNotThrow(string groups)
+    {
+        using var interop = NewInterop();
+
+        var act = () =>
+        {
+            using var finder = interop.CreateFinder(groups);
+            _ = interop.GetCurrentSources(finder);
+        };
+
+        act.Should().NotThrow(because: $"groups='{groups}' must round-trip into NDIlib_find_create_v2 cleanly");
+    }
+
+    [Fact]
+    [Trait("requires", "ndi")]
+    public void Sender_DefaultGroups_CreatesAndDisposesCleanly()
+    {
+        using var interop = NewInterop();
+
+        using (var sender = interop.CreateSender("TEAMSISO_TEST_DEFAULT"))
+        {
+            sender.Should().NotBeNull();
+        }
+    }
+
+    [Fact]
+    [Trait("requires", "ndi")]
+    public void Sender_CustomGroups_CreatesAndDisposesCleanly()
+    {
+        using var interop = NewInterop();
+
+        using (var sender = interop.CreateSender("TEAMSISO_TEST_GROUPED", "teamsiso-test-output"))
+        {
+            sender.Should().NotBeNull();
+        }
+    }
+
+    [Fact]
+    [Trait("requires", "ndi")]
+    public async Task LoopbackDiscovery_FindsOurOwnSenderWithinFiveSeconds()
+    {
+        // End-to-end check: a sender we create on the local machine must be visible
+        // to a finder running in the same process, within a reasonable window. Catches
+        // network-layer regressions (firewall, mDNS, multicast disable).
+        using var interop = NewInterop();
+
+        var uniqueName = $"TEAMSISO_LOOP_{Guid.NewGuid().ToString("N")[..8].ToUpperInvariant()}";
+        using var sender = interop.CreateSender(uniqueName);
+        using var finder = interop.CreateFinder();
+
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        bool found = false;
+        while (DateTime.UtcNow < deadline)
+        {
+            var sources = interop.GetCurrentSources(finder);
+            if (sources.Any(s => s.Contains(uniqueName, StringComparison.Ordinal)))
+            {
+                found = true;
+                break;
+            }
+            await Task.Delay(250);
+        }
+
+        found.Should().BeTrue(
+            because: $"local sender '{uniqueName}' must be discovered by a same-process finder within 5s");
     }
 }