diff --git a/tests/NATS.Server.Tests/LeafNodes/LeafClusterRegistrationTests.cs b/tests/NATS.Server.Tests/LeafNodes/LeafClusterRegistrationTests.cs
new file mode 100644
index 0000000..b49b0ad
--- /dev/null
+++ b/tests/NATS.Server.Tests/LeafNodes/LeafClusterRegistrationTests.cs
@@ -0,0 +1,149 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Configuration;
+using NATS.Server.LeafNodes;
+
+namespace NATS.Server.Tests.LeafNodes;
+
+/// <summary>
+/// Unit tests for leaf cluster topology registration (Gap 12.6).
+/// Verifies that <see cref="LeafNodeManager.RegisterLeafNodeCluster"/>,
+/// <see cref="LeafNodeManager.UnregisterLeafNodeCluster"/>,
+/// <see cref="LeafNodeManager.HasLeafNodeCluster"/>,
+/// <see cref="LeafNodeManager.GetLeafNodeCluster"/>,
+/// <see cref="LeafNodeManager.GetAllLeafClusters"/>,
+/// <see cref="LeafNodeManager.LeafClusterCount"/>, and
+/// <see cref="LeafNodeManager.UpdateLeafClusterConnectionCount"/>
+/// correctly manage leaf cluster entries.
+/// Go reference: leafnode.go registerLeafNodeCluster.
+/// </summary>
+public class LeafClusterRegistrationTests
+{
+    private static LeafNodeManager CreateManager() =>
+        new(
+            options: new LeafNodeOptions { Host = "127.0.0.1", Port = 0 },
+            stats: new ServerStats(),
+            serverId: "test-server",
+            remoteSubSink: _ => { },
+            messageSink: _ => { },
+            logger: NullLogger<LeafNodeManager>.Instance);
+
+    // Go: leafnode.go registerLeafNodeCluster — first registration succeeds
+    [Fact]
+    public void RegisterLeafNodeCluster_NewCluster_ReturnsTrue()
+    {
+        var manager = CreateManager();
+
+        var result = manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 3);
+
+        result.ShouldBeTrue();
+    }
+
+    // Go: leafnode.go registerLeafNodeCluster — duplicate name returns false
+    [Fact]
+    public void RegisterLeafNodeCluster_Duplicate_ReturnsFalse()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 3);
+
+        var result = manager.RegisterLeafNodeCluster("cluster-A", "nats://other:7222", 1);
+
+        result.ShouldBeFalse();
+    }
+
+    // Go: leafnode.go — cluster removal for existing entry returns true
+    [Fact]
+    public void UnregisterLeafNodeCluster_Existing_ReturnsTrue()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 3);
+
+        var result = manager.UnregisterLeafNodeCluster("cluster-A");
+
+        result.ShouldBeTrue();
+    }
+
+    // Go: leafnode.go — cluster removal for absent entry returns false
+    [Fact]
+    public void UnregisterLeafNodeCluster_NonExistent_ReturnsFalse()
+    {
+        var manager = CreateManager();
+
+        var result = manager.UnregisterLeafNodeCluster("cluster-X");
+
+        result.ShouldBeFalse();
+    }
+
+    // Go: leafnode.go — HasLeafNodeCluster true after registration
+    [Fact]
+    public void HasLeafNodeCluster_Registered_ReturnsTrue()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 3);
+
+        manager.HasLeafNodeCluster("cluster-A").ShouldBeTrue();
+    }
+
+    // Go: leafnode.go — HasLeafNodeCluster false when not registered
+    [Fact]
+    public void HasLeafNodeCluster_NotRegistered_ReturnsFalse()
+    {
+        var manager = CreateManager();
+
+        manager.HasLeafNodeCluster("cluster-X").ShouldBeFalse();
+    }
+
+    // Go: leafnode.go — GetLeafNodeCluster returns registered info
+    [Fact]
+    public void GetLeafNodeCluster_Found_ReturnsInfo()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 5);
+
+        var info = manager.GetLeafNodeCluster("cluster-A");
+
+        info.ShouldNotBeNull();
+        info.ClusterName.ShouldBe("cluster-A");
+        info.GatewayUrl.ShouldBe("nats://gateway:7222");
+        info.ConnectionCount.ShouldBe(5);
+    }
+
+    // Go: leafnode.go — GetLeafNodeCluster returns null for absent cluster
+    [Fact]
+    public void GetLeafNodeCluster_NotFound_ReturnsNull()
+    {
+        var manager = CreateManager();
+
+        var info = manager.GetLeafNodeCluster("cluster-X");
+
+        info.ShouldBeNull();
+    }
+
+    // Go: leafnode.go — GetAllLeafClusters returns all registered entries
+    [Fact]
+    public void GetAllLeafClusters_ReturnsAll()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gw-a:7222", 2);
+        manager.RegisterLeafNodeCluster("cluster-B", "nats://gw-b:7222", 4);
+
+        var all = manager.GetAllLeafClusters();
+
+        all.Count.ShouldBe(2);
+        all.Select(c => c.ClusterName).ShouldContain("cluster-A");
+        all.Select(c => c.ClusterName).ShouldContain("cluster-B");
+    }
+
+    // Go: leafnode.go — UpdateLeafClusterConnectionCount updates the count on existing entry
+    [Fact]
+    public void UpdateLeafClusterConnectionCount_UpdatesCount()
+    {
+        var manager = CreateManager();
+        manager.RegisterLeafNodeCluster("cluster-A", "nats://gateway:7222", 1);
+
+        manager.UpdateLeafClusterConnectionCount("cluster-A", 7);
+
+        var info = manager.GetLeafNodeCluster("cluster-A");
+        info.ShouldNotBeNull();
+        info.ConnectionCount.ShouldBe(7);
+    }
+}