diff --git a/src/NATS.Server/ClientClosedReason.cs b/src/NATS.Server/ClientClosedReason.cs
new file mode 100644
index 0000000..01eec58
--- /dev/null
+++ b/src/NATS.Server/ClientClosedReason.cs
@@ -0,0 +1,51 @@
+namespace NATS.Server;
+
+/// <summary>
+/// Reason a client connection was closed.
+/// Corresponds to Go server/client.go ClosedState (subset for single-server scope).
+/// </summary>
+public enum ClientClosedReason
+{
+    None = 0,
+    ClientClosed,
+    AuthenticationTimeout,
+    AuthenticationViolation,
+    TlsHandshakeError,
+    SlowConsumerPendingBytes,
+    SlowConsumerWriteDeadline,
+    WriteError,
+    ReadError,
+    ParseError,
+    StaleConnection,
+    ProtocolViolation,
+    MaxPayloadExceeded,
+    MaxSubscriptionsExceeded,
+    ServerShutdown,
+    MsgHeaderViolation,
+    NoRespondersRequiresHeaders,
+}
+
+public static class ClientClosedReasonExtensions
+{
+    public static string ToReasonString(this ClientClosedReason reason) => reason switch
+    {
+        ClientClosedReason.None => "",
+        ClientClosedReason.ClientClosed => "Client Closed",
+        ClientClosedReason.AuthenticationTimeout => "Authentication Timeout",
+        ClientClosedReason.AuthenticationViolation => "Authorization Violation",
+        ClientClosedReason.TlsHandshakeError => "TLS Handshake Error",
+        ClientClosedReason.SlowConsumerPendingBytes => "Slow Consumer (Pending Bytes)",
+        ClientClosedReason.SlowConsumerWriteDeadline => "Slow Consumer (Write Deadline)",
+        ClientClosedReason.WriteError => "Write Error",
+        ClientClosedReason.ReadError => "Read Error",
+        ClientClosedReason.ParseError => "Parse Error",
+        ClientClosedReason.StaleConnection => "Stale Connection",
+        ClientClosedReason.ProtocolViolation => "Protocol Violation",
+        ClientClosedReason.MaxPayloadExceeded => "Maximum Payload Exceeded",
+        ClientClosedReason.MaxSubscriptionsExceeded => "Maximum Subscriptions Exceeded",
+        ClientClosedReason.ServerShutdown => "Server Shutdown",
+        ClientClosedReason.MsgHeaderViolation => "Message Header Violation",
+        ClientClosedReason.NoRespondersRequiresHeaders => "No Responders Requires Headers",
+        _ => reason.ToString(),
+    };
+}
diff --git a/tests/NATS.Server.Tests/ClientClosedReasonTests.cs b/tests/NATS.Server.Tests/ClientClosedReasonTests.cs
new file mode 100644
index 0000000..16edf19
--- /dev/null
+++ b/tests/NATS.Server.Tests/ClientClosedReasonTests.cs
@@ -0,0 +1,24 @@
+namespace NATS.Server.Tests;
+
+public class ClientClosedReasonTests
+{
+    [Fact]
+    public void All_expected_close_reasons_exist()
+    {
+        // Verify all 17 enum values exist and are distinct (None + 16 named reasons)
+        var values = Enum.GetValues<ClientClosedReason>();
+        values.Length.ShouldBe(17);
+        values.Distinct().Count().ShouldBe(17);
+    }
+
+    [Theory]
+    [InlineData(ClientClosedReason.ClientClosed, "Client Closed")]
+    [InlineData(ClientClosedReason.SlowConsumerPendingBytes, "Slow Consumer (Pending Bytes)")]
+    [InlineData(ClientClosedReason.SlowConsumerWriteDeadline, "Slow Consumer (Write Deadline)")]
+    [InlineData(ClientClosedReason.StaleConnection, "Stale Connection")]
+    [InlineData(ClientClosedReason.ServerShutdown, "Server Shutdown")]
+    public void ToReasonString_returns_human_readable_description(ClientClosedReason reason, string expected)
+    {
+        reason.ToReasonString().ShouldBe(expected);
+    }
+}