diff --git a/src/NATS.Server/Monitoring/ClosedClient.cs b/src/NATS.Server/Monitoring/ClosedClient.cs
new file mode 100644
index 0000000..0710d19
--- /dev/null
+++ b/src/NATS.Server/Monitoring/ClosedClient.cs
@@ -0,0 +1,25 @@
+namespace NATS.Server.Monitoring;
+
+/// <summary>
+/// Snapshot of a closed client connection for /connz reporting.
+/// </summary>
+public sealed record ClosedClient
+{
+    public required ulong Cid { get; init; }
+    public string Ip { get; init; } = "";
+    public int Port { get; init; }
+    public DateTime Start { get; init; }
+    public DateTime Stop { get; init; }
+    public string Reason { get; init; } = "";
+    public string Name { get; init; } = "";
+    public string Lang { get; init; } = "";
+    public string Version { get; init; } = "";
+    public long InMsgs { get; init; }
+    public long OutMsgs { get; init; }
+    public long InBytes { get; init; }
+    public long OutBytes { get; init; }
+    public uint NumSubs { get; init; }
+    public TimeSpan Rtt { get; init; }
+    public string TlsVersion { get; init; } = "";
+    public string TlsCipherSuite { get; init; } = "";
+}
diff --git a/src/NATS.Server/Monitoring/Connz.cs b/src/NATS.Server/Monitoring/Connz.cs
index 2147715..aae62ed 100644
--- a/src/NATS.Server/Monitoring/Connz.cs
+++ b/src/NATS.Server/Monitoring/Connz.cs
@@ -169,6 +169,8 @@ public enum SortOpt
     ByIdle,
     ByUptime,
     ByRtt,
+    ByStop,
+    ByReason,
 }
 
 /// <summary>
diff --git a/src/NATS.Server/Monitoring/ConnzHandler.cs b/src/NATS.Server/Monitoring/ConnzHandler.cs
index f532e7c..8ecf512 100644
--- a/src/NATS.Server/Monitoring/ConnzHandler.cs
+++ b/src/NATS.Server/Monitoring/ConnzHandler.cs
@@ -12,9 +12,25 @@ public sealed class ConnzHandler(NatsServer server)
     {
         var opts = ParseQueryParams(ctx);
         var now = DateTime.UtcNow;
-        var clients = server.GetClients().ToArray();
 
-        var connInfos = clients.Select(c => BuildConnInfo(c, now, opts)).ToList();
+        var connInfos = new List<ConnInfo>();
+
+        // Collect open connections
+        if (opts.State is ConnState.Open or ConnState.All)
+        {
+            var clients = server.GetClients().ToArray();
+            connInfos.AddRange(clients.Select(c => BuildConnInfo(c, now, opts)));
+        }
+
+        // Collect closed connections
+        if (opts.State is ConnState.Closed or ConnState.All)
+        {
+            connInfos.AddRange(server.GetClosedClients().Select(c => BuildClosedConnInfo(c, now, opts)));
+        }
+
+        // Validate sort options that require closed state
+        if (opts.Sort is SortOpt.ByStop or SortOpt.ByReason && opts.State == ConnState.Open)
+            opts.Sort = SortOpt.ByCid; // Fallback
 
         // Sort
         connInfos = opts.Sort switch
@@ -30,6 +46,8 @@ public sealed class ConnzHandler(NatsServer server)
             SortOpt.ByLast => connInfos.OrderByDescending(c => c.LastActivity).ToList(),
             SortOpt.ByIdle => connInfos.OrderByDescending(c => now - c.LastActivity).ToList(),
             SortOpt.ByUptime => connInfos.OrderByDescending(c => now - c.Start).ToList(),
+            SortOpt.ByStop => connInfos.OrderByDescending(c => c.Stop ?? DateTime.MinValue).ToList(),
+            SortOpt.ByReason => connInfos.OrderBy(c => c.Reason).ToList(),
             SortOpt.ByRtt => connInfos.OrderBy(c => c.Rtt).ToList(),
             _ => connInfos.OrderBy(c => c.Cid).ToList(),
         };
@@ -74,7 +92,7 @@ public sealed class ConnzHandler(NatsServer server)
             Reason = client.CloseReason.ToReasonString(),
             TlsVersion = client.TlsState?.TlsVersion ?? "",
             TlsCipherSuite = client.TlsState?.CipherSuite ?? "",
-            Rtt = client.Rtt == TimeSpan.Zero ? "" : $"{client.Rtt.TotalMilliseconds:F3}ms",
+            Rtt = FormatRtt(client.Rtt),
         };
 
         if (opts.Subscriptions)
@@ -98,6 +116,35 @@ public sealed class ConnzHandler(NatsServer server)
         return info;
     }
 
+    private static ConnInfo BuildClosedConnInfo(ClosedClient closed, DateTime now, ConnzOptions opts)
+    {
+        return new ConnInfo
+        {
+            Cid = closed.Cid,
+            Kind = "Client",
+            Type = "Client",
+            Ip = closed.Ip,
+            Port = closed.Port,
+            Start = closed.Start,
+            Stop = closed.Stop,
+            LastActivity = closed.Stop,
+            Uptime = FormatDuration(closed.Stop - closed.Start),
+            Idle = FormatDuration(now - closed.Stop),
+            InMsgs = closed.InMsgs,
+            OutMsgs = closed.OutMsgs,
+            InBytes = closed.InBytes,
+            OutBytes = closed.OutBytes,
+            NumSubs = closed.NumSubs,
+            Name = closed.Name,
+            Lang = closed.Lang,
+            Version = closed.Version,
+            Reason = closed.Reason,
+            Rtt = FormatRtt(closed.Rtt),
+            TlsVersion = closed.TlsVersion,
+            TlsCipherSuite = closed.TlsCipherSuite,
+        };
+    }
+
     private static ConnzOptions ParseQueryParams(HttpContext ctx)
     {
         var q = ctx.Request.Query;
@@ -119,6 +166,8 @@ public sealed class ConnzHandler(NatsServer server)
                 "idle" => SortOpt.ByIdle,
                 "uptime" => SortOpt.ByUptime,
                 "rtt" => SortOpt.ByRtt,
+                "stop" => SortOpt.ByStop,
+                "reason" => SortOpt.ByReason,
                 _ => SortOpt.ByCid,
             };
         }
@@ -131,6 +180,17 @@ public sealed class ConnzHandler(NatsServer server)
                 opts.Subscriptions = true;
         }
 
+        if (q.TryGetValue("state", out var state))
+        {
+            opts.State = state.ToString().ToLowerInvariant() switch
+            {
+                "open" => ConnState.Open,
+                "closed" => ConnState.Closed,
+                "all" => ConnState.All,
+                _ => ConnState.Open,
+            };
+        }
+
         if (q.TryGetValue("offset", out var offset) && int.TryParse(offset, out var o))
             opts.Offset = o;
 
@@ -140,6 +200,16 @@ public sealed class ConnzHandler(NatsServer server)
         return opts;
     }
 
+    private static string FormatRtt(TimeSpan rtt)
+    {
+        if (rtt == TimeSpan.Zero) return "";
+        if (rtt.TotalMilliseconds < 1)
+            return $"{rtt.TotalMicroseconds:F3}\u00b5s";
+        if (rtt.TotalSeconds < 1)
+            return $"{rtt.TotalMilliseconds:F3}ms";
+        return $"{rtt.TotalSeconds:F3}s";
+    }
+
     private static string FormatDuration(TimeSpan ts)
     {
         if (ts.TotalDays >= 1)
diff --git a/src/NATS.Server/NatsServer.cs b/src/NATS.Server/NatsServer.cs
index 2eff8de..64b1cf5 100644
--- a/src/NATS.Server/NatsServer.cs
+++ b/src/NATS.Server/NatsServer.cs
@@ -19,6 +19,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 {
     private readonly NatsOptions _options;
     private readonly ConcurrentDictionary<ulong, NatsClient> _clients = new();
+    private readonly ConcurrentQueue<ClosedClient> _closedClients = new();
+    private const int MaxClosedClients = 10_000;
     private readonly ServerInfo _serverInfo;
     private readonly ILogger<NatsServer> _logger;
     private readonly ILoggerFactory _loggerFactory;
@@ -64,6 +66,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public Action? ReOpenLogFile { get; set; }
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 
+    public IEnumerable<ClosedClient> GetClosedClients() => _closedClients;
+
     public IEnumerable<Auth.Account> GetAccounts() => _accounts.Values;
 
     public Task WaitForReadyAsync() => _listeningStarted.Task;
@@ -580,6 +584,33 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     {
         _clients.TryRemove(client.Id, out _);
         _logger.LogDebug("Removed client {ClientId}", client.Id);
+
+        // Snapshot for closed-connections tracking
+        _closedClients.Enqueue(new ClosedClient
+        {
+            Cid = client.Id,
+            Ip = client.RemoteIp ?? "",
+            Port = client.RemotePort,
+            Start = client.StartTime,
+            Stop = DateTime.UtcNow,
+            Reason = client.CloseReason.ToReasonString(),
+            Name = client.ClientOpts?.Name ?? "",
+            Lang = client.ClientOpts?.Lang ?? "",
+            Version = client.ClientOpts?.Version ?? "",
+            InMsgs = Interlocked.Read(ref client.InMsgs),
+            OutMsgs = Interlocked.Read(ref client.OutMsgs),
+            InBytes = Interlocked.Read(ref client.InBytes),
+            OutBytes = Interlocked.Read(ref client.OutBytes),
+            NumSubs = (uint)client.Subscriptions.Count,
+            Rtt = client.Rtt,
+            TlsVersion = client.TlsState?.TlsVersion ?? "",
+            TlsCipherSuite = client.TlsState?.CipherSuite ?? "",
+        });
+
+        // Cap closed clients list
+        while (_closedClients.Count > MaxClosedClients)
+            _closedClients.TryDequeue(out _);
+
         var subList = client.Account?.SubList ?? _globalAccount.SubList;
         client.RemoveAllSubscriptions(subList);
         client.Account?.RemoveClient(client.Id);
diff --git a/tests/NATS.Server.Tests/MonitorTests.cs b/tests/NATS.Server.Tests/MonitorTests.cs
index 65a1399..e89a0db 100644
--- a/tests/NATS.Server.Tests/MonitorTests.cs
+++ b/tests/NATS.Server.Tests/MonitorTests.cs
@@ -179,6 +179,53 @@ public class MonitorTests : IAsyncLifetime
         conn.Subs.ShouldContain("bar");
     }
 
+    [Fact]
+    public async Task Connz_state_closed_returns_disconnected_clients()
+    {
+        // Connect then disconnect a client
+        var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await sock.ConnectAsync(new IPEndPoint(IPAddress.Loopback, _natsPort));
+        using var stream = new NetworkStream(sock);
+        var buf = new byte[4096];
+        _ = await stream.ReadAsync(buf);
+        await stream.WriteAsync("CONNECT {\"name\":\"closing-client\"}\r\n"u8.ToArray());
+        await Task.Delay(200);
+        sock.Shutdown(SocketShutdown.Both);
+        sock.Dispose();
+        await Task.Delay(500);
+
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?state=closed");
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+        connz.ShouldNotBeNull();
+        connz.Conns.ShouldContain(c => c.Name == "closing-client");
+        var closed = connz.Conns.First(c => c.Name == "closing-client");
+        closed.Stop.ShouldNotBeNull();
+        closed.Reason.ShouldNotBeNullOrEmpty();
+    }
+
+    [Fact]
+    public async Task Connz_sort_by_stop_requires_closed_state()
+    {
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?sort=stop&state=open");
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+        connz.ShouldNotBeNull();
+    }
+
+    [Fact]
+    public async Task Connz_sort_by_reason()
+    {
+        var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await sock.ConnectAsync(new IPEndPoint(IPAddress.Loopback, _natsPort));
+        var buf = new byte[4096];
+        _ = await sock.ReceiveAsync(buf);
+        sock.Shutdown(SocketShutdown.Both);
+        sock.Dispose();
+        await Task.Delay(500);
+
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?sort=reason&state=closed");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+    }
+
     private static int GetFreePort()
     {
         using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);