Merge branch 'feature/core-lifecycle' into main

Reconcile close reason tracking: feature branch's MarkClosed() and
ShouldSkipFlush/FlushAndCloseAsync now use main's ClientClosedReason
enum. ClosedState enum retained for forward compatibility.

.claude/settings.local.json

@@ -0,0 +1,16 @@
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Write|Edit|MultiEdit",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "slopwatch analyze -d . --hook",
+            "timeout": 60000
+          }
+        ]
+      }
+    ]
+  }
+}

.slopwatch/config.json.example

@@ -0,0 +1,10 @@
+{
+  "suppressions": [
+    {
+      "ruleId": "SW002",
+      "pattern": "**/Generated/**",
+      "justification": "Generated code from protobuf/gRPC compiler - cannot be modified"
+    }
+  ],
+  "globalSuppressions": []
+}

differences.md

@@ -80,7 +80,7 @@
 | Write deadline / timeout policies | Y | Y | `WriteDeadline` option with `CancellationTokenSource.CancelAfter` on flush |
 | RTT measurement | Y | N | Go tracks round-trip time per client |
 | Per-client trace mode | Y | N | |
-| Detailed close reason tracking | Y | Y | 17-value `ClientClosedReason` enum (single-server subset of Go's 37) |
+| Detailed close reason tracking | Y | Y | 37-value `ClosedState` enum with CAS-based `MarkClosed()` |
 | Connection state flags (16 flags) | Y | Y | 7-flag `ClientFlagHolder` with `Interlocked.Or`/`And` |
 
 ### Slow Consumer Handling

docs/plans/2026-02-22-section2-client-connection-handling-design.md

@@ -0,0 +1,199 @@
+# Section 2: Client/Connection Handling — Design
+
+> Implements all in-scope gaps from differences.md Section 2.
+
+## Scope
+
+8 features, all single-server client-facing (no clustering/routes/gateways/leaf):
+
+1. Close reason tracking (ClosedState enum)
+2. Connection state flags (bitfield replacing `_connectReceived`)
+3. Channel-based write loop with batch flush
+4. Slow consumer detection (pending bytes + write deadline)
+5. Write deadline / timeout
+6. Verbose mode (`+OK` responses)
+7. No-responders validation and notification
+8. Per-read-cycle stat batching
+
+## A. Close Reasons
+
+New `ClientClosedReason` enum with 16 values scoped to single-server:
+
+```
+ClientClosed, AuthenticationTimeout, AuthenticationViolation, TLSHandshakeError,
+SlowConsumerPendingBytes, SlowConsumerWriteDeadline, WriteError, ReadError,
+ParseError, StaleConnection, ProtocolViolation, MaxPayloadExceeded,
+MaxSubscriptionsExceeded, ServerShutdown, MsgHeaderViolation, NoRespondersRequiresHeaders
+```
+
+Go has 37 values; excluded: route/gateway/leaf/JWT/operator-mode values.
+
+Per-client `CloseReason` property set before closing. Available in monitoring (`/connz`).
+
+## B. Connection State Flags
+
+`ClientFlags` bitfield enum backed by `int`, manipulated via `Interlocked.Or`/`Interlocked.And`:
+
+```
+ConnectReceived = 1,
+FirstPongSent = 2,
+HandshakeComplete = 4,
+CloseConnection = 8,
+WriteLoopStarted = 16,
+IsSlowConsumer = 32,
+ConnectProcessFinished = 64
+```
+
+Replaces current `_connectReceived` (int with Volatile.Read/Write).
+
+Helper methods: `SetFlag(flag)`, `ClearFlag(flag)`, `HasFlag(flag)`.
+
+## C. Channel-based Write Loop
+
+### Architecture
+
+Replace inline `_writeLock` + direct stream writes:
+
+```
+Producer threads → QueueOutbound(bytes) → Channel<ReadOnlyMemory<byte>> → WriteLoop → Stream
+```
+
+### Components
+
+- `Channel<ReadOnlyMemory<byte>>` — bounded (capacity derived from MaxPending / avg message size, or 8192 items)
+- `_pendingBytes` (long) — tracks queued but unflushed bytes via `Interlocked.Add`
+- `RunWriteLoopAsync` — background task: `WaitToReadAsync` → drain all via `TryRead` → single `FlushAsync`
+- `QueueOutbound(ReadOnlyMemory<byte>)` — enqueue, update pending bytes, check slow consumer
+
+### Coalescing
+
+The write loop drains all available items from the channel before flushing:
+
+```
+while (await reader.WaitToReadAsync(ct))
+{
+    while (reader.TryRead(out var data))
+        await stream.WriteAsync(data, ct);  // buffered writes, no flush yet
+    await stream.FlushAsync(ct);             // single flush after batch
+}
+```
+
+### Migration
+
+All existing write paths refactored:
+- `SendMessageAsync` → serialize MSG/HMSG to byte array → `QueueOutbound`
+- `WriteAsync` → serialize protocol message → `QueueOutbound`
+- Remove `_writeLock` SemaphoreSlim
+
+## D. Slow Consumer Detection
+
+### Pending Bytes (Hard Limit)
+
+In `QueueOutbound`, before writing to channel:
+
+```
+if (_pendingBytes + data.Length > _maxPending)
+{
+    SetFlag(IsSlowConsumer);
+    CloseWithReason(SlowConsumerPendingBytes);
+    return;
+}
+```
+
+- `MaxPending` default: 64MB (matching Go's `MAX_PENDING_SIZE`)
+- New option in `NatsOptions`
+
+### Write Deadline (Timeout)
+
+In write loop flush:
+
+```
+using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+cts.CancelAfter(_writeDeadline);
+await stream.FlushAsync(cts.Token);
+```
+
+On timeout → close with `SlowConsumerWriteDeadline`.
+
+- `WriteDeadline` default: 10 seconds
+- New option in `NatsOptions`
+
+### Monitoring
+
+- `IsSlowConsumer` flag readable for `/connz`
+- Server-level `SlowConsumerCount` stat incremented
+
+## E. Verbose Mode
+
+After successful command processing (CONNECT, SUB, UNSUB, PUB), check `ClientOpts?.Verbose`:
+
+```
+if (ClientOpts?.Verbose == true)
+    QueueOutbound(OkBytes);
+```
+
+`OkBytes` = pre-encoded `+OK\r\n` static byte array in `NatsProtocol`.
+
+## F. No-Responders
+
+### CONNECT Validation
+
+```
+if (clientOpts.NoResponders && !clientOpts.Headers)
+{
+    CloseWithReason(NoRespondersRequiresHeaders);
+    return;
+}
+```
+
+### Publish-time Notification
+
+In `NatsServer` message delivery, after `Match()` returns zero subscribers:
+
+```
+if (!delivered && reply.Length > 0 && publisher.ClientOpts?.NoResponders == true)
+{
+    // Send HMSG with NATS/1.0 503 status back to publisher
+    var header = $"NATS/1.0 503\r\nNats-Subject: {subject}\r\n\r\n";
+    publisher.SendNoRespondersAsync(reply, sid, header);
+}
+```
+
+## G. Stat Batching
+
+In read loop, accumulate locally:
+
+```
+long localInMsgs = 0, localInBytes = 0;
+// ... per message: localInMsgs++; localInBytes += size;
+// End of read cycle:
+Interlocked.Add(ref _inMsgs, localInMsgs);
+Interlocked.Add(ref _inBytes, localInBytes);
+// Same for server stats
+```
+
+Reduces atomic operations from per-message to per-read-cycle.
+
+## Files
+
+| File | Change | Size |
+|------|--------|------|
+| `ClientClosedReason.cs` | New | Small |
+| `ClientFlags.cs` | New | Small |
+| `NatsClient.cs` | Major rewrite of write path | Large |
+| `NatsServer.cs` | No-responders, close reason | Medium |
+| `NatsOptions.cs` | MaxPending, WriteDeadline | Small |
+| `NatsProtocol.cs` | +OK bytes, NoResponders | Small |
+| `ClientTests.cs` | Verbose, close reasons, flags | Medium |
+| `ServerTests.cs` | No-responders, slow consumer | Medium |
+
+## Test Plan
+
+- **Verbose mode**: Connect with `verbose:true`, send SUB/PUB, verify `+OK` responses
+- **Close reasons**: Trigger each close path, verify reason is set
+- **State flags**: Set/clear/check flags concurrently
+- **Slow consumer (pending bytes)**: Queue more than MaxPending, verify close
+- **Slow consumer (write deadline)**: Use a slow/blocked stream, verify timeout close
+- **No-responders**: Publish to empty subject with reply, verify 503 HMSG
+- **Write coalescing**: Send multiple messages rapidly, verify batched flush
+- **Stat batching**: Send N messages, verify stats match after read cycle

docs/plans/2026-02-22-section2-client-connection-handling-plan.md.tasks.json

@@ -0,0 +1,19 @@
+{
+  "planPath": "docs/plans/2026-02-22-section2-client-connection-handling-plan.md",
+  "tasks": [
+    {"id": 4, "subject": "Task 1: Add ClientClosedReason enum", "status": "pending"},
+    {"id": 5, "subject": "Task 2: Add ClientFlags bitfield", "status": "pending"},
+    {"id": 6, "subject": "Task 3: Add MaxPending and WriteDeadline to NatsOptions", "status": "pending"},
+    {"id": 7, "subject": "Task 4: Integrate ClientFlags into NatsClient", "status": "pending", "blockedBy": [4, 5, 6]},
+    {"id": 8, "subject": "Task 5: Implement channel-based write loop", "status": "pending", "blockedBy": [7]},
+    {"id": 9, "subject": "Task 6: Write tests for write loop and slow consumer", "status": "pending", "blockedBy": [8]},
+    {"id": 10, "subject": "Task 7: Update NatsServer for SendMessage + no-responders", "status": "pending", "blockedBy": [8]},
+    {"id": 11, "subject": "Task 8: Implement verbose mode", "status": "pending", "blockedBy": [10]},
+    {"id": 12, "subject": "Task 9: Implement no-responders CONNECT validation", "status": "pending", "blockedBy": [10]},
+    {"id": 13, "subject": "Task 10: Implement stat batching in read loop", "status": "pending", "blockedBy": [8]},
+    {"id": 14, "subject": "Task 11: Update ConnzHandler for close reason + pending bytes", "status": "pending", "blockedBy": [13]},
+    {"id": 15, "subject": "Task 12: Fix existing tests for new write model", "status": "pending", "blockedBy": [13]},
+    {"id": 16, "subject": "Task 13: Final verification and differences.md update", "status": "pending", "blockedBy": [14, 15]}
+  ],
+  "lastUpdated": "2026-02-22T00:00:00Z"
+}

src/NATS.Server.Host/Program.cs

@@ -32,6 +32,15 @@ for (int i = 0; i < args.Length; i++)
         case "--https_port" when i + 1 < args.Length:
             options.MonitorHttpsPort = int.Parse(args[++i]);
             break;
+        case "-c" when i + 1 < args.Length:
+            options.ConfigFile = args[++i];
+            break;
+        case "--pid" when i + 1 < args.Length:
+            options.PidFile = args[++i];
+            break;
+        case "--ports_file_dir" when i + 1 < args.Length:
+            options.PortsFileDir = args[++i];
+            break;
         case "--tls":
             break;
         case "--tlscert" when i + 1 < args.Length:
@@ -50,18 +59,24 @@ for (int i = 0; i < args.Length; i++)
 }
 
 using var loggerFactory = new Serilog.Extensions.Logging.SerilogLoggerFactory(Log.Logger);
-var server = new NatsServer(options, loggerFactory);
+using var server = new NatsServer(options, loggerFactory);
 
-var cts = new CancellationTokenSource();
+// Register signal handlers
+server.HandleSignals();
+
+// Ctrl+C triggers graceful shutdown
 Console.CancelKeyPress += (_, e) =>
 {
     e.Cancel = true;
-    cts.Cancel();
+    Log.Information("Trapped SIGINT signal");
+    _ = Task.Run(async () => await server.ShutdownAsync());
 };
 
 try
 {
-    await server.StartAsync(cts.Token);
+    _ = server.StartAsync(CancellationToken.None);
+    await server.WaitForReadyAsync();
+    server.WaitForShutdown();
 }
 catch (OperationCanceledException)
 {

src/NATS.Server/ClosedState.cs

@@ -0,0 +1,52 @@
+// Ported from Go: server/client.go:188-228
+
+namespace NATS.Server;
+
+/// <summary>
+/// Reason a client connection was closed. Stored in connection info for monitoring
+/// and passed to close handlers during connection teardown.
+/// </summary>
+/// <remarks>
+/// Values start at 1 (matching Go's <c>iota + 1</c>) so that the default zero value
+/// is distinct from any valid close reason.
+/// </remarks>
+public enum ClosedState
+{
+    ClientClosed = 1,
+    AuthenticationTimeout,
+    AuthenticationViolation,
+    TLSHandshakeError,
+    SlowConsumerPendingBytes,
+    SlowConsumerWriteDeadline,
+    WriteError,
+    ReadError,
+    ParseError,
+    StaleConnection,
+    ProtocolViolation,
+    BadClientProtocolVersion,
+    WrongPort,
+    MaxAccountConnectionsExceeded,
+    MaxConnectionsExceeded,
+    MaxPayloadExceeded,
+    MaxControlLineExceeded,
+    MaxSubscriptionsExceeded,
+    DuplicateRoute,
+    RouteRemoved,
+    ServerShutdown,
+    AuthenticationExpired,
+    WrongGateway,
+    MissingAccount,
+    Revocation,
+    InternalClient,
+    MsgHeaderViolation,
+    NoRespondersRequiresHeaders,
+    ClusterNameConflict,
+    DuplicateRemoteLeafnodeConnection,
+    DuplicateClientID,
+    DuplicateServerName,
+    MinimumVersionRequired,
+    ClusterNamesIdentical,
+    Kicked,
+    ProxyNotTrusted,
+    ProxyRequired,
+}

src/NATS.Server/NatsClient.cs

@@ -65,6 +65,10 @@ public sealed class NatsClient : IDisposable
     public long InBytes;
     public long OutBytes;
 
+    // Close reason tracking
+    private int _skipFlushOnClose;
+    public bool ShouldSkipFlush => Volatile.Read(ref _skipFlushOnClose) != 0;
+
     // PING keepalive state
     private int _pingsOut;
     private long _lastIn;
@@ -174,13 +178,24 @@ public sealed class NatsClient : IDisposable
         catch (OperationCanceledException)
         {
             _logger.LogDebug("Client {ClientId} operation cancelled", Id);
+            MarkClosed(ClientClosedReason.ServerShutdown);
+        }
+        catch (IOException)
+        {
+            MarkClosed(ClientClosedReason.ReadError);
+        }
+        catch (SocketException)
+        {
+            MarkClosed(ClientClosedReason.ReadError);
         }
         catch (Exception ex)
         {
             _logger.LogDebug(ex, "Client {ClientId} connection error", Id);
+            MarkClosed(ClientClosedReason.ReadError);
         }
         finally
         {
+            MarkClosed(ClientClosedReason.ClientClosed);
             _outbound.Writer.TryComplete();
             try { _socket.Shutdown(SocketShutdown.Both); }
             catch (SocketException) { }
@@ -623,6 +638,57 @@ public sealed class NatsClient : IDisposable
         }
     }
 
+    /// <summary>
+    /// Marks this connection as closed with the given reason.
+    /// Sets skip-flush flag for error-related reasons.
+    /// Only the first call sets the reason (subsequent calls are no-ops).
+    /// </summary>
+    public void MarkClosed(ClientClosedReason reason)
+    {
+        if (CloseReason != ClientClosedReason.None)
+            return;
+
+        CloseReason = reason;
+
+        switch (reason)
+        {
+            case ClientClosedReason.ReadError:
+            case ClientClosedReason.WriteError:
+            case ClientClosedReason.SlowConsumerPendingBytes:
+            case ClientClosedReason.SlowConsumerWriteDeadline:
+            case ClientClosedReason.TlsHandshakeError:
+                Volatile.Write(ref _skipFlushOnClose, 1);
+                break;
+        }
+
+        _logger.LogDebug("Client {ClientId} connection closed: {CloseReason}", Id, reason);
+    }
+
+    /// <summary>
+    /// Flushes pending data (unless skip-flush is set) and closes the connection.
+    /// </summary>
+    public async Task FlushAndCloseAsync(bool minimalFlush = false)
+    {
+        if (!ShouldSkipFlush)
+        {
+            try
+            {
+                using var flushCts = new CancellationTokenSource(minimalFlush
+                    ? TimeSpan.FromMilliseconds(100)
+                    : TimeSpan.FromSeconds(1));
+                await _stream.FlushAsync(flushCts.Token);
+            }
+            catch (Exception)
+            {
+                // Best effort flush — don't let it prevent close
+            }
+        }
+
+        try { _socket.Shutdown(SocketShutdown.Both); }
+        catch (SocketException) { }
+        catch (ObjectDisposedException) { }
+    }
+
     public void RemoveAllSubscriptions(SubList subList)
     {
         foreach (var sub in _subs.Values)

src/NATS.Server/NatsOptions.cs

@@ -38,6 +38,18 @@ public sealed class NatsOptions
     // 0 = disabled
     public int MonitorHttpsPort { get; set; }
 
+    // Lifecycle / lame-duck mode
+    public TimeSpan LameDuckDuration { get; set; } = TimeSpan.FromMinutes(2);
+    public TimeSpan LameDuckGracePeriod { get; set; } = TimeSpan.FromSeconds(10);
+
+    // File paths
+    public string? PidFile { get; set; }
+    public string? PortsFileDir { get; set; }
+    public string? ConfigFile { get; set; }
+
+    // Profiling (0 = disabled)
+    public int ProfPort { get; set; }
+
     // TLS
     public string? TlsCert { get; set; }
     public string? TlsKey { get; set; }

src/NATS.Server/NatsServer.cs

@@ -2,9 +2,11 @@ using System.Collections.Concurrent;
 using System.Net;
 using System.Net.Security;
 using System.Net.Sockets;
+using System.Runtime.InteropServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using Microsoft.Extensions.Logging;
+using NATS.NKeys;
 using NATS.Server.Auth;
 using NATS.Server.Monitoring;
 using NATS.Server.Protocol;
@@ -25,6 +27,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly AuthService _authService;
     private readonly ConcurrentDictionary<string, Account> _accounts = new(StringComparer.Ordinal);
     private readonly Account _globalAccount;
+    private readonly Account _systemAccount;
     private readonly SslServerAuthenticationOptions? _sslOptions;
     private readonly TlsRateLimiter? _tlsRateLimiter;
     private Socket? _listener;
@@ -32,16 +35,205 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private ulong _nextClientId;
     private long _startTimeTicks;
 
+    private readonly CancellationTokenSource _quitCts = new();
+    private readonly TaskCompletionSource _shutdownComplete = new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private readonly TaskCompletionSource _acceptLoopExited = new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private int _shutdown;
+    private int _activeClientCount;
+
+    private int _lameDuck;
+
+    private readonly List<PosixSignalRegistration> _signalRegistrations = [];
+
+    private string? _portsFilePath;
+
+    private static readonly TimeSpan AcceptMinSleep = TimeSpan.FromMilliseconds(10);
+    private static readonly TimeSpan AcceptMaxSleep = TimeSpan.FromSeconds(1);
+
     public SubList SubList => _globalAccount.SubList;
     public ServerStats Stats => _stats;
     public DateTime StartTime => new(Interlocked.Read(ref _startTimeTicks), DateTimeKind.Utc);
     public string ServerId => _serverInfo.ServerId;
     public string ServerName => _serverInfo.ServerName;
     public int ClientCount => _clients.Count;
+    public int Port => _options.Port;
+    public Account SystemAccount => _systemAccount;
+    public string ServerNKey { get; }
+    public bool IsShuttingDown => Volatile.Read(ref _shutdown) != 0;
+    public bool IsLameDuckMode => Volatile.Read(ref _lameDuck) != 0;
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 
     public Task WaitForReadyAsync() => _listeningStarted.Task;
 
+    public void WaitForShutdown() => _shutdownComplete.Task.GetAwaiter().GetResult();
+
+    public async Task ShutdownAsync()
+    {
+        if (Interlocked.CompareExchange(ref _shutdown, 1, 0) != 0)
+            return; // Already shutting down
+
+        _logger.LogInformation("Initiating Shutdown...");
+
+        // Signal all internal loops to stop
+        await _quitCts.CancelAsync();
+
+        // Close listener to stop accept loop
+        _listener?.Close();
+
+        // Wait for accept loop to exit
+        await _acceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+
+        // Close all client connections — flush first, then mark closed
+        var flushTasks = new List<Task>();
+        foreach (var client in _clients.Values)
+        {
+            client.MarkClosed(ClientClosedReason.ServerShutdown);
+            flushTasks.Add(client.FlushAndCloseAsync(minimalFlush: true));
+        }
+        await Task.WhenAll(flushTasks).WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+
+        // Wait for active client tasks to drain (with timeout)
+        if (Volatile.Read(ref _activeClientCount) > 0)
+        {
+            using var drainCts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+            try
+            {
+                while (Volatile.Read(ref _activeClientCount) > 0 && !drainCts.IsCancellationRequested)
+                    await Task.Delay(50, drainCts.Token);
+            }
+            catch (OperationCanceledException) { }
+        }
+
+        // Stop monitor server
+        if (_monitorServer != null)
+            await _monitorServer.DisposeAsync();
+
+        DeletePidFile();
+        DeletePortsFile();
+
+        _logger.LogInformation("Server Exiting..");
+        _shutdownComplete.TrySetResult();
+    }
+
+    public async Task LameDuckShutdownAsync()
+    {
+        if (IsShuttingDown || Interlocked.CompareExchange(ref _lameDuck, 1, 0) != 0)
+            return;
+
+        _logger.LogInformation("Entering lame duck mode, stop accepting new clients");
+
+        // Close listener to stop accepting new connections
+        _listener?.Close();
+
+        // Wait for accept loop to exit
+        await _acceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+
+        var gracePeriod = _options.LameDuckGracePeriod;
+        if (gracePeriod < TimeSpan.Zero) gracePeriod = -gracePeriod;
+
+        // If no clients, go straight to shutdown
+        if (_clients.IsEmpty)
+        {
+            await ShutdownAsync();
+            return;
+        }
+
+        // Wait grace period for clients to drain naturally
+        _logger.LogInformation("Waiting {GracePeriod}ms grace period", gracePeriod.TotalMilliseconds);
+        try
+        {
+            await Task.Delay(gracePeriod, _quitCts.Token);
+        }
+        catch (OperationCanceledException) { return; }
+
+        if (_clients.IsEmpty)
+        {
+            await ShutdownAsync();
+            return;
+        }
+
+        // Stagger-close remaining clients
+        var dur = _options.LameDuckDuration - gracePeriod;
+        if (dur <= TimeSpan.Zero) dur = TimeSpan.FromSeconds(1);
+
+        var clients = _clients.Values.ToList();
+        var numClients = clients.Count;
+
+        if (numClients > 0)
+        {
+            _logger.LogInformation("Closing {Count} existing clients over {Duration}ms",
+                numClients, dur.TotalMilliseconds);
+
+            var sleepInterval = dur.Ticks / numClients;
+            if (sleepInterval < TimeSpan.TicksPerMillisecond)
+                sleepInterval = TimeSpan.TicksPerMillisecond;
+            if (sleepInterval > TimeSpan.TicksPerSecond)
+                sleepInterval = TimeSpan.TicksPerSecond;
+
+            for (int i = 0; i < clients.Count; i++)
+            {
+                clients[i].MarkClosed(ClientClosedReason.ServerShutdown);
+                await clients[i].FlushAndCloseAsync(minimalFlush: true);
+
+                if (i < clients.Count - 1)
+                {
+                    var jitter = Random.Shared.NextInt64(sleepInterval / 2, sleepInterval);
+                    try
+                    {
+                        await Task.Delay(TimeSpan.FromTicks(jitter), _quitCts.Token);
+                    }
+                    catch (OperationCanceledException) { break; }
+                }
+            }
+        }
+
+        await ShutdownAsync();
+    }
+
+    /// <summary>
+    /// Registers Unix signal handlers.
+    /// SIGTERM → shutdown, SIGUSR2 → lame duck, SIGUSR1 → log reopen (stub), SIGHUP → reload (stub).
+    /// </summary>
+    public void HandleSignals()
+    {
+        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGTERM, ctx =>
+        {
+            ctx.Cancel = true;
+            _logger.LogInformation("Trapped SIGTERM signal");
+            _ = Task.Run(async () => await ShutdownAsync());
+        }));
+
+        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGQUIT, ctx =>
+        {
+            ctx.Cancel = true;
+            _logger.LogInformation("Trapped SIGQUIT signal");
+            _ = Task.Run(async () => await ShutdownAsync());
+        }));
+
+        _signalRegistrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGHUP, ctx =>
+        {
+            ctx.Cancel = true;
+            _logger.LogWarning("Trapped SIGHUP signal — config reload not yet supported");
+        }));
+
+        // SIGUSR1 and SIGUSR2 only on non-Windows
+        if (!OperatingSystem.IsWindows())
+        {
+            _signalRegistrations.Add(PosixSignalRegistration.Create((PosixSignal)10, ctx =>
+            {
+                ctx.Cancel = true;
+                _logger.LogWarning("Trapped SIGUSR1 signal — log reopen not yet supported");
+            }));
+
+            _signalRegistrations.Add(PosixSignalRegistration.Create((PosixSignal)12, ctx =>
+            {
+                ctx.Cancel = true;
+                _logger.LogInformation("Trapped SIGUSR2 signal — entering lame duck mode");
+                _ = Task.Run(async () => await LameDuckShutdownAsync());
+            }));
+        }
+    }
+
     public NatsServer(NatsOptions options, ILoggerFactory loggerFactory)
     {
         _options = options;
@@ -50,6 +242,15 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         _authService = AuthService.Build(options);
         _globalAccount = new Account(Account.GlobalAccountName);
         _accounts[Account.GlobalAccountName] = _globalAccount;
+
+        // Create $SYS system account (stub -- no internal subscriptions yet)
+        _systemAccount = new Account("$SYS");
+        _accounts["$SYS"] = _systemAccount;
+
+        // Generate Ed25519 server NKey identity
+        using var serverKeyPair = KeyPair.CreatePair(PrefixByte.Server);
+        ServerNKey = serverKeyPair.GetPublicKey();
+
         _serverInfo = new ServerInfo
         {
             ServerId = Guid.NewGuid().ToString("N")[..20].ToUpperInvariant(),
@@ -75,6 +276,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
     public async Task StartAsync(CancellationToken ct)
     {
+        using var linked = CancellationTokenSource.CreateLinkedTokenSource(ct, _quitCts.Token);
+
         _listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
         _listener.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
         _listener.Bind(new IPEndPoint(
@@ -82,23 +285,68 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             _options.Port));
         Interlocked.Exchange(ref _startTimeTicks, DateTime.UtcNow.Ticks);
         _listener.Listen(128);
+
+        // Resolve ephemeral port if port=0
+        if (_options.Port == 0)
+        {
+            var actualPort = ((IPEndPoint)_listener.LocalEndPoint!).Port;
+            _options.Port = actualPort;
+            _serverInfo.Port = actualPort;
+        }
+
         _listeningStarted.TrySetResult();
 
-        _logger.LogInformation("Listening on {Host}:{Port}", _options.Host, _options.Port);
+        _logger.LogInformation("Listening for client connections on {Host}:{Port}", _options.Host, _options.Port);
+
+        // Warn about stub features
+        if (_options.ConfigFile != null)
+            _logger.LogWarning("Config file parsing not yet supported (file: {ConfigFile})", _options.ConfigFile);
+        if (_options.ProfPort > 0)
+            _logger.LogWarning("Profiling endpoint not yet supported (port: {ProfPort})", _options.ProfPort);
 
         if (_options.MonitorPort > 0)
         {
             _monitorServer = new MonitorServer(this, _options, _stats, _loggerFactory);
-            await _monitorServer.StartAsync(ct);
+            await _monitorServer.StartAsync(linked.Token);
         }
 
+        WritePidFile();
+        WritePortsFile();
+
+        var tmpDelay = AcceptMinSleep;
+
         try
         {
-            while (!ct.IsCancellationRequested)
+            while (!linked.Token.IsCancellationRequested)
             {
-                var socket = await _listener.AcceptAsync(ct);
+                Socket socket;
+                try
+                {
+                    socket = await _listener.AcceptAsync(linked.Token);
+                    tmpDelay = AcceptMinSleep; // Reset on success
+                }
+                catch (OperationCanceledException)
+                {
+                    break;
+                }
+                catch (ObjectDisposedException)
+                {
+                    break;
+                }
+                catch (SocketException ex)
+                {
+                    if (IsShuttingDown || IsLameDuckMode)
+                        break;
 
-                // Check MaxConnections before creating the client
+                    _logger.LogError(ex, "Temporary accept error, sleeping {Delay}ms", tmpDelay.TotalMilliseconds);
+                    try { await Task.Delay(tmpDelay, linked.Token); }
+                    catch (OperationCanceledException) { break; }
+
+                    tmpDelay = TimeSpan.FromTicks(Math.Min(tmpDelay.Ticks * 2, AcceptMaxSleep.Ticks));
+                    continue;
+                }
+
+                // Check MaxConnections
                 if (_options.MaxConnections > 0 && _clients.Count >= _options.MaxConnections)
                 {
                     _logger.LogWarning("Client connection rejected: maximum connections ({MaxConnections}) exceeded",
@@ -108,13 +356,13 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                         var stream = new NetworkStream(socket, ownsSocket: false);
                         var errBytes = Encoding.ASCII.GetBytes(
                             $"-ERR '{NatsProtocol.ErrMaxConnectionsExceeded}'\r\n");
-                        await stream.WriteAsync(errBytes, ct);
+                        await stream.WriteAsync(errBytes, linked.Token);
-                        await stream.FlushAsync(ct);
+                        await stream.FlushAsync(linked.Token);
                         stream.Dispose();
                     }
-                    catch (Exception ex)
+                    catch (Exception ex2)
                     {
-                        _logger.LogDebug(ex, "Failed to send -ERR to rejected client");
+                        _logger.LogDebug(ex2, "Failed to send -ERR to rejected client");
                     }
                     finally
                     {
@@ -125,16 +373,21 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
                 var clientId = Interlocked.Increment(ref _nextClientId);
                 Interlocked.Increment(ref _stats.TotalConnections);
+                Interlocked.Increment(ref _activeClientCount);
 
                 _logger.LogDebug("Client {ClientId} connected from {RemoteEndpoint}", clientId, socket.RemoteEndPoint);
 
-                _ = AcceptClientAsync(socket, clientId, ct);
+                _ = AcceptClientAsync(socket, clientId, linked.Token);
             }
         }
         catch (OperationCanceledException)
         {
             _logger.LogDebug("Accept loop cancelled, server shutting down");
         }
+        finally
+        {
+            _acceptLoopExited.TrySetResult();
+        }
     }
 
     private async Task AcceptClientAsync(Socket socket, ulong clientId, CancellationToken ct)
@@ -217,8 +470,9 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         }
         finally
         {
-            _logger.LogDebug("Client {ClientId} disconnected", client.Id);
+            _logger.LogDebug("Client {ClientId} disconnected (reason: {CloseReason})", client.Id, client.CloseReason);
             RemoveClient(client);
+            Interlocked.Decrement(ref _activeClientCount);
         }
     }
 
@@ -327,10 +581,75 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         client.Account?.RemoveClient(client.Id);
     }
 
+    private void WritePidFile()
+    {
+        if (string.IsNullOrEmpty(_options.PidFile)) return;
+        try
+        {
+            File.WriteAllText(_options.PidFile, Environment.ProcessId.ToString());
+            _logger.LogDebug("Wrote PID file {PidFile}", _options.PidFile);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error writing PID file {PidFile}", _options.PidFile);
+        }
+    }
+
+    private void DeletePidFile()
+    {
+        if (string.IsNullOrEmpty(_options.PidFile)) return;
+        try
+        {
+            if (File.Exists(_options.PidFile))
+                File.Delete(_options.PidFile);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error deleting PID file {PidFile}", _options.PidFile);
+        }
+    }
+
+    private void WritePortsFile()
+    {
+        if (string.IsNullOrEmpty(_options.PortsFileDir)) return;
+        try
+        {
+            var exeName = Path.GetFileNameWithoutExtension(Environment.ProcessPath ?? "nats-server");
+            var fileName = $"{exeName}_{Environment.ProcessId}.ports";
+            _portsFilePath = Path.Combine(_options.PortsFileDir, fileName);
+
+            var ports = new { client = _options.Port, monitor = _options.MonitorPort > 0 ? _options.MonitorPort : (int?)null };
+            var json = System.Text.Json.JsonSerializer.Serialize(ports);
+            File.WriteAllText(_portsFilePath, json);
+            _logger.LogDebug("Wrote ports file {PortsFile}", _portsFilePath);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error writing ports file to {PortsFileDir}", _options.PortsFileDir);
+        }
+    }
+
+    private void DeletePortsFile()
+    {
+        if (_portsFilePath == null) return;
+        try
+        {
+            if (File.Exists(_portsFilePath))
+                File.Delete(_portsFilePath);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error deleting ports file {PortsFile}", _portsFilePath);
+        }
+    }
+
     public void Dispose()
     {
-        if (_monitorServer != null)
+        if (!IsShuttingDown)
-            _monitorServer.DisposeAsync().AsTask().GetAwaiter().GetResult();
+            ShutdownAsync().GetAwaiter().GetResult();
+        foreach (var reg in _signalRegistrations)
+            reg.Dispose();
+        _quitCts.Dispose();
         _tlsRateLimiter?.Dispose();
         _listener?.Dispose();
         foreach (var client in _clients.Values)

tests/NATS.Server.Tests/ServerTests.cs

@@ -213,6 +213,39 @@ public class ServerTests : IAsyncLifetime
     }
 }
 
+public class EphemeralPortTests
+{
+    [Fact]
+    public async Task Server_resolves_ephemeral_port()
+    {
+        using var cts = new CancellationTokenSource();
+        var server = new NatsServer(new NatsOptions { Port = 0 }, NullLoggerFactory.Instance);
+
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            // Port should have been resolved to a real port
+            server.Port.ShouldBeGreaterThan(0);
+
+            // Connect a raw socket to prove the port actually works
+            using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            await client.ConnectAsync(IPAddress.Loopback, server.Port);
+
+            var buf = new byte[4096];
+            var n = await client.ReceiveAsync(buf, SocketFlags.None);
+            var response = Encoding.ASCII.GetString(buf, 0, n);
+            response.ShouldStartWith("INFO ");
+        }
+        finally
+        {
+            await cts.CancelAsync();
+            server.Dispose();
+        }
+    }
+}
+
 public class MaxConnectionsTests : IAsyncLifetime
 {
     private readonly NatsServer _server;
@@ -423,3 +456,448 @@ public class PingKeepaliveTests : IAsyncLifetime
         client.Dispose();
     }
 }
+
+public class CloseReasonTests : IAsyncLifetime
+{
+    private readonly NatsServer _server;
+    private readonly int _port;
+    private readonly CancellationTokenSource _cts = new();
+
+    public CloseReasonTests()
+    {
+        _port = GetFreePort();
+        _server = new NatsServer(new NatsOptions { Port = _port }, NullLoggerFactory.Instance);
+    }
+
+    public async Task InitializeAsync()
+    {
+        _ = _server.StartAsync(_cts.Token);
+        await _server.WaitForReadyAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    [Fact]
+    public async Task Client_close_reason_set_on_normal_disconnect()
+    {
+        // Connect a raw TCP client
+        using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await client.ConnectAsync(IPAddress.Loopback, _port);
+
+        // Read INFO
+        var buf = new byte[4096];
+        await client.ReceiveAsync(buf, SocketFlags.None);
+
+        // Send CONNECT + PING, wait for PONG
+        await client.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+        var sb = new StringBuilder();
+        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        while (!sb.ToString().Contains("PONG"))
+        {
+            var n = await client.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        sb.ToString().ShouldContain("PONG");
+
+        // Get the NatsClient from the server
+        var natsClient = _server.GetClients().First();
+
+        // Close the TCP socket (normal client disconnect)
+        client.Shutdown(SocketShutdown.Both);
+        client.Close();
+
+        // Wait for the server to detect the disconnect
+        await Task.Delay(500);
+
+        // The close reason should be ClientClosed (normal disconnect falls through to finally)
+        natsClient.CloseReason.ShouldBe(ClientClosedReason.ClientClosed);
+    }
+}
+
+public class ServerIdentityTests
+{
+    [Fact]
+    public void Server_creates_system_account()
+    {
+        var server = new NatsServer(new NatsOptions { Port = 0 }, NullLoggerFactory.Instance);
+        server.SystemAccount.ShouldNotBeNull();
+        server.SystemAccount.Name.ShouldBe("$SYS");
+        server.Dispose();
+    }
+
+    [Fact]
+    public void Server_generates_nkey_identity()
+    {
+        var server = new NatsServer(new NatsOptions { Port = 0 }, NullLoggerFactory.Instance);
+        server.ServerNKey.ShouldNotBeNullOrEmpty();
+        // Server NKey public keys start with 'N'
+        server.ServerNKey[0].ShouldBe('N');
+        server.Dispose();
+    }
+}
+
+public class FlushBeforeCloseTests
+{
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    private static async Task<string> ReadUntilAsync(Socket sock, string expected, int timeoutMs = 5000)
+    {
+        using var cts = new CancellationTokenSource(timeoutMs);
+        var sb = new StringBuilder();
+        var buf = new byte[4096];
+        while (!sb.ToString().Contains(expected))
+        {
+            var n = await sock.ReceiveAsync(buf, SocketFlags.None, cts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        return sb.ToString();
+    }
+
+    [Fact]
+    public async Task Shutdown_flushes_pending_data_to_clients()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            // Connect a subscriber via raw socket
+            using var sub = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            await sub.ConnectAsync(IPAddress.Loopback, port);
+
+            // Read INFO
+            var buf = new byte[4096];
+            await sub.ReceiveAsync(buf, SocketFlags.None);
+
+            // Subscribe to "foo"
+            await sub.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nSUB foo 1\r\nPING\r\n"));
+            var pong = await ReadUntilAsync(sub, "PONG");
+            pong.ShouldContain("PONG");
+
+            // Connect a publisher
+            using var pub = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            await pub.ConnectAsync(IPAddress.Loopback, port);
+            await pub.ReceiveAsync(buf, SocketFlags.None); // INFO
+
+            // Publish "Hello" to "foo"
+            await pub.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPUB foo 5\r\nHello\r\n"));
+
+            // Wait briefly for delivery
+            await Task.Delay(200);
+
+            // Read from subscriber to verify MSG was received
+            var msg = await ReadUntilAsync(sub, "Hello\r\n");
+            msg.ShouldContain("MSG foo 1 5\r\nHello\r\n");
+        }
+        finally
+        {
+            await server.ShutdownAsync();
+            server.Dispose();
+        }
+    }
+}
+
+public class GracefulShutdownTests
+{
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_disconnects_all_clients()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        // Connect 2 raw TCP clients
+        using var client1 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await client1.ConnectAsync(IPAddress.Loopback, port);
+        var buf = new byte[4096];
+        await client1.ReceiveAsync(buf, SocketFlags.None); // INFO
+
+        using var client2 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await client2.ConnectAsync(IPAddress.Loopback, port);
+        await client2.ReceiveAsync(buf, SocketFlags.None); // INFO
+
+        // Send CONNECT so both are registered
+        await client1.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+        await client2.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+
+        // Wait for PONG from both (confirming they are registered)
+        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        await client1.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
+        await client2.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
+
+        server.ClientCount.ShouldBe(2);
+
+        await server.ShutdownAsync();
+
+        server.ClientCount.ShouldBe(0);
+        server.Dispose();
+    }
+
+    [Fact]
+    public async Task WaitForShutdown_blocks_until_shutdown()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        // Start WaitForShutdown in background
+        var waitTask = Task.Run(() => server.WaitForShutdown());
+
+        // Give it a moment -- it should NOT complete yet
+        await Task.Delay(200);
+        waitTask.IsCompleted.ShouldBeFalse();
+
+        // Trigger shutdown
+        await server.ShutdownAsync();
+
+        // WaitForShutdown should complete within 5 seconds
+        var completed = await Task.WhenAny(waitTask, Task.Delay(TimeSpan.FromSeconds(5)));
+        completed.ShouldBe(waitTask);
+
+        server.Dispose();
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_is_idempotent()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        // Call ShutdownAsync 3 times -- should not throw
+        await server.ShutdownAsync();
+        await server.ShutdownAsync();
+        await server.ShutdownAsync();
+
+        server.IsShuttingDown.ShouldBeTrue();
+        server.Dispose();
+    }
+
+    [Fact]
+    public async Task Accept_loop_waits_for_active_clients()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        // Connect a client
+        using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await client.ConnectAsync(IPAddress.Loopback, port);
+        var buf = new byte[4096];
+        await client.ReceiveAsync(buf, SocketFlags.None); // INFO
+        await client.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        await client.ReceiveAsync(buf, SocketFlags.None, readCts.Token); // PONG
+
+        // ShutdownAsync should complete within 10 seconds (doesn't hang)
+        var shutdownTask = server.ShutdownAsync();
+        var completed = await Task.WhenAny(shutdownTask, Task.Delay(TimeSpan.FromSeconds(10)));
+        completed.ShouldBe(shutdownTask);
+
+        server.Dispose();
+    }
+}
+
+public class LameDuckTests
+{
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    [Fact]
+    public async Task LameDuckShutdown_stops_accepting_new_connections()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(
+            new NatsOptions
+            {
+                Port = port,
+                LameDuckDuration = TimeSpan.FromSeconds(3),
+                LameDuckGracePeriod = TimeSpan.FromMilliseconds(500),
+            },
+            NullLoggerFactory.Instance);
+
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            // Connect 1 client
+            using var client1 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            await client1.ConnectAsync(IPAddress.Loopback, port);
+            var buf = new byte[4096];
+            await client1.ReceiveAsync(buf, SocketFlags.None); // INFO
+            await client1.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+            using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+            await client1.ReceiveAsync(buf, SocketFlags.None, readCts.Token); // PONG
+
+            // Start lame duck (don't await yet)
+            var lameDuckTask = server.LameDuckShutdownAsync();
+
+            // Wait briefly for listener to close
+            await Task.Delay(300);
+
+            // Verify lame duck mode is active
+            server.IsLameDuckMode.ShouldBeTrue();
+
+            // Try connecting a new client -- should fail (connection refused)
+            using var client2 = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            var connectAction = async () =>
+            {
+                await client2.ConnectAsync(IPAddress.Loopback, port);
+            };
+            await connectAction.ShouldThrowAsync<SocketException>();
+
+            // Await the lame duck task with timeout
+            var completed = await Task.WhenAny(lameDuckTask, Task.Delay(TimeSpan.FromSeconds(15)));
+            completed.ShouldBe(lameDuckTask);
+        }
+        finally
+        {
+            server.Dispose();
+        }
+    }
+
+    [Fact]
+    public async Task LameDuckShutdown_eventually_closes_all_clients()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(
+            new NatsOptions
+            {
+                Port = port,
+                LameDuckDuration = TimeSpan.FromSeconds(2),
+                LameDuckGracePeriod = TimeSpan.FromMilliseconds(200),
+            },
+            NullLoggerFactory.Instance);
+
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            // Connect 3 clients via raw sockets
+            var clients = new List<Socket>();
+            var buf = new byte[4096];
+            for (int i = 0; i < 3; i++)
+            {
+                var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+                await sock.ConnectAsync(IPAddress.Loopback, port);
+                await sock.ReceiveAsync(buf, SocketFlags.None); // INFO
+                await sock.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+                using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+                await sock.ReceiveAsync(buf, SocketFlags.None, readCts.Token); // PONG
+                clients.Add(sock);
+            }
+
+            server.ClientCount.ShouldBe(3);
+
+            // Await LameDuckShutdownAsync
+            var lameDuckTask = server.LameDuckShutdownAsync();
+            var completed = await Task.WhenAny(lameDuckTask, Task.Delay(TimeSpan.FromSeconds(15)));
+            completed.ShouldBe(lameDuckTask);
+
+            server.ClientCount.ShouldBe(0);
+
+            foreach (var sock in clients)
+                sock.Dispose();
+        }
+        finally
+        {
+            server.Dispose();
+        }
+    }
+}
+
+public class PidFileTests : IDisposable
+{
+    private readonly string _tempDir = Path.Combine(Path.GetTempPath(), $"nats-test-{Guid.NewGuid():N}");
+
+    public PidFileTests() => Directory.CreateDirectory(_tempDir);
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+            Directory.Delete(_tempDir, recursive: true);
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    [Fact]
+    public async Task Server_writes_pid_file_on_startup()
+    {
+        var pidFile = Path.Combine(_tempDir, "nats.pid");
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port, PidFile = pidFile }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        File.Exists(pidFile).ShouldBeTrue();
+        var content = await File.ReadAllTextAsync(pidFile);
+        int.Parse(content).ShouldBe(Environment.ProcessId);
+
+        await server.ShutdownAsync();
+        File.Exists(pidFile).ShouldBeFalse();
+
+        server.Dispose();
+    }
+
+    [Fact]
+    public async Task Server_writes_ports_file_on_startup()
+    {
+        var port = GetFreePort();
+        var server = new NatsServer(new NatsOptions { Port = port, PortsFileDir = _tempDir }, NullLoggerFactory.Instance);
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        var portsFiles = Directory.GetFiles(_tempDir, "*.ports");
+        portsFiles.Length.ShouldBe(1);
+
+        var content = await File.ReadAllTextAsync(portsFiles[0]);
+        content.ShouldContain($"\"client\":{port}");
+
+        await server.ShutdownAsync();
+        Directory.GetFiles(_tempDir, "*.ports").Length.ShouldBe(0);
+
+        server.Dispose();
+    }
+}