Issue #22: implement pipe client and frame protocol

Verified with dotnet build src\\MxGateway.sln and dotnet test src\\MxGateway.sln.

docs/gateway-process-design.md

@@ -411,7 +411,7 @@ session ids as protocol faults and close the session.
 
 `WorkerClient` is the gateway-side object that owns one worker connection.
 
-Suggested public shape:
+Current public shape:
 
 ```csharp
 public interface IWorkerClient : IAsyncDisposable
@@ -419,6 +419,7 @@ public interface IWorkerClient : IAsyncDisposable
     string SessionId { get; }
     int? ProcessId { get; }
     WorkerClientState State { get; }
+    DateTimeOffset LastHeartbeatAt { get; }
 
     Task StartAsync(CancellationToken cancellationToken);
     Task<WorkerCommandReply> InvokeAsync(
@@ -438,12 +439,17 @@ Internally it owns:
 - pipe stream,
 - read loop,
 - write loop,
-- bounded outbound command/control channel,
+- outbound command/control channel serialized by the write loop,
 - bounded inbound event channel,
 - pending command dictionary keyed by correlation id,
 - heartbeat monitor,
 - terminal fault source.
 
+`StartAsync` sends `GatewayHello`, verifies the `WorkerHello` protocol version
+and nonce, waits for `WorkerReady`, and only then exposes `Ready` state. The
+read loop starts after readiness so the handshake has a single owner for its
+ordered frames.
+
 ### Read Loop
 
 The read loop:

src/MxGateway.Server/Workers/IWorkerClient.cs

@@ -0,0 +1,27 @@
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Server.Workers;
+
+public interface IWorkerClient : IAsyncDisposable
+{
+    string SessionId { get; }
+
+    int? ProcessId { get; }
+
+    WorkerClientState State { get; }
+
+    DateTimeOffset LastHeartbeatAt { get; }
+
+    Task StartAsync(CancellationToken cancellationToken);
+
+    Task<WorkerCommandReply> InvokeAsync(
+        WorkerCommand command,
+        TimeSpan timeout,
+        CancellationToken cancellationToken);
+
+    IAsyncEnumerable<WorkerEvent> ReadEventsAsync(CancellationToken cancellationToken);
+
+    Task ShutdownAsync(TimeSpan timeout, CancellationToken cancellationToken);
+
+    void Kill(string reason);
+}

src/MxGateway.Server/Workers/WorkerClient.cs

@@ -0,0 +1,755 @@
+using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+using Google.Protobuf.WellKnownTypes;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Server.Metrics;
+
+namespace MxGateway.Server.Workers;
+
+public sealed class WorkerClient : IWorkerClient
+{
+    private const string GatewayVersionFallback = "unknown";
+    private readonly object _syncRoot = new();
+    private readonly WorkerClientConnection _connection;
+    private readonly WorkerClientOptions _options;
+    private readonly GatewayMetrics? _metrics;
+    private readonly TimeProvider _timeProvider;
+    private readonly ILogger<WorkerClient> _logger;
+    private readonly WorkerFrameReader _reader;
+    private readonly WorkerFrameWriter _writer;
+    private readonly Channel<WorkerEnvelope> _outboundEnvelopes;
+    private readonly Channel<WorkerEvent> _events;
+    private readonly ConcurrentDictionary<string, PendingCommand> _pendingCommands = new(StringComparer.Ordinal);
+    private readonly CancellationTokenSource _stopCts = new();
+    private long _nextSequence;
+    private WorkerClientState _state;
+    private DateTimeOffset _lastHeartbeatAt;
+    private int? _processId;
+    private Task? _readLoopTask;
+    private Task? _writeLoopTask;
+    private Task? _heartbeatLoopTask;
+    private bool _disposed;
+
+    public WorkerClient(
+        WorkerClientConnection connection,
+        WorkerClientOptions? options = null,
+        GatewayMetrics? metrics = null,
+        TimeProvider? timeProvider = null,
+        ILogger<WorkerClient>? logger = null)
+    {
+        _connection = connection ?? throw new ArgumentNullException(nameof(connection));
+        _options = options ?? new WorkerClientOptions();
+        _metrics = metrics;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _logger = logger ?? NullLogger<WorkerClient>.Instance;
+        _reader = new WorkerFrameReader(connection.Stream, connection.FrameOptions);
+        _writer = new WorkerFrameWriter(connection.Stream, connection.FrameOptions);
+        _outboundEnvelopes = Channel.CreateUnbounded<WorkerEnvelope>(
+            new UnboundedChannelOptions
+            {
+                SingleReader = true,
+                SingleWriter = false,
+                AllowSynchronousContinuations = false,
+            });
+        _events = Channel.CreateBounded<WorkerEvent>(
+            new BoundedChannelOptions(_options.EventChannelCapacity)
+            {
+                SingleReader = false,
+                SingleWriter = true,
+                FullMode = BoundedChannelFullMode.Wait,
+                AllowSynchronousContinuations = false,
+            });
+        _lastHeartbeatAt = _timeProvider.GetUtcNow();
+    }
+
+    public string SessionId => _connection.SessionId;
+
+    public int? ProcessId
+    {
+        get
+        {
+            lock (_syncRoot)
+            {
+                return _processId;
+            }
+        }
+    }
+
+    public WorkerClientState State
+    {
+        get
+        {
+            lock (_syncRoot)
+            {
+                return _state;
+            }
+        }
+    }
+
+    public DateTimeOffset LastHeartbeatAt
+    {
+        get
+        {
+            lock (_syncRoot)
+            {
+                return _lastHeartbeatAt;
+            }
+        }
+    }
+
+    public async Task StartAsync(CancellationToken cancellationToken)
+    {
+        ThrowIfDisposed();
+        TransitionFromCreatedToHandshaking();
+
+        _writeLoopTask = Task.Run(WriteLoopAsync);
+        await EnqueueAsync(CreateGatewayHelloEnvelope(), cancellationToken).ConfigureAwait(false);
+
+        WorkerEnvelope helloEnvelope = await ReadHandshakeEnvelopeAsync(
+            WorkerEnvelope.BodyOneofCase.WorkerHello,
+            cancellationToken).ConfigureAwait(false);
+        ValidateWorkerHello(helloEnvelope.WorkerHello);
+
+        WorkerEnvelope readyEnvelope = await ReadHandshakeEnvelopeAsync(
+            WorkerEnvelope.BodyOneofCase.WorkerReady,
+            cancellationToken).ConfigureAwait(false);
+        MarkReady(readyEnvelope.WorkerReady);
+
+        _readLoopTask = Task.Run(ReadLoopAsync);
+        _heartbeatLoopTask = Task.Run(HeartbeatLoopAsync);
+    }
+
+    public async Task<WorkerCommandReply> InvokeAsync(
+        WorkerCommand command,
+        TimeSpan timeout,
+        CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+        ThrowIfDisposed();
+        EnsureReady();
+
+        if (timeout <= TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(timeout), timeout, "Command timeout must be greater than zero.");
+        }
+
+        string correlationId = Guid.NewGuid().ToString("N");
+        string method = GetCommandMethod(command);
+        PendingCommand pendingCommand = new(
+            correlationId,
+            method,
+            _timeProvider.GetTimestamp());
+
+        if (!_pendingCommands.TryAdd(correlationId, pendingCommand))
+        {
+            throw new InvalidOperationException("Generated a duplicate command correlation id.");
+        }
+
+        _metrics?.CommandStarted(method);
+
+        try
+        {
+            await EnqueueAsync(CreateCommandEnvelope(correlationId, command), cancellationToken).ConfigureAwait(false);
+            using CancellationTokenSource timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            Task timeoutTask = Task.Delay(timeout, timeoutCts.Token);
+            Task<WorkerCommandReply> replyTask = pendingCommand.Task;
+            Task completedTask = await Task.WhenAny(replyTask, timeoutTask).ConfigureAwait(false);
+
+            if (completedTask == replyTask)
+            {
+                await timeoutCts.CancelAsync().ConfigureAwait(false);
+                return await replyTask.ConfigureAwait(false);
+            }
+
+            if (cancellationToken.IsCancellationRequested)
+            {
+                RemovePendingCommandAsFailed(
+                    correlationId,
+                    pendingCommand,
+                    WorkerClientErrorCode.GatewayShutdown,
+                    "Command wait was canceled.");
+                cancellationToken.ThrowIfCancellationRequested();
+            }
+
+            RemovePendingCommandAsFailed(
+                correlationId,
+                pendingCommand,
+                WorkerClientErrorCode.CommandTimeout,
+                $"Worker command {method} timed out after {timeout}.");
+
+            throw new WorkerClientException(
+                WorkerClientErrorCode.CommandTimeout,
+                $"Worker command {method} timed out after {timeout}.");
+        }
+        catch
+        {
+            _pendingCommands.TryRemove(correlationId, out _);
+            throw;
+        }
+    }
+
+    public async IAsyncEnumerable<WorkerEvent> ReadEventsAsync(
+        [EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        await foreach (WorkerEvent workerEvent in _events.Reader.ReadAllAsync(cancellationToken).ConfigureAwait(false))
+        {
+            yield return workerEvent;
+        }
+    }
+
+    public async Task ShutdownAsync(TimeSpan timeout, CancellationToken cancellationToken)
+    {
+        ThrowIfDisposed();
+        if (timeout <= TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(nameof(timeout), timeout, "Shutdown timeout must be greater than zero.");
+        }
+
+        WorkerClientState state = State;
+        if (state is WorkerClientState.Closed or WorkerClientState.Faulted)
+        {
+            return;
+        }
+
+        MarkClosing();
+        await EnqueueAsync(CreateShutdownEnvelope(timeout, "gateway-shutdown"), cancellationToken).ConfigureAwait(false);
+        _outboundEnvelopes.Writer.TryComplete();
+
+        using CancellationTokenSource timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+        timeoutCts.CancelAfter(timeout);
+        try
+        {
+            await WaitForBackgroundTasksAsync(timeoutCts.Token).ConfigureAwait(false);
+            MarkClosed("shutdown");
+        }
+        catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
+        {
+            SetFaulted(
+                WorkerClientErrorCode.ShutdownTimeout,
+                "Worker shutdown timed out.",
+                null);
+            throw new WorkerClientException(
+                WorkerClientErrorCode.ShutdownTimeout,
+                $"Worker shutdown timed out after {timeout}.");
+        }
+    }
+
+    public void Kill(string reason)
+    {
+        ThrowIfDisposed();
+        _connection.ProcessHandle?.Process.Kill(entireProcessTree: true);
+        _metrics?.WorkerKilled(reason);
+        SetFaulted(
+            WorkerClientErrorCode.WorkerFaulted,
+            $"Worker was killed by the gateway: {reason}.",
+            null);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_disposed)
+        {
+            return;
+        }
+
+        _disposed = true;
+        _stopCts.Cancel();
+        _outboundEnvelopes.Writer.TryComplete();
+        _events.Writer.TryComplete();
+        CompletePendingCommands(
+            new WorkerClientException(
+                WorkerClientErrorCode.GatewayShutdown,
+                "Worker client was disposed."));
+
+        await WaitForBackgroundTasksAsync(CancellationToken.None).ConfigureAwait(false);
+        await _connection.Stream.DisposeAsync().ConfigureAwait(false);
+        _connection.ProcessHandle?.Dispose();
+        _stopCts.Dispose();
+    }
+
+    private async Task WriteLoopAsync()
+    {
+        try
+        {
+            await foreach (WorkerEnvelope envelope in _outboundEnvelopes.Reader.ReadAllAsync(_stopCts.Token).ConfigureAwait(false))
+            {
+                await _writer.WriteAsync(envelope, _stopCts.Token).ConfigureAwait(false);
+            }
+        }
+        catch (OperationCanceledException) when (_stopCts.IsCancellationRequested || IsTerminalState())
+        {
+        }
+        catch (Exception exception)
+        {
+            SetFaulted(
+                WorkerClientErrorCode.WriteFailed,
+                "Worker pipe write failed.",
+                exception);
+        }
+    }
+
+    private async Task ReadLoopAsync()
+    {
+        try
+        {
+            while (!_stopCts.IsCancellationRequested)
+            {
+                WorkerEnvelope envelope = await _reader.ReadAsync(_stopCts.Token).ConfigureAwait(false);
+                await DispatchEnvelopeAsync(envelope, _stopCts.Token).ConfigureAwait(false);
+            }
+        }
+        catch (OperationCanceledException) when (_stopCts.IsCancellationRequested || IsTerminalState())
+        {
+        }
+        catch (WorkerFrameProtocolException exception) when (exception.ErrorCode == WorkerFrameProtocolErrorCode.EndOfStream)
+        {
+            SetFaulted(
+                WorkerClientErrorCode.PipeDisconnected,
+                "Worker pipe disconnected.",
+                exception);
+        }
+        catch (Exception exception)
+        {
+            SetFaulted(
+                WorkerClientErrorCode.ProtocolViolation,
+                "Worker read loop failed.",
+                exception);
+        }
+    }
+
+    private async Task HeartbeatLoopAsync()
+    {
+        try
+        {
+            while (!_stopCts.IsCancellationRequested)
+            {
+                await Task.Delay(_options.HeartbeatCheckInterval, _stopCts.Token).ConfigureAwait(false);
+                if (State != WorkerClientState.Ready)
+                {
+                    continue;
+                }
+
+                DateTimeOffset lastHeartbeatAt = LastHeartbeatAt;
+                DateTimeOffset now = _timeProvider.GetUtcNow();
+                if (now - lastHeartbeatAt <= _options.HeartbeatGrace)
+                {
+                    continue;
+                }
+
+                _metrics?.HeartbeatFailed(SessionId);
+                SetFaulted(
+                    WorkerClientErrorCode.HeartbeatExpired,
+                    $"Worker heartbeat expired. Last heartbeat was at {lastHeartbeatAt:O}.",
+                    null);
+            }
+        }
+        catch (OperationCanceledException) when (_stopCts.IsCancellationRequested || IsTerminalState())
+        {
+        }
+    }
+
+    private async Task DispatchEnvelopeAsync(
+        WorkerEnvelope envelope,
+        CancellationToken cancellationToken)
+    {
+        switch (envelope.BodyCase)
+        {
+            case WorkerEnvelope.BodyOneofCase.WorkerCommandReply:
+                CompleteCommand(envelope);
+                break;
+            case WorkerEnvelope.BodyOneofCase.WorkerEvent:
+                await EnqueueWorkerEventAsync(envelope.WorkerEvent, cancellationToken).ConfigureAwait(false);
+                break;
+            case WorkerEnvelope.BodyOneofCase.WorkerHeartbeat:
+                MarkHeartbeat(envelope.WorkerHeartbeat);
+                break;
+            case WorkerEnvelope.BodyOneofCase.WorkerFault:
+                SetFaulted(
+                    WorkerClientErrorCode.WorkerFaulted,
+                    CreateWorkerFaultMessage(envelope.WorkerFault),
+                    null);
+                break;
+            case WorkerEnvelope.BodyOneofCase.WorkerShutdownAck:
+                MarkClosed("worker-shutdown-ack");
+                break;
+            default:
+                SetFaulted(
+                    WorkerClientErrorCode.ProtocolViolation,
+                    $"Worker sent unexpected envelope body {envelope.BodyCase}.",
+                    null);
+                break;
+        }
+    }
+
+    private async Task EnqueueWorkerEventAsync(
+        WorkerEvent workerEvent,
+        CancellationToken cancellationToken)
+    {
+        if (workerEvent.Event is not null)
+        {
+            _metrics?.EventReceived(SessionId, workerEvent.Event.Family.ToString());
+        }
+
+        if (!await _events.Writer.WaitToWriteAsync(cancellationToken).ConfigureAwait(false))
+        {
+            return;
+        }
+
+        if (!_events.Writer.TryWrite(workerEvent))
+        {
+            _metrics?.QueueOverflow("worker-events");
+            SetFaulted(
+                WorkerClientErrorCode.ProtocolViolation,
+                "Worker event channel rejected an event.",
+                null);
+        }
+    }
+
+    private void CompleteCommand(WorkerEnvelope envelope)
+    {
+        string correlationId = envelope.CorrelationId;
+        if (string.IsNullOrWhiteSpace(correlationId))
+        {
+            correlationId = envelope.WorkerCommandReply.Reply?.CorrelationId ?? string.Empty;
+        }
+
+        if (!_pendingCommands.TryRemove(correlationId, out PendingCommand? pendingCommand))
+        {
+            _logger.LogDebug(
+                "Ignoring late or unknown worker command reply for session {SessionId} and correlation {CorrelationId}.",
+                SessionId,
+                correlationId);
+            return;
+        }
+
+        TimeSpan duration = _timeProvider.GetElapsedTime(pendingCommand.StartTimestamp);
+        _metrics?.CommandSucceeded(pendingCommand.Method, duration);
+        pendingCommand.SetResult(envelope.WorkerCommandReply);
+    }
+
+    private void RemovePendingCommandAsFailed(
+        string correlationId,
+        PendingCommand pendingCommand,
+        WorkerClientErrorCode errorCode,
+        string message)
+    {
+        if (!_pendingCommands.TryRemove(correlationId, out _))
+        {
+            return;
+        }
+
+        TimeSpan duration = _timeProvider.GetElapsedTime(pendingCommand.StartTimestamp);
+        _metrics?.CommandFailed(pendingCommand.Method, errorCode.ToString(), duration);
+        pendingCommand.SetException(new WorkerClientException(errorCode, message));
+    }
+
+    private async Task<WorkerEnvelope> ReadHandshakeEnvelopeAsync(
+        WorkerEnvelope.BodyOneofCase expectedBody,
+        CancellationToken cancellationToken)
+    {
+        WorkerEnvelope envelope = await _reader.ReadAsync(cancellationToken).ConfigureAwait(false);
+        if (envelope.BodyCase != expectedBody)
+        {
+            throw new WorkerClientException(
+                WorkerClientErrorCode.ProtocolViolation,
+                $"Worker handshake expected {expectedBody} but received {envelope.BodyCase}.");
+        }
+
+        return envelope;
+    }
+
+    private void ValidateWorkerHello(WorkerHello workerHello)
+    {
+        if (workerHello.ProtocolVersion != _connection.FrameOptions.ProtocolVersion)
+        {
+            throw new WorkerClientException(
+                WorkerClientErrorCode.ProtocolViolation,
+                "Worker hello protocol version does not match the gateway protocol version.");
+        }
+
+        if (!string.Equals(workerHello.Nonce, _connection.Nonce, StringComparison.Ordinal))
+        {
+            throw new WorkerClientException(
+                WorkerClientErrorCode.ProtocolViolation,
+                "Worker hello nonce does not match the gateway nonce.");
+        }
+
+        lock (_syncRoot)
+        {
+            _processId = workerHello.WorkerProcessId == 0
+                ? _connection.ProcessHandle?.ProcessId
+                : workerHello.WorkerProcessId;
+        }
+    }
+
+    private void MarkReady(WorkerReady ready)
+    {
+        lock (_syncRoot)
+        {
+            _processId = ready.WorkerProcessId == 0
+                ? _processId ?? _connection.ProcessHandle?.ProcessId
+                : ready.WorkerProcessId;
+            _lastHeartbeatAt = _timeProvider.GetUtcNow();
+            _state = WorkerClientState.Ready;
+        }
+
+        DateTimeOffset readyAt = _timeProvider.GetUtcNow();
+        DateTimeOffset launchedAt = _connection.ProcessHandle?.LaunchedAt ?? readyAt;
+        _metrics?.WorkerStarted(readyAt - launchedAt);
+    }
+
+    private void MarkHeartbeat(WorkerHeartbeat heartbeat)
+    {
+        lock (_syncRoot)
+        {
+            _lastHeartbeatAt = _timeProvider.GetUtcNow();
+            if (heartbeat.WorkerProcessId != 0)
+            {
+                _processId = heartbeat.WorkerProcessId;
+            }
+        }
+    }
+
+    private void MarkClosing()
+    {
+        lock (_syncRoot)
+        {
+            if (_state is WorkerClientState.Closed or WorkerClientState.Faulted)
+            {
+                return;
+            }
+
+            _state = WorkerClientState.Closing;
+        }
+    }
+
+    private void MarkClosed(string reason)
+    {
+        lock (_syncRoot)
+        {
+            if (_state == WorkerClientState.Closed)
+            {
+                return;
+            }
+
+            _state = WorkerClientState.Closed;
+        }
+
+        _stopCts.Cancel();
+        _outboundEnvelopes.Writer.TryComplete();
+        _events.Writer.TryComplete();
+        CompletePendingCommands(
+            new WorkerClientException(
+                WorkerClientErrorCode.GatewayShutdown,
+                $"Worker client closed because {reason}."));
+        _metrics?.WorkerStopped(reason);
+    }
+
+    private void SetFaulted(
+        WorkerClientErrorCode errorCode,
+        string message,
+        Exception? exception)
+    {
+        WorkerClientException fault = exception is null
+            ? new WorkerClientException(errorCode, message)
+            : new WorkerClientException(errorCode, message, exception);
+
+        lock (_syncRoot)
+        {
+            if (_state is WorkerClientState.Faulted or WorkerClientState.Closed)
+            {
+                return;
+            }
+
+            _state = WorkerClientState.Faulted;
+        }
+
+        _stopCts.Cancel();
+        _outboundEnvelopes.Writer.TryComplete(fault);
+        _events.Writer.TryComplete(fault);
+        CompletePendingCommands(fault);
+        _metrics?.Fault(errorCode.ToString());
+        _logger.LogWarning(exception, "Worker client faulted for session {SessionId}: {Message}", SessionId, message);
+    }
+
+    private void CompletePendingCommands(Exception exception)
+    {
+        foreach (KeyValuePair<string, PendingCommand> item in _pendingCommands.ToArray())
+        {
+            if (_pendingCommands.TryRemove(item.Key, out PendingCommand? pendingCommand))
+            {
+                TimeSpan duration = _timeProvider.GetElapsedTime(pendingCommand.StartTimestamp);
+                _metrics?.CommandFailed(pendingCommand.Method, exception.GetType().Name, duration);
+                pendingCommand.SetException(exception);
+            }
+        }
+    }
+
+    private void TransitionFromCreatedToHandshaking()
+    {
+        lock (_syncRoot)
+        {
+            if (_state != WorkerClientState.Created)
+            {
+                throw new WorkerClientException(
+                    WorkerClientErrorCode.InvalidState,
+                    $"Worker client cannot start from state {_state}.");
+            }
+
+            _state = WorkerClientState.Handshaking;
+        }
+    }
+
+    private void EnsureReady()
+    {
+        WorkerClientState state = State;
+        if (state != WorkerClientState.Ready)
+        {
+            throw new WorkerClientException(
+                WorkerClientErrorCode.InvalidState,
+                $"Worker client is not ready. Current state is {state}.");
+        }
+    }
+
+    private bool IsTerminalState()
+    {
+        WorkerClientState state = State;
+        return state is WorkerClientState.Closing or WorkerClientState.Closed or WorkerClientState.Faulted;
+    }
+
+    private async Task EnqueueAsync(
+        WorkerEnvelope envelope,
+        CancellationToken cancellationToken)
+    {
+        try
+        {
+            await _outboundEnvelopes.Writer.WriteAsync(envelope, cancellationToken).ConfigureAwait(false);
+        }
+        catch (ChannelClosedException exception)
+        {
+            throw new WorkerClientException(
+                WorkerClientErrorCode.WriteFailed,
+                "Worker outbound channel is closed.",
+                exception);
+        }
+    }
+
+    private WorkerEnvelope CreateGatewayHelloEnvelope()
+    {
+        return CreateEnvelope(
+            correlationId: string.Empty,
+            envelope => envelope.GatewayHello = new GatewayHello
+            {
+                SupportedProtocolVersion = _connection.FrameOptions.ProtocolVersion,
+                Nonce = _connection.Nonce,
+                GatewayVersion = typeof(GatewayContractInfo).Assembly.GetName().Version?.ToString() ?? GatewayVersionFallback,
+            });
+    }
+
+    private WorkerEnvelope CreateCommandEnvelope(
+        string correlationId,
+        WorkerCommand command)
+    {
+        return CreateEnvelope(
+            correlationId,
+            envelope => envelope.WorkerCommand = command.Clone());
+    }
+
+    private WorkerEnvelope CreateShutdownEnvelope(
+        TimeSpan timeout,
+        string reason)
+    {
+        return CreateEnvelope(
+            correlationId: string.Empty,
+            envelope => envelope.WorkerShutdown = new WorkerShutdown
+            {
+                GracePeriod = Duration.FromTimeSpan(timeout),
+                Reason = reason,
+            });
+    }
+
+    private WorkerEnvelope CreateEnvelope(
+        string correlationId,
+        Action<WorkerEnvelope> setBody)
+    {
+        WorkerEnvelope envelope = new()
+        {
+            ProtocolVersion = _connection.FrameOptions.ProtocolVersion,
+            SessionId = SessionId,
+            Sequence = (ulong)Interlocked.Increment(ref _nextSequence),
+            CorrelationId = correlationId,
+        };
+        setBody(envelope);
+
+        return envelope;
+    }
+
+    private static string GetCommandMethod(WorkerCommand command)
+    {
+        return command.Command?.Kind.ToString() ?? MxCommandKind.Unspecified.ToString();
+    }
+
+    private static string CreateWorkerFaultMessage(WorkerFault fault)
+    {
+        return string.IsNullOrWhiteSpace(fault.DiagnosticMessage)
+            ? $"Worker faulted with category {fault.Category}."
+            : $"Worker faulted with category {fault.Category}: {fault.DiagnosticMessage}";
+    }
+
+    private async Task WaitForBackgroundTasksAsync(CancellationToken cancellationToken)
+    {
+        Task[] tasks = new[] { _readLoopTask, _writeLoopTask, _heartbeatLoopTask }
+            .Where(task => task is not null)
+            .Cast<Task>()
+            .ToArray();
+
+        if (tasks.Length == 0)
+        {
+            return;
+        }
+
+        await Task.WhenAll(tasks).WaitAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    private void ThrowIfDisposed()
+    {
+        ObjectDisposedException.ThrowIf(_disposed, this);
+    }
+
+    private sealed class PendingCommand
+    {
+        private readonly TaskCompletionSource<WorkerCommandReply> _completion = new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        public PendingCommand(
+            string correlationId,
+            string method,
+            long startTimestamp)
+        {
+            CorrelationId = correlationId;
+            Method = method;
+            StartTimestamp = startTimestamp;
+        }
+
+        public string CorrelationId { get; }
+
+        public string Method { get; }
+
+        public long StartTimestamp { get; }
+
+        public Task<WorkerCommandReply> Task => _completion.Task;
+
+        public void SetResult(WorkerCommandReply reply)
+        {
+            _completion.TrySetResult(reply);
+        }
+
+        public void SetException(Exception exception)
+        {
+            _completion.TrySetException(exception);
+        }
+    }
+}

src/MxGateway.Server/Workers/WorkerClientConnection.cs

@@ -0,0 +1,38 @@
+namespace MxGateway.Server.Workers;
+
+public sealed class WorkerClientConnection
+{
+    public WorkerClientConnection(
+        string sessionId,
+        string nonce,
+        Stream stream,
+        WorkerFrameProtocolOptions frameOptions,
+        WorkerProcessHandle? processHandle = null)
+    {
+        if (string.IsNullOrWhiteSpace(sessionId))
+        {
+            throw new ArgumentException("Session id is required.", nameof(sessionId));
+        }
+
+        if (string.IsNullOrWhiteSpace(nonce))
+        {
+            throw new ArgumentException("Worker nonce is required.", nameof(nonce));
+        }
+
+        SessionId = sessionId;
+        Nonce = nonce;
+        Stream = stream ?? throw new ArgumentNullException(nameof(stream));
+        FrameOptions = frameOptions ?? throw new ArgumentNullException(nameof(frameOptions));
+        ProcessHandle = processHandle;
+    }
+
+    public string SessionId { get; }
+
+    public string Nonce { get; }
+
+    public Stream Stream { get; }
+
+    public WorkerFrameProtocolOptions FrameOptions { get; }
+
+    public WorkerProcessHandle? ProcessHandle { get; }
+}

src/MxGateway.Server/Workers/WorkerClientErrorCode.cs

@@ -0,0 +1,14 @@
+namespace MxGateway.Server.Workers;
+
+public enum WorkerClientErrorCode
+{
+    InvalidState,
+    ProtocolViolation,
+    PipeDisconnected,
+    CommandTimeout,
+    WorkerFaulted,
+    HeartbeatExpired,
+    ShutdownTimeout,
+    GatewayShutdown,
+    WriteFailed,
+}

src/MxGateway.Server/Workers/WorkerClientException.cs

@@ -0,0 +1,23 @@
+namespace MxGateway.Server.Workers;
+
+public sealed class WorkerClientException : Exception
+{
+    public WorkerClientException(
+        WorkerClientErrorCode errorCode,
+        string message)
+        : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+
+    public WorkerClientException(
+        WorkerClientErrorCode errorCode,
+        string message,
+        Exception innerException)
+        : base(message, innerException)
+    {
+        ErrorCode = errorCode;
+    }
+
+    public WorkerClientErrorCode ErrorCode { get; }
+}

src/MxGateway.Server/Workers/WorkerClientOptions.cs

@@ -0,0 +1,24 @@
+namespace MxGateway.Server.Workers;
+
+public sealed class WorkerClientOptions
+{
+    public static readonly TimeSpan DefaultHeartbeatGrace = TimeSpan.FromSeconds(15);
+    public static readonly TimeSpan DefaultHeartbeatCheckInterval = TimeSpan.FromSeconds(1);
+    public static readonly TimeSpan DefaultEventChannelFullModeTimeout = TimeSpan.FromSeconds(5);
+
+    public WorkerClientOptions()
+    {
+        HeartbeatGrace = DefaultHeartbeatGrace;
+        HeartbeatCheckInterval = DefaultHeartbeatCheckInterval;
+        EventChannelCapacity = 1_024;
+        EventChannelFullModeTimeout = DefaultEventChannelFullModeTimeout;
+    }
+
+    public TimeSpan HeartbeatGrace { get; init; }
+
+    public TimeSpan HeartbeatCheckInterval { get; init; }
+
+    public int EventChannelCapacity { get; init; }
+
+    public TimeSpan EventChannelFullModeTimeout { get; init; }
+}

src/MxGateway.Server/Workers/WorkerClientState.cs

@@ -0,0 +1,11 @@
+namespace MxGateway.Server.Workers;
+
+public enum WorkerClientState
+{
+    Created,
+    Handshaking,
+    Ready,
+    Closing,
+    Closed,
+    Faulted,
+}

src/MxGateway.Tests/Gateway/Workers/WorkerClientTests.cs

@@ -0,0 +1,341 @@
+using System.IO.Pipes;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Server.Workers;
+
+namespace MxGateway.Tests.Gateway.Workers;
+
+public sealed class WorkerClientTests
+{
+    private const string SessionId = "session-worker-client";
+    private const string Nonce = "nonce-worker-client";
+    private const int WorkerProcessId = 4321;
+    private static readonly TimeSpan TestTimeout = TimeSpan.FromSeconds(5);
+
+    [Fact]
+    public async Task StartAsync_WithWorkerHelloAndReady_EntersReadyState()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+
+        await CompleteHandshakeAsync(client, pipePair);
+
+        Assert.Equal(WorkerClientState.Ready, client.State);
+        Assert.Equal(WorkerProcessId, client.ProcessId);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_WithMatchingReply_CompletesPendingCommand()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+
+        Task<WorkerCommandReply> invokeTask = client.InvokeAsync(
+            CreateCommand(MxCommandKind.Ping),
+            TestTimeout,
+            CancellationToken.None);
+
+        WorkerEnvelope commandEnvelope = await pipePair.WorkerReader.ReadAsync().AsTask().WaitAsync(TestTimeout);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerCommand, commandEnvelope.BodyCase);
+        Assert.False(string.IsNullOrWhiteSpace(commandEnvelope.CorrelationId));
+
+        await pipePair.WorkerWriter.WriteAsync(
+            CreateCommandReplyEnvelope(commandEnvelope.CorrelationId, MxCommandKind.Ping));
+
+        WorkerCommandReply reply = await invokeTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(commandEnvelope.CorrelationId, reply.Reply.CorrelationId);
+        Assert.Equal(MxCommandKind.Ping, reply.Reply.Kind);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_WithLateReply_IgnoresLateReplyAndKeepsClientReady()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+
+        Task<WorkerCommandReply> timedOutInvokeTask = client.InvokeAsync(
+            CreateCommand(MxCommandKind.Ping),
+            TimeSpan.FromMilliseconds(50),
+            CancellationToken.None);
+        WorkerEnvelope timedOutCommand = await pipePair.WorkerReader.ReadAsync().AsTask().WaitAsync(TestTimeout);
+
+        WorkerClientException exception = await Assert.ThrowsAsync<WorkerClientException>(
+            async () => await timedOutInvokeTask);
+        Assert.Equal(WorkerClientErrorCode.CommandTimeout, exception.ErrorCode);
+
+        await pipePair.WorkerWriter.WriteAsync(
+            CreateCommandReplyEnvelope(timedOutCommand.CorrelationId, MxCommandKind.Ping));
+        await Task.Delay(TimeSpan.FromMilliseconds(50));
+
+        Task<WorkerCommandReply> secondInvokeTask = client.InvokeAsync(
+            CreateCommand(MxCommandKind.GetWorkerInfo),
+            TestTimeout,
+            CancellationToken.None);
+        WorkerEnvelope secondCommand = await pipePair.WorkerReader.ReadAsync().AsTask().WaitAsync(TestTimeout);
+        await pipePair.WorkerWriter.WriteAsync(
+            CreateCommandReplyEnvelope(secondCommand.CorrelationId, MxCommandKind.GetWorkerInfo));
+
+        WorkerCommandReply reply = await secondInvokeTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(WorkerClientState.Ready, client.State);
+        Assert.Equal(MxCommandKind.GetWorkerInfo, reply.Reply.Kind);
+    }
+
+    [Fact]
+    public async Task ReadEventsAsync_WithWorkerEvents_YieldsEventsInPipeOrder()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+        using CancellationTokenSource cancellationTokenSource = new(TestTimeout);
+
+        await using IAsyncEnumerator<WorkerEvent> events =
+            client.ReadEventsAsync(cancellationTokenSource.Token).GetAsyncEnumerator(cancellationTokenSource.Token);
+
+        await pipePair.WorkerWriter.WriteAsync(
+            CreateEventEnvelope(sequence: 11, MxEventFamily.OnDataChange));
+        await pipePair.WorkerWriter.WriteAsync(
+            CreateEventEnvelope(sequence: 12, MxEventFamily.OperationComplete));
+
+        Assert.True(await events.MoveNextAsync());
+        Assert.Equal((ulong)11, events.Current.Event.WorkerSequence);
+        Assert.Equal(MxEventFamily.OnDataChange, events.Current.Event.Family);
+
+        Assert.True(await events.MoveNextAsync());
+        Assert.Equal((ulong)12, events.Current.Event.WorkerSequence);
+        Assert.Equal(MxEventFamily.OperationComplete, events.Current.Event.Family);
+    }
+
+    [Fact]
+    public async Task ReadLoop_WhenPipeDisconnects_FaultsClient()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+
+        await pipePair.DisposeWorkerSideAsync();
+
+        await WaitUntilAsync(
+            () => client.State == WorkerClientState.Faulted,
+            TestTimeout);
+
+        Assert.Equal(WorkerClientState.Faulted, client.State);
+    }
+
+    [Fact]
+    public async Task HeartbeatMonitor_WhenHeartbeatExpires_FaultsClient()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(
+            pipePair,
+            new WorkerClientOptions
+            {
+                HeartbeatGrace = TimeSpan.FromMilliseconds(80),
+                HeartbeatCheckInterval = TimeSpan.FromMilliseconds(20),
+                EventChannelCapacity = 8,
+            });
+        await CompleteHandshakeAsync(client, pipePair);
+
+        await WaitUntilAsync(
+            () => client.State == WorkerClientState.Faulted,
+            TestTimeout);
+
+        Assert.Equal(WorkerClientState.Faulted, client.State);
+    }
+
+    private static WorkerClient CreateClient(
+        PipePair pipePair,
+        WorkerClientOptions? options = null)
+    {
+        WorkerFrameProtocolOptions frameOptions = new(SessionId);
+        WorkerClientConnection connection = new(
+            SessionId,
+            Nonce,
+            pipePair.GatewayStream,
+            frameOptions);
+
+        return new WorkerClient(connection, options);
+    }
+
+    private static async Task CompleteHandshakeAsync(
+        WorkerClient client,
+        PipePair pipePair)
+    {
+        Task startTask = client.StartAsync(CancellationToken.None);
+
+        WorkerEnvelope gatewayHello = await pipePair.WorkerReader.ReadAsync().AsTask().WaitAsync(TestTimeout);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, gatewayHello.BodyCase);
+        Assert.Equal(Nonce, gatewayHello.GatewayHello.Nonce);
+        Assert.Equal(GatewayContractInfo.WorkerProtocolVersion, gatewayHello.GatewayHello.SupportedProtocolVersion);
+
+        await pipePair.WorkerWriter.WriteAsync(CreateWorkerHelloEnvelope());
+        await pipePair.WorkerWriter.WriteAsync(CreateWorkerReadyEnvelope());
+        await startTask.WaitAsync(TestTimeout);
+    }
+
+    private static WorkerCommand CreateCommand(MxCommandKind kind)
+    {
+        return new WorkerCommand
+        {
+            Command = new MxCommand
+            {
+                Kind = kind,
+            },
+        };
+    }
+
+    private static WorkerEnvelope CreateWorkerHelloEnvelope()
+    {
+        return CreateWorkerEnvelope(
+            correlationId: string.Empty,
+            sequence: 1,
+            envelope => envelope.WorkerHello = new WorkerHello
+            {
+                ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+                Nonce = Nonce,
+                WorkerProcessId = WorkerProcessId,
+                WorkerVersion = "fake-worker",
+            });
+    }
+
+    private static WorkerEnvelope CreateWorkerReadyEnvelope()
+    {
+        return CreateWorkerEnvelope(
+            correlationId: string.Empty,
+            sequence: 2,
+            envelope => envelope.WorkerReady = new WorkerReady
+            {
+                WorkerProcessId = WorkerProcessId,
+                MxaccessProgid = "LMXProxy.LMXProxyServer.1",
+                MxaccessClsid = "{C30B52F5-2CB5-4760-AF0A-3A344A7EB5DC}",
+            });
+    }
+
+    private static WorkerEnvelope CreateCommandReplyEnvelope(
+        string correlationId,
+        MxCommandKind kind)
+    {
+        return CreateWorkerEnvelope(
+            correlationId,
+            sequence: 10,
+            envelope => envelope.WorkerCommandReply = new WorkerCommandReply
+            {
+                Reply = new MxCommandReply
+                {
+                    SessionId = SessionId,
+                    CorrelationId = correlationId,
+                    Kind = kind,
+                },
+            });
+    }
+
+    private static WorkerEnvelope CreateEventEnvelope(
+        ulong sequence,
+        MxEventFamily family)
+    {
+        return CreateWorkerEnvelope(
+            correlationId: string.Empty,
+            sequence,
+            envelope => envelope.WorkerEvent = new WorkerEvent
+            {
+                Event = new MxEvent
+                {
+                    SessionId = SessionId,
+                    Family = family,
+                    WorkerSequence = sequence,
+                },
+            });
+    }
+
+    private static WorkerEnvelope CreateWorkerEnvelope(
+        string correlationId,
+        ulong sequence,
+        Action<WorkerEnvelope> setBody)
+    {
+        WorkerEnvelope envelope = new()
+        {
+            ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+            SessionId = SessionId,
+            Sequence = sequence,
+            CorrelationId = correlationId,
+        };
+        setBody(envelope);
+
+        return envelope;
+    }
+
+    private static async Task WaitUntilAsync(
+        Func<bool> predicate,
+        TimeSpan timeout)
+    {
+        using CancellationTokenSource cancellationTokenSource = new(timeout);
+        while (!predicate())
+        {
+            await Task.Delay(TimeSpan.FromMilliseconds(10), cancellationTokenSource.Token);
+        }
+    }
+
+    private sealed class PipePair : IAsyncDisposable
+    {
+        private readonly NamedPipeClientStream _workerStream;
+        private bool _workerSideDisposed;
+
+        private PipePair(
+            NamedPipeServerStream gatewayStream,
+            NamedPipeClientStream workerStream)
+        {
+            GatewayStream = gatewayStream;
+            _workerStream = workerStream;
+            WorkerReader = new WorkerFrameReader(_workerStream, new WorkerFrameProtocolOptions(SessionId));
+            WorkerWriter = new WorkerFrameWriter(_workerStream, new WorkerFrameProtocolOptions(SessionId));
+        }
+
+        public NamedPipeServerStream GatewayStream { get; }
+
+        public WorkerFrameReader WorkerReader { get; }
+
+        public WorkerFrameWriter WorkerWriter { get; }
+
+        public static async Task<PipePair> CreateAsync()
+        {
+            string pipeName = $"mxaccessgw-workerclient-tests-{Guid.NewGuid():N}";
+            NamedPipeServerStream gatewayStream = new(
+                pipeName,
+                PipeDirection.InOut,
+                maxNumberOfServerInstances: 1,
+                PipeTransmissionMode.Byte,
+                PipeOptions.Asynchronous);
+            NamedPipeClientStream workerStream = new(
+                ".",
+                pipeName,
+                PipeDirection.InOut,
+                PipeOptions.Asynchronous);
+
+            Task waitForConnectionTask = gatewayStream.WaitForConnectionAsync();
+            await workerStream.ConnectAsync();
+            await waitForConnectionTask;
+
+            return new PipePair(gatewayStream, workerStream);
+        }
+
+        public async ValueTask DisposeWorkerSideAsync()
+        {
+            if (_workerSideDisposed)
+            {
+                return;
+            }
+
+            await _workerStream.DisposeAsync();
+            _workerSideDisposed = true;
+        }
+
+        public async ValueTask DisposeAsync()
+        {
+            await DisposeWorkerSideAsync();
+            await GatewayStream.DisposeAsync();
+        }
+    }
+}

src/MxGateway.Worker.Tests/Bootstrap/WorkerApplicationTests.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Threading;
+using System.Threading.Tasks;
 using MxGateway.Contracts;
 using MxGateway.Worker.Bootstrap;
+using MxGateway.Worker.Ipc;
 
 namespace MxGateway.Worker.Tests.Bootstrap;
 
@@ -15,16 +18,19 @@ public sealed class WorkerApplicationTests
         int exitCode = MxGateway.Worker.WorkerApplication.Run(
             ValidArgs(),
             environment,
-            logger);
+            logger,
+            new SucceedingPipeClient());
 
         Assert.Equal((int)WorkerExitCode.Success, exitCode);
-        MemoryWorkerLogEntry entry = Assert.Single(logger.Entries);
+        Assert.Equal(2, logger.Entries.Count);
+        MemoryWorkerLogEntry entry = logger.Entries[0];
         Assert.Equal("Information", entry.Level);
         Assert.Equal("WorkerBootstrapSucceeded", entry.EventName);
         Assert.Equal("session-1", entry.Fields["session_id"]);
         Assert.Equal("mxaccess-gateway-123-session-1", entry.Fields["pipe_name"]);
         Assert.Equal(GatewayContractInfo.WorkerProtocolVersion, entry.Fields["protocol_version"]);
         Assert.Equal("[redacted]", entry.Fields["nonce"]);
+        Assert.Equal("WorkerPipeHandshakeSucceeded", logger.Entries[1].EventName);
     }
 
     [Fact]
@@ -73,6 +79,24 @@ public sealed class WorkerApplicationTests
         Assert.Equal((int)WorkerExitCode.MissingNonce, exitCode);
     }
 
+    [Fact]
+    public void Run_WithPipeProtocolFailure_ReturnsProtocolViolation()
+    {
+        MemoryWorkerEnvironment environment = CreateEnvironment("nonce-secret");
+        MemoryWorkerLogger logger = new();
+
+        int exitCode = MxGateway.Worker.WorkerApplication.Run(
+            ValidArgs(),
+            environment,
+            logger,
+            new ThrowingPipeClient(new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.NonceMismatch,
+                "Bad nonce.")));
+
+        Assert.Equal((int)WorkerExitCode.ProtocolViolation, exitCode);
+        Assert.Equal("WorkerPipeProtocolFailure", logger.Entries[1].EventName);
+    }
+
     [Fact]
     public void Run_WithUnexpectedBootstrapFailure_ReturnsUnexpectedFailure()
     {
@@ -110,4 +134,31 @@ public sealed class WorkerApplicationTests
         environment.Set(WorkerOptions.NonceEnvironmentVariableName, nonce);
         return environment;
     }
+
+    private sealed class SucceedingPipeClient : IWorkerPipeClient
+    {
+        public Task RunAsync(
+            WorkerOptions options,
+            CancellationToken cancellationToken = default)
+        {
+            return Task.CompletedTask;
+        }
+    }
+
+    private sealed class ThrowingPipeClient : IWorkerPipeClient
+    {
+        private readonly Exception _exception;
+
+        public ThrowingPipeClient(Exception exception)
+        {
+            _exception = exception;
+        }
+
+        public Task RunAsync(
+            WorkerOptions options,
+            CancellationToken cancellationToken = default)
+        {
+            throw _exception;
+        }
+    }
 }

src/MxGateway.Worker.Tests/Ipc/WorkerFrameProtocolTests.cs

@@ -0,0 +1,163 @@
+using System;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Google.Protobuf;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.Ipc;
+
+namespace MxGateway.Worker.Tests.Ipc;
+
+public sealed class WorkerFrameProtocolTests
+{
+    private const string SessionId = "session-1";
+    private const string Nonce = "nonce-secret";
+
+    [Fact]
+    public async Task WriteAndReadAsync_WithValidEnvelope_RoundTripsFrame()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream stream = new();
+        WorkerEnvelope original = CreateGatewayHelloEnvelope();
+
+        WorkerFrameWriter writer = new(stream, options);
+        await writer.WriteAsync(original);
+        stream.Position = 0;
+
+        WorkerFrameReader reader = new(stream, options);
+        WorkerEnvelope parsed = await reader.ReadAsync();
+
+        Assert.Equal(original, parsed);
+    }
+
+    [Fact]
+    public async Task ReadAsync_WithWrongProtocolVersion_ThrowsProtocolVersionMismatch()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        WorkerEnvelope envelope = CreateGatewayHelloEnvelope();
+        envelope.ProtocolVersion++;
+        MemoryStream stream = new(CreateFrame(envelope));
+
+        WorkerFrameReader reader = new(stream, options);
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await reader.ReadAsync());
+
+        Assert.Equal(WorkerFrameProtocolErrorCode.ProtocolVersionMismatch, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task ReadAsync_WithWrongSessionId_ThrowsSessionMismatch()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        WorkerEnvelope envelope = CreateGatewayHelloEnvelope();
+        envelope.SessionId = "different-session";
+        MemoryStream stream = new(CreateFrame(envelope));
+
+        WorkerFrameReader reader = new(stream, options);
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await reader.ReadAsync());
+
+        Assert.Equal(WorkerFrameProtocolErrorCode.SessionMismatch, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task ReadAsync_WithMalformedLength_ThrowsMalformedLength()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream stream = new(new byte[sizeof(uint)]);
+
+        WorkerFrameReader reader = new(stream, options);
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await reader.ReadAsync());
+
+        Assert.Equal(WorkerFrameProtocolErrorCode.MalformedLength, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task ReadAsync_WithMalformedPayload_ThrowsInvalidEnvelope()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream stream = new(CreateFrame(new byte[] { 0x80 }));
+
+        WorkerFrameReader reader = new(stream, options);
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await reader.ReadAsync());
+
+        Assert.Equal(WorkerFrameProtocolErrorCode.InvalidEnvelope, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task WriteAsync_WithConcurrentCalls_SerializesCompleteFrames()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream stream = new();
+        WorkerFrameWriter writer = new(stream, options);
+
+        await Task.WhenAll(
+            writer.WriteAsync(CreateGatewayHelloEnvelope(sequence: 1)),
+            writer.WriteAsync(CreateGatewayHelloEnvelope(sequence: 2)),
+            writer.WriteAsync(CreateGatewayHelloEnvelope(sequence: 3)));
+
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+
+        WorkerEnvelope first = await reader.ReadAsync();
+        WorkerEnvelope second = await reader.ReadAsync();
+        WorkerEnvelope third = await reader.ReadAsync();
+
+        Assert.Equal(new ulong[] { 1, 2, 3 }, new[] { first.Sequence, second.Sequence, third.Sequence }.OrderBy(sequence => sequence));
+    }
+
+    private static WorkerFrameProtocolOptions CreateOptions()
+    {
+        return new WorkerFrameProtocolOptions(
+            SessionId,
+            GatewayContractInfo.WorkerProtocolVersion,
+            Nonce);
+    }
+
+    private static WorkerEnvelope CreateGatewayHelloEnvelope(ulong sequence = 1)
+    {
+        return new WorkerEnvelope
+        {
+            ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+            SessionId = SessionId,
+            Sequence = sequence,
+            GatewayHello = new GatewayHello
+            {
+                SupportedProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+                Nonce = Nonce,
+                GatewayVersion = "test-gateway",
+            },
+        };
+    }
+
+    private static byte[] CreateFrame(IMessage message)
+    {
+        return CreateFrame(message.ToByteArray());
+    }
+
+    private static byte[] CreateFrame(byte[] payload)
+    {
+        byte[] frame = new byte[sizeof(uint) + payload.Length];
+        WriteUInt32LittleEndian(frame, (uint)payload.Length);
+        payload.CopyTo(frame, sizeof(uint));
+
+        return frame;
+    }
+
+    private static void WriteUInt32LittleEndian(
+        byte[] buffer,
+        uint value)
+    {
+        buffer[0] = (byte)value;
+        buffer[1] = (byte)(value >> 8);
+        buffer[2] = (byte)(value >> 16);
+        buffer[3] = (byte)(value >> 24);
+    }
+}

src/MxGateway.Worker.Tests/Ipc/WorkerPipeClientTests.cs

@@ -0,0 +1,61 @@
+using System;
+using System.IO.Pipes;
+using System.Threading.Tasks;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.Bootstrap;
+using MxGateway.Worker.Ipc;
+
+namespace MxGateway.Worker.Tests.Ipc;
+
+public sealed class WorkerPipeClientTests
+{
+    [Fact]
+    public async Task RunAsync_ConnectsToPipeAndCompletesHandshake()
+    {
+        string pipeName = $"mxaccess-gateway-test-{Guid.NewGuid():N}";
+        WorkerOptions workerOptions = new(
+            "session-1",
+            pipeName,
+            GatewayContractInfo.WorkerProtocolVersion,
+            "nonce-secret");
+        WorkerFrameProtocolOptions frameOptions = new(workerOptions);
+
+        using NamedPipeServerStream server = new(
+            pipeName,
+            PipeDirection.InOut,
+            1,
+            PipeTransmissionMode.Byte,
+            PipeOptions.Asynchronous);
+
+        WorkerPipeClient client = new(connectTimeoutMilliseconds: 5000);
+        Task clientTask = client.RunAsync(workerOptions);
+
+        await Task.Factory.FromAsync(server.BeginWaitForConnection, server.EndWaitForConnection, null);
+
+        WorkerFrameReader reader = new(server, frameOptions);
+        WorkerFrameWriter writer = new(server, frameOptions);
+
+        await writer.WriteAsync(new WorkerEnvelope
+        {
+            ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+            SessionId = "session-1",
+            Sequence = 1,
+            GatewayHello = new GatewayHello
+            {
+                SupportedProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+                Nonce = "nonce-secret",
+                GatewayVersion = "test-gateway",
+            },
+        });
+
+        WorkerEnvelope hello = await reader.ReadAsync();
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerHello, hello.BodyCase);
+        Assert.Equal("nonce-secret", hello.WorkerHello.Nonce);
+
+        WorkerEnvelope ready = await reader.ReadAsync();
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerReady, ready.BodyCase);
+
+        await clientTask;
+    }
+}

src/MxGateway.Worker.Tests/Ipc/WorkerPipeSessionTests.cs

@@ -0,0 +1,192 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.Ipc;
+
+namespace MxGateway.Worker.Tests.Ipc;
+
+public sealed class WorkerPipeSessionTests
+{
+    private const string SessionId = "session-1";
+    private const string Nonce = "nonce-secret";
+
+    [Fact]
+    public async Task CompleteStartupHandshakeAsync_WithValidGatewayHello_SendsHelloThenReady()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream inbound = new();
+        await new WorkerFrameWriter(inbound, options).WriteAsync(CreateGatewayHelloEnvelope());
+        inbound.Position = 0;
+        MemoryStream outbound = new();
+        WorkerPipeSession session = CreateSession(inbound, outbound, options);
+        bool initialized = false;
+
+        await session.CompleteStartupHandshakeAsync(
+            _ =>
+            {
+                initialized = true;
+                return Task.CompletedTask;
+            });
+
+        Assert.True(initialized);
+        WorkerEnvelope[] written = ReadWrittenFrames(outbound, options);
+        Assert.Equal(2, written.Length);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerHello, written[0].BodyCase);
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerReady, written[1].BodyCase);
+        Assert.Equal(Nonce, written[0].WorkerHello.Nonce);
+    }
+
+    [Fact]
+    public async Task CompleteStartupHandshakeAsync_WithWrongNonce_FaultsBeforeInitialization()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream inbound = new();
+        await new WorkerFrameWriter(inbound, options).WriteAsync(CreateGatewayHelloEnvelope(nonce: "wrong"));
+        inbound.Position = 0;
+        MemoryStream outbound = new();
+        WorkerPipeSession session = CreateSession(inbound, outbound, options);
+        bool initialized = false;
+
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await session.CompleteStartupHandshakeAsync(
+                    _ =>
+                    {
+                        initialized = true;
+                        return Task.CompletedTask;
+                    }));
+
+        Assert.False(initialized);
+        Assert.Equal(WorkerFrameProtocolErrorCode.NonceMismatch, exception.ErrorCode);
+        WorkerEnvelope fault = Assert.Single(ReadWrittenFrames(outbound, options));
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerFault, fault.BodyCase);
+        Assert.Equal(WorkerFaultCategory.ProtocolViolation, fault.WorkerFault.Category);
+    }
+
+    [Fact]
+    public async Task CompleteStartupHandshakeAsync_WithWrongProtocol_FaultsBeforeInitialization()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream inbound = new();
+        await new WorkerFrameWriter(inbound, options).WriteAsync(CreateGatewayHelloEnvelope(supportedProtocolVersion: 999));
+        inbound.Position = 0;
+        MemoryStream outbound = new();
+        WorkerPipeSession session = CreateSession(inbound, outbound, options);
+        bool initialized = false;
+
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await session.CompleteStartupHandshakeAsync(
+                    _ =>
+                    {
+                        initialized = true;
+                        return Task.CompletedTask;
+                    }));
+
+        Assert.False(initialized);
+        Assert.Equal(WorkerFrameProtocolErrorCode.ProtocolVersionMismatch, exception.ErrorCode);
+        WorkerEnvelope fault = Assert.Single(ReadWrittenFrames(outbound, options));
+        Assert.Equal(WorkerFaultCategory.ProtocolMismatch, fault.WorkerFault.Category);
+    }
+
+    [Fact]
+    public async Task CompleteStartupHandshakeAsync_WithMalformedFrame_WritesWorkerFault()
+    {
+        WorkerFrameProtocolOptions options = CreateOptions();
+        MemoryStream inbound = new(CreateFrame(new byte[] { 0x80 }));
+        MemoryStream outbound = new();
+        WorkerPipeSession session = CreateSession(inbound, outbound, options);
+        bool initialized = false;
+
+        WorkerFrameProtocolException exception =
+            await Assert.ThrowsAsync<WorkerFrameProtocolException>(
+                async () => await session.CompleteStartupHandshakeAsync(
+                    _ =>
+                    {
+                        initialized = true;
+                        return Task.CompletedTask;
+                    }));
+
+        Assert.False(initialized);
+        Assert.Equal(WorkerFrameProtocolErrorCode.InvalidEnvelope, exception.ErrorCode);
+        WorkerEnvelope fault = Assert.Single(ReadWrittenFrames(outbound, options));
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerFault, fault.BodyCase);
+        Assert.Equal(WorkerFaultCategory.ProtocolViolation, fault.WorkerFault.Category);
+    }
+
+    private static WorkerPipeSession CreateSession(
+        Stream inbound,
+        Stream outbound,
+        WorkerFrameProtocolOptions options)
+    {
+        return new WorkerPipeSession(
+            new WorkerFrameReader(inbound, options),
+            new WorkerFrameWriter(outbound, options),
+            options,
+            () => 1234);
+    }
+
+    private static WorkerFrameProtocolOptions CreateOptions()
+    {
+        return new WorkerFrameProtocolOptions(
+            SessionId,
+            GatewayContractInfo.WorkerProtocolVersion,
+            Nonce);
+    }
+
+    private static WorkerEnvelope CreateGatewayHelloEnvelope(
+        string nonce = Nonce,
+        uint supportedProtocolVersion = GatewayContractInfo.WorkerProtocolVersion)
+    {
+        return new WorkerEnvelope
+        {
+            ProtocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+            SessionId = SessionId,
+            Sequence = 1,
+            GatewayHello = new GatewayHello
+            {
+                SupportedProtocolVersion = supportedProtocolVersion,
+                Nonce = nonce,
+                GatewayVersion = "test-gateway",
+            },
+        };
+    }
+
+    private static WorkerEnvelope[] ReadWrittenFrames(
+        MemoryStream stream,
+        WorkerFrameProtocolOptions options)
+    {
+        stream.Position = 0;
+        WorkerFrameReader reader = new(stream, options);
+        List<WorkerEnvelope> envelopes = new();
+
+        while (stream.Position < stream.Length)
+        {
+            envelopes.Add(reader.ReadAsync(CancellationToken.None).GetAwaiter().GetResult());
+        }
+
+        return envelopes.ToArray();
+    }
+
+    private static byte[] CreateFrame(byte[] payload)
+    {
+        byte[] frame = new byte[sizeof(uint) + payload.Length];
+        WriteUInt32LittleEndian(frame, (uint)payload.Length);
+        payload.CopyTo(frame, sizeof(uint));
+
+        return frame;
+    }
+
+    private static void WriteUInt32LittleEndian(
+        byte[] buffer,
+        uint value)
+    {
+        buffer[0] = (byte)value;
+        buffer[1] = (byte)(value >> 8);
+        buffer[2] = (byte)(value >> 16);
+        buffer[3] = (byte)(value >> 24);
+    }
+}

src/MxGateway.Worker/Bootstrap/WorkerExitCode.cs

@@ -7,4 +7,6 @@ public enum WorkerExitCode
     InvalidArguments = 2,
     InvalidProtocolVersion = 3,
     MissingNonce = 4,
+    PipeConnectionFailed = 5,
+    ProtocolViolation = 6,
 }

src/MxGateway.Worker/Ipc/IWorkerPipeClient.cs

@@ -0,0 +1,12 @@
+using System.Threading;
+using System.Threading.Tasks;
+using MxGateway.Worker.Bootstrap;
+
+namespace MxGateway.Worker.Ipc;
+
+public interface IWorkerPipeClient
+{
+    Task RunAsync(
+        WorkerOptions options,
+        CancellationToken cancellationToken = default);
+}

src/MxGateway.Worker/Ipc/WorkerEnvelopeValidator.cs

@@ -0,0 +1,33 @@
+using System;
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Worker.Ipc;
+
+internal static class WorkerEnvelopeValidator
+{
+    public static void Validate(
+        WorkerEnvelope envelope,
+        WorkerFrameProtocolOptions options)
+    {
+        if (envelope.ProtocolVersion != options.ProtocolVersion)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.ProtocolVersionMismatch,
+                $"Worker envelope protocol version {envelope.ProtocolVersion} does not match expected version {options.ProtocolVersion}.");
+        }
+
+        if (!string.Equals(envelope.SessionId, options.SessionId, StringComparison.Ordinal))
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.SessionMismatch,
+                "Worker envelope session id does not match the owning worker session.");
+        }
+
+        if (envelope.BodyCase == WorkerEnvelope.BodyOneofCase.None)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidEnvelope,
+                "Worker envelope must include a typed body.");
+        }
+    }
+}

src/MxGateway.Worker/Ipc/WorkerFrameProtocolErrorCode.cs

@@ -0,0 +1,15 @@
+namespace MxGateway.Worker.Ipc;
+
+public enum WorkerFrameProtocolErrorCode
+{
+    Unknown = 0,
+    InvalidConfiguration = 1,
+    EndOfStream = 2,
+    MalformedLength = 3,
+    MessageTooLarge = 4,
+    InvalidEnvelope = 5,
+    ProtocolVersionMismatch = 6,
+    SessionMismatch = 7,
+    NonceMismatch = 8,
+    UnexpectedEnvelopeBody = 9,
+}

src/MxGateway.Worker/Ipc/WorkerFrameProtocolException.cs

@@ -0,0 +1,25 @@
+using System;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerFrameProtocolException : Exception
+{
+    public WorkerFrameProtocolException(
+        WorkerFrameProtocolErrorCode errorCode,
+        string message)
+        : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+
+    public WorkerFrameProtocolException(
+        WorkerFrameProtocolErrorCode errorCode,
+        string message,
+        Exception innerException)
+        : base(message, innerException)
+    {
+        ErrorCode = errorCode;
+    }
+
+    public WorkerFrameProtocolErrorCode ErrorCode { get; }
+}

src/MxGateway.Worker/Ipc/WorkerFrameProtocolOptions.cs

@@ -0,0 +1,86 @@
+using System;
+using MxGateway.Contracts;
+using MxGateway.Worker.Bootstrap;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerFrameProtocolOptions
+{
+    public const int DefaultMaxMessageBytes = 16 * 1024 * 1024;
+
+    public WorkerFrameProtocolOptions(WorkerOptions options)
+        : this(
+            options?.SessionId ?? throw new ArgumentNullException(nameof(options)),
+            options.ProtocolVersion,
+            options.Nonce,
+            DefaultMaxMessageBytes)
+    {
+    }
+
+    public WorkerFrameProtocolOptions(
+        string sessionId,
+        uint protocolVersion,
+        string nonce)
+        : this(
+            sessionId,
+            protocolVersion,
+            nonce,
+            DefaultMaxMessageBytes)
+    {
+    }
+
+    public WorkerFrameProtocolOptions(
+        string sessionId,
+        uint protocolVersion,
+        string nonce,
+        int maxMessageBytes)
+    {
+        if (string.IsNullOrWhiteSpace(sessionId))
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidConfiguration,
+                "Worker frame protocol requires a session id.");
+        }
+
+        if (protocolVersion == 0)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidConfiguration,
+                "Worker frame protocol requires a non-zero protocol version.");
+        }
+
+        if (protocolVersion != GatewayContractInfo.WorkerProtocolVersion)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.ProtocolVersionMismatch,
+                $"Worker frame protocol version {protocolVersion} is not supported.");
+        }
+
+        if (string.IsNullOrWhiteSpace(nonce))
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidConfiguration,
+                "Worker frame protocol requires a nonce.");
+        }
+
+        if (maxMessageBytes <= 0)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidConfiguration,
+                "Worker frame protocol max message size must be greater than zero.");
+        }
+
+        SessionId = sessionId;
+        ProtocolVersion = protocolVersion;
+        Nonce = nonce;
+        MaxMessageBytes = maxMessageBytes;
+    }
+
+    public string SessionId { get; }
+
+    public uint ProtocolVersion { get; }
+
+    public string Nonce { get; }
+
+    public int MaxMessageBytes { get; }
+}

src/MxGateway.Worker/Ipc/WorkerFrameReader.cs

@@ -0,0 +1,93 @@
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using Google.Protobuf;
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerFrameReader
+{
+    private readonly WorkerFrameProtocolOptions _options;
+    private readonly Stream _stream;
+
+    public WorkerFrameReader(
+        Stream stream,
+        WorkerFrameProtocolOptions options)
+    {
+        _stream = stream ?? throw new ArgumentNullException(nameof(stream));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+    }
+
+    public async Task<WorkerEnvelope> ReadAsync(CancellationToken cancellationToken = default)
+    {
+        byte[] lengthPrefix = new byte[sizeof(uint)];
+        await ReadExactlyOrThrowAsync(lengthPrefix, cancellationToken).ConfigureAwait(false);
+
+        uint payloadLength = ReadUInt32LittleEndian(lengthPrefix);
+        if (payloadLength == 0)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.MalformedLength,
+                "Worker frame payload length must be greater than zero.");
+        }
+
+        if (payloadLength > _options.MaxMessageBytes)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.MessageTooLarge,
+                $"Worker frame payload length {payloadLength} exceeds the configured maximum of {_options.MaxMessageBytes} bytes.");
+        }
+
+        byte[] payload = new byte[payloadLength];
+        await ReadExactlyOrThrowAsync(payload, cancellationToken).ConfigureAwait(false);
+
+        WorkerEnvelope envelope;
+        try
+        {
+            envelope = WorkerEnvelope.Parser.ParseFrom(payload);
+        }
+        catch (InvalidProtocolBufferException exception)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidEnvelope,
+                "Worker frame payload is not a valid WorkerEnvelope protobuf message.",
+                exception);
+        }
+
+        WorkerEnvelopeValidator.Validate(envelope, _options);
+
+        return envelope;
+    }
+
+    private static uint ReadUInt32LittleEndian(byte[] buffer)
+    {
+        return (uint)buffer[0]
+            | ((uint)buffer[1] << 8)
+            | ((uint)buffer[2] << 16)
+            | ((uint)buffer[3] << 24);
+    }
+
+    private async Task ReadExactlyOrThrowAsync(
+        byte[] buffer,
+        CancellationToken cancellationToken)
+    {
+        int offset = 0;
+        while (offset < buffer.Length)
+        {
+            int bytesRead = await _stream
+                .ReadAsync(buffer, offset, buffer.Length - offset, cancellationToken)
+                .ConfigureAwait(false);
+
+            if (bytesRead == 0)
+            {
+                throw new WorkerFrameProtocolException(
+                    WorkerFrameProtocolErrorCode.EndOfStream,
+                    "Worker frame ended before the expected number of bytes were read.");
+            }
+
+            offset += bytesRead;
+        }
+    }
+}

src/MxGateway.Worker/Ipc/WorkerFrameWriter.cs

@@ -0,0 +1,76 @@
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using Google.Protobuf;
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerFrameWriter
+{
+    private readonly WorkerFrameProtocolOptions _options;
+    private readonly SemaphoreSlim _writeLock = new(1, 1);
+    private readonly Stream _stream;
+
+    public WorkerFrameWriter(
+        Stream stream,
+        WorkerFrameProtocolOptions options)
+    {
+        _stream = stream ?? throw new ArgumentNullException(nameof(stream));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+    }
+
+    public async Task WriteAsync(
+        WorkerEnvelope envelope,
+        CancellationToken cancellationToken = default)
+    {
+        if (envelope is null)
+        {
+            throw new ArgumentNullException(nameof(envelope));
+        }
+
+        WorkerEnvelopeValidator.Validate(envelope, _options);
+
+        int payloadLength = envelope.CalculateSize();
+        if (payloadLength == 0)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.InvalidEnvelope,
+                "Worker envelope cannot serialize to an empty payload.");
+        }
+
+        if (payloadLength > _options.MaxMessageBytes)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.MessageTooLarge,
+                $"Worker envelope payload length {payloadLength} exceeds the configured maximum of {_options.MaxMessageBytes} bytes.");
+        }
+
+        byte[] payload = envelope.ToByteArray();
+        byte[] lengthPrefix = new byte[sizeof(uint)];
+        WriteUInt32LittleEndian(lengthPrefix, (uint)payloadLength);
+
+        await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            await _stream.WriteAsync(lengthPrefix, 0, lengthPrefix.Length, cancellationToken).ConfigureAwait(false);
+            await _stream.WriteAsync(payload, 0, payload.Length, cancellationToken).ConfigureAwait(false);
+            await _stream.FlushAsync(cancellationToken).ConfigureAwait(false);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+    }
+
+    private static void WriteUInt32LittleEndian(
+        byte[] buffer,
+        uint value)
+    {
+        buffer[0] = (byte)value;
+        buffer[1] = (byte)(value >> 8);
+        buffer[2] = (byte)(value >> 16);
+        buffer[3] = (byte)(value >> 24);
+    }
+}

src/MxGateway.Worker/Ipc/WorkerPipeClient.cs

@@ -0,0 +1,67 @@
+using System;
+using System.IO.Pipes;
+using System.Threading;
+using System.Threading.Tasks;
+using MxGateway.Worker.Bootstrap;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerPipeClient : IWorkerPipeClient
+{
+    public const int DefaultConnectTimeoutMilliseconds = 30000;
+
+    private readonly int _connectTimeoutMilliseconds;
+
+    public WorkerPipeClient()
+        : this(DefaultConnectTimeoutMilliseconds)
+    {
+    }
+
+    public WorkerPipeClient(int connectTimeoutMilliseconds)
+    {
+        if (connectTimeoutMilliseconds <= 0)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(connectTimeoutMilliseconds),
+                "Worker pipe connect timeout must be greater than zero.");
+        }
+
+        _connectTimeoutMilliseconds = connectTimeoutMilliseconds;
+    }
+
+    public async Task RunAsync(
+        WorkerOptions options,
+        CancellationToken cancellationToken = default)
+    {
+        if (options is null)
+        {
+            throw new ArgumentNullException(nameof(options));
+        }
+
+        WorkerFrameProtocolOptions frameOptions = new(options);
+
+        using NamedPipeClientStream pipe = new(
+            ".",
+            options.PipeName,
+            PipeDirection.InOut,
+            PipeOptions.Asynchronous);
+
+        await ConnectAsync(pipe, cancellationToken).ConfigureAwait(false);
+
+        WorkerPipeSession session = new(pipe, frameOptions);
+        await session.CompleteStartupHandshakeAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    private Task ConnectAsync(
+        NamedPipeClientStream pipe,
+        CancellationToken cancellationToken)
+    {
+        return Task.Run(
+            () =>
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                pipe.Connect(_connectTimeoutMilliseconds);
+            },
+            cancellationToken);
+    }
+}

src/MxGateway.Worker/Ipc/WorkerPipeSession.cs

@@ -0,0 +1,218 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using Google.Protobuf.WellKnownTypes;
+using MxGateway.Contracts.Proto;
+using MxGateway.Worker.MxAccess;
+
+namespace MxGateway.Worker.Ipc;
+
+public sealed class WorkerPipeSession
+{
+    private readonly WorkerFrameProtocolOptions _options;
+    private readonly Func<int> _processIdProvider;
+    private readonly WorkerFrameReader _reader;
+    private readonly WorkerFrameWriter _writer;
+    private long _nextSequence;
+
+    public WorkerPipeSession(
+        Stream stream,
+        WorkerFrameProtocolOptions options)
+        : this(
+            new WorkerFrameReader(stream, options),
+            new WorkerFrameWriter(stream, options),
+            options,
+            () => Process.GetCurrentProcess().Id)
+    {
+    }
+
+    public WorkerPipeSession(
+        WorkerFrameReader reader,
+        WorkerFrameWriter writer,
+        WorkerFrameProtocolOptions options,
+        Func<int> processIdProvider)
+    {
+        _reader = reader ?? throw new ArgumentNullException(nameof(reader));
+        _writer = writer ?? throw new ArgumentNullException(nameof(writer));
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+        _processIdProvider = processIdProvider ?? throw new ArgumentNullException(nameof(processIdProvider));
+    }
+
+    public Task CompleteStartupHandshakeAsync(CancellationToken cancellationToken = default)
+    {
+        return CompleteStartupHandshakeAsync(_ => Task.CompletedTask, cancellationToken);
+    }
+
+    public async Task CompleteStartupHandshakeAsync(
+        Func<CancellationToken, Task> initializeMxAccessAsync,
+        CancellationToken cancellationToken = default)
+    {
+        if (initializeMxAccessAsync is null)
+        {
+            throw new ArgumentNullException(nameof(initializeMxAccessAsync));
+        }
+
+        try
+        {
+            WorkerEnvelope envelope = await _reader.ReadAsync(cancellationToken).ConfigureAwait(false);
+            ValidateGatewayHello(envelope);
+
+            await WriteWorkerHelloAsync(cancellationToken).ConfigureAwait(false);
+            await initializeMxAccessAsync(cancellationToken).ConfigureAwait(false);
+            await WriteWorkerReadyAsync(cancellationToken).ConfigureAwait(false);
+        }
+        catch (WorkerFrameProtocolException exception)
+        {
+            await TryWriteFaultAsync(exception, cancellationToken).ConfigureAwait(false);
+            throw;
+        }
+    }
+
+    private void ValidateGatewayHello(WorkerEnvelope envelope)
+    {
+        if (envelope.BodyCase != WorkerEnvelope.BodyOneofCase.GatewayHello)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.UnexpectedEnvelopeBody,
+                "Worker expected GatewayHello during startup handshake.");
+        }
+
+        GatewayHello gatewayHello = envelope.GatewayHello;
+        if (gatewayHello.SupportedProtocolVersion != _options.ProtocolVersion)
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.ProtocolVersionMismatch,
+                $"GatewayHello supported protocol version {gatewayHello.SupportedProtocolVersion} does not match expected version {_options.ProtocolVersion}.");
+        }
+
+        if (!string.Equals(gatewayHello.Nonce, _options.Nonce, StringComparison.Ordinal))
+        {
+            throw new WorkerFrameProtocolException(
+                WorkerFrameProtocolErrorCode.NonceMismatch,
+                "GatewayHello nonce does not match the worker launch nonce.");
+        }
+    }
+
+    private Task WriteWorkerHelloAsync(CancellationToken cancellationToken)
+    {
+        return _writer.WriteAsync(
+            CreateEnvelope(new WorkerHello
+            {
+                ProtocolVersion = _options.ProtocolVersion,
+                Nonce = _options.Nonce,
+                WorkerProcessId = _processIdProvider(),
+                WorkerVersion = typeof(WorkerPipeSession).Assembly.GetName().Version?.ToString() ?? string.Empty,
+            }),
+            cancellationToken);
+    }
+
+    private Task WriteWorkerReadyAsync(CancellationToken cancellationToken)
+    {
+        return _writer.WriteAsync(
+            CreateEnvelope(new WorkerReady
+            {
+                WorkerProcessId = _processIdProvider(),
+                MxaccessProgid = MxAccessInteropInfo.ProgId,
+                MxaccessClsid = MxAccessInteropInfo.Clsid,
+                ReadyTimestamp = Timestamp.FromDateTime(DateTime.UtcNow),
+            }),
+            cancellationToken);
+    }
+
+    private async Task TryWriteFaultAsync(
+        WorkerFrameProtocolException exception,
+        CancellationToken cancellationToken)
+    {
+        try
+        {
+            await _writer
+                .WriteAsync(CreateEnvelope(CreateFault(exception)), cancellationToken)
+                .ConfigureAwait(false);
+        }
+        catch (Exception faultWriteException) when (
+            faultWriteException is IOException
+            || faultWriteException is ObjectDisposedException
+            || faultWriteException is WorkerFrameProtocolException)
+        {
+            // The original protocol failure is the actionable error.
+        }
+    }
+
+    private WorkerEnvelope CreateEnvelope(WorkerHello hello)
+    {
+        return CreateBaseEnvelope(hello);
+    }
+
+    private WorkerEnvelope CreateEnvelope(WorkerReady ready)
+    {
+        return CreateBaseEnvelope(ready);
+    }
+
+    private WorkerEnvelope CreateEnvelope(WorkerFault fault)
+    {
+        return CreateBaseEnvelope(fault);
+    }
+
+    private WorkerEnvelope CreateBaseEnvelope(WorkerHello body)
+    {
+        WorkerEnvelope envelope = CreateBaseEnvelope();
+        envelope.WorkerHello = body;
+        return envelope;
+    }
+
+    private WorkerEnvelope CreateBaseEnvelope(WorkerReady body)
+    {
+        WorkerEnvelope envelope = CreateBaseEnvelope();
+        envelope.WorkerReady = body;
+        return envelope;
+    }
+
+    private WorkerEnvelope CreateBaseEnvelope(WorkerFault body)
+    {
+        WorkerEnvelope envelope = CreateBaseEnvelope();
+        envelope.WorkerFault = body;
+        return envelope;
+    }
+
+    private WorkerEnvelope CreateBaseEnvelope()
+    {
+        return new WorkerEnvelope
+        {
+            ProtocolVersion = _options.ProtocolVersion,
+            SessionId = _options.SessionId,
+            Sequence = NextSequence(),
+        };
+    }
+
+    private ulong NextSequence()
+    {
+        return unchecked((ulong)Interlocked.Increment(ref _nextSequence));
+    }
+
+    private static WorkerFault CreateFault(WorkerFrameProtocolException exception)
+    {
+        return new WorkerFault
+        {
+            Category = MapFaultCategory(exception.ErrorCode),
+            ExceptionType = exception.GetType().FullName ?? string.Empty,
+            DiagnosticMessage = exception.Message,
+            ProtocolStatus = new ProtocolStatus
+            {
+                Code = ProtocolStatusCode.ProtocolViolation,
+                Message = exception.Message,
+            },
+        };
+    }
+
+    private static WorkerFaultCategory MapFaultCategory(WorkerFrameProtocolErrorCode errorCode)
+    {
+        return errorCode switch
+        {
+            WorkerFrameProtocolErrorCode.ProtocolVersionMismatch => WorkerFaultCategory.ProtocolMismatch,
+            WorkerFrameProtocolErrorCode.EndOfStream => WorkerFaultCategory.PipeDisconnected,
+            _ => WorkerFaultCategory.ProtocolViolation,
+        };
+    }
+}

src/MxGateway.Worker/WorkerApplication.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using MxGateway.Worker.Bootstrap;
+using MxGateway.Worker.Ipc;
 
 namespace MxGateway.Worker;
 
@@ -11,13 +13,27 @@ public static class WorkerApplication
         return Run(
             args,
             new EnvironmentVariableWorkerEnvironment(),
-            new WorkerConsoleLogger(Console.Error));
+            new WorkerConsoleLogger(Console.Error),
+            new WorkerPipeClient());
     }
 
     public static int Run(
         string[] args,
         IWorkerEnvironment environment,
         IWorkerLogger logger)
+    {
+        return Run(
+            args,
+            environment,
+            logger,
+            new WorkerPipeClient());
+    }
+
+    public static int Run(
+        string[] args,
+        IWorkerEnvironment environment,
+        IWorkerLogger logger,
+        IWorkerPipeClient pipeClient)
     {
         if (args is null)
         {
@@ -34,6 +50,11 @@ public static class WorkerApplication
             throw new ArgumentNullException(nameof(logger));
         }
 
+        if (pipeClient is null)
+        {
+            throw new ArgumentNullException(nameof(pipeClient));
+        }
+
         try
         {
             WorkerOptionsParser parser = new(environment);
@@ -61,8 +82,38 @@ public static class WorkerApplication
                 ["nonce"] = options.Nonce,
             });
 
+            pipeClient.RunAsync(options).GetAwaiter().GetResult();
+
+            logger.Information("WorkerPipeHandshakeSucceeded", new Dictionary<string, object?>
+            {
+                ["session_id"] = options.SessionId,
+                ["pipe_name"] = options.PipeName,
+                ["protocol_version"] = options.ProtocolVersion,
+            });
+
             return (int)WorkerExitCode.Success;
         }
+        catch (WorkerFrameProtocolException exception)
+        {
+            logger.Error("WorkerPipeProtocolFailure", new Dictionary<string, object?>
+            {
+                ["exit_code"] = WorkerExitCode.ProtocolViolation,
+                ["error_code"] = exception.ErrorCode,
+                ["exception_type"] = exception.GetType().FullName,
+            });
+
+            return (int)WorkerExitCode.ProtocolViolation;
+        }
+        catch (Exception exception) when (exception is IOException or TimeoutException)
+        {
+            logger.Error("WorkerPipeConnectionFailed", new Dictionary<string, object?>
+            {
+                ["exit_code"] = WorkerExitCode.PipeConnectionFailed,
+                ["exception_type"] = exception.GetType().FullName,
+            });
+
+            return (int)WorkerExitCode.PipeConnectionFailed;
+        }
         catch (Exception exception)
         {
             logger.Error("WorkerBootstrapUnexpectedFailure", new Dictionary<string, object?>