diff --git a/docs/mxaccess-worker-instance-design.md b/docs/mxaccess-worker-instance-design.md
index 0682494..60eb2d0 100644
--- a/docs/mxaccess-worker-instance-design.md
+++ b/docs/mxaccess-worker-instance-design.md
@@ -250,6 +250,17 @@ The loop should update a heartbeat timestamp after:
 - finishing a command,
 - processing an MXAccess event.
 
+`StaRuntime` implements this runtime boundary in the worker. It starts one
+background thread named `MxGateway.Worker.STA`, sets it to `ApartmentState.STA`,
+initializes COM through `StaComApartmentInitializer`, and runs
+`StaMessagePump`. Commands are scheduled through `InvokeAsync`; the command
+queue signals an `AutoResetEvent` so `MsgWaitForMultipleObjectsEx` can wake the
+STA without busy-waiting. `LastActivityUtc` records pump, command, startup, and
+shutdown activity so the future heartbeat/watchdog can report whether the STA
+is still responsive. Shutdown marks the runtime as closing, wakes the pump,
+rejects new commands, cancels queued work, uninitializes COM on the STA, and
+waits for the thread to exit.
+
 ## COM Creation
 
 The MXAccess analysis source at `C:\Users\dohertj2\Desktop\mxaccess` identifies
diff --git a/src/MxGateway.Worker.Tests/Sta/StaRuntimeTests.cs b/src/MxGateway.Worker.Tests/Sta/StaRuntimeTests.cs
new file mode 100644
index 0000000..43a1014
--- /dev/null
+++ b/src/MxGateway.Worker.Tests/Sta/StaRuntimeTests.cs
@@ -0,0 +1,152 @@
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using MxGateway.Worker.Sta;
+
+namespace MxGateway.Worker.Tests.Sta;
+
+public sealed class StaRuntimeTests
+{
+    [Fact]
+    public async Task InvokeAsync_ExecutesCommandOnStaThread()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+
+        runtime.Start();
+
+        StaCommandObservation observation = await runtime.InvokeAsync(
+            () => new StaCommandObservation(
+                Thread.CurrentThread.ManagedThreadId,
+                Thread.CurrentThread.GetApartmentState()));
+
+        Assert.Equal(runtime.StaThreadId, observation.ThreadId);
+        Assert.Equal(initializer.InitializeThreadId, observation.ThreadId);
+        Assert.Equal(ApartmentState.STA, observation.ApartmentState);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_WakesIdlePumpForQueuedCommand()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = new(
+            initializer,
+            new StaMessagePump(),
+            TimeSpan.FromSeconds(30));
+        runtime.Start();
+        Stopwatch stopwatch = Stopwatch.StartNew();
+
+        int threadId = await runtime.InvokeAsync(() => Thread.CurrentThread.ManagedThreadId);
+
+        stopwatch.Stop();
+        Assert.Equal(runtime.StaThreadId, threadId);
+        Assert.True(
+            stopwatch.Elapsed < TimeSpan.FromSeconds(2),
+            $"Command took {stopwatch.Elapsed} to execute, so the command wake event did not wake the STA promptly.");
+    }
+
+    [Fact]
+    public void Shutdown_StopsThreadAndUninitializesComApartment()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+        runtime.Start();
+
+        bool stopped = runtime.Shutdown(TimeSpan.FromSeconds(2));
+
+        Assert.True(stopped);
+        Assert.False(runtime.IsRunning);
+        Assert.Equal(1, initializer.InitializeCount);
+        Assert.Equal(1, initializer.UninitializeCount);
+        Assert.Equal(initializer.InitializeThreadId, initializer.UninitializeThreadId);
+    }
+
+    [Fact]
+    public void LastActivityUtc_UpdatesWhilePumpIsIdle()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+        runtime.Start();
+        DateTimeOffset firstActivity = runtime.LastActivityUtc;
+
+        bool updated = SpinWait.SpinUntil(
+            () => runtime.LastActivityUtc > firstActivity,
+            TimeSpan.FromSeconds(2));
+
+        Assert.True(updated);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_CommandException_FaultsReturnedTaskWithoutStoppingRuntime()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+        runtime.Start();
+
+        InvalidOperationException exception = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => runtime.InvokeAsync<int>(() => throw new InvalidOperationException("command failed")));
+
+        int threadId = await runtime.InvokeAsync(() => Thread.CurrentThread.ManagedThreadId);
+        Assert.Equal("command failed", exception.Message);
+        Assert.Equal(runtime.StaThreadId, threadId);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_AfterShutdown_ReturnsFaultedTask()
+    {
+        RecordingComApartmentInitializer initializer = new();
+        using StaRuntime runtime = CreateRuntime(initializer);
+        runtime.Start();
+        runtime.Shutdown(TimeSpan.FromSeconds(2));
+
+        InvalidOperationException exception = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => runtime.InvokeAsync(() => Thread.CurrentThread.ManagedThreadId));
+
+        Assert.Contains("shutting down", exception.Message);
+    }
+
+    private static StaRuntime CreateRuntime(RecordingComApartmentInitializer initializer)
+    {
+        return new StaRuntime(
+            initializer,
+            new StaMessagePump(),
+            TimeSpan.FromMilliseconds(25));
+    }
+
+    private sealed class StaCommandObservation
+    {
+        public StaCommandObservation(int threadId, ApartmentState apartmentState)
+        {
+            ThreadId = threadId;
+            ApartmentState = apartmentState;
+        }
+
+        public int ThreadId { get; }
+
+        public ApartmentState ApartmentState { get; }
+    }
+
+    private sealed class RecordingComApartmentInitializer : IStaComApartmentInitializer
+    {
+        public int InitializeCount { get; private set; }
+
+        public int UninitializeCount { get; private set; }
+
+        public int? InitializeThreadId { get; private set; }
+
+        public int? UninitializeThreadId { get; private set; }
+
+        public void Initialize()
+        {
+            InitializeCount++;
+            InitializeThreadId = Thread.CurrentThread.ManagedThreadId;
+        }
+
+        public void Uninitialize()
+        {
+            UninitializeCount++;
+            UninitializeThreadId = Thread.CurrentThread.ManagedThreadId;
+        }
+    }
+}
diff --git a/src/MxGateway.Worker/Sta/IStaComApartmentInitializer.cs b/src/MxGateway.Worker/Sta/IStaComApartmentInitializer.cs
new file mode 100644
index 0000000..782a8eb
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/IStaComApartmentInitializer.cs
@@ -0,0 +1,8 @@
+namespace MxGateway.Worker.Sta;
+
+public interface IStaComApartmentInitializer
+{
+    void Initialize();
+
+    void Uninitialize();
+}
diff --git a/src/MxGateway.Worker/Sta/IStaWorkItem.cs b/src/MxGateway.Worker/Sta/IStaWorkItem.cs
new file mode 100644
index 0000000..735385a
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/IStaWorkItem.cs
@@ -0,0 +1,8 @@
+namespace MxGateway.Worker.Sta;
+
+internal interface IStaWorkItem
+{
+    void CancelBeforeExecution();
+
+    void Execute();
+}
diff --git a/src/MxGateway.Worker/Sta/StaComApartmentInitializer.cs b/src/MxGateway.Worker/Sta/StaComApartmentInitializer.cs
new file mode 100644
index 0000000..2f6b2e0
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/StaComApartmentInitializer.cs
@@ -0,0 +1,31 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace MxGateway.Worker.Sta;
+
+public sealed class StaComApartmentInitializer : IStaComApartmentInitializer
+{
+    private const uint CoInitializeApartmentThreaded = 0x2;
+    private const int SOk = 0;
+    private const int SFalse = 1;
+
+    public void Initialize()
+    {
+        int hresult = CoInitializeEx(IntPtr.Zero, CoInitializeApartmentThreaded);
+        if (hresult != SOk && hresult != SFalse)
+        {
+            throw new COMException("Failed to initialize the worker STA COM apartment.", hresult);
+        }
+    }
+
+    public void Uninitialize()
+    {
+        CoUninitialize();
+    }
+
+    [DllImport("ole32.dll")]
+    private static extern int CoInitializeEx(IntPtr reserved, uint coInit);
+
+    [DllImport("ole32.dll")]
+    private static extern void CoUninitialize();
+}
diff --git a/src/MxGateway.Worker/Sta/StaMessagePump.cs b/src/MxGateway.Worker/Sta/StaMessagePump.cs
new file mode 100644
index 0000000..e0a0f21
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/StaMessagePump.cs
@@ -0,0 +1,111 @@
+using System;
+using System.Runtime.InteropServices;
+using System.Threading;
+using Microsoft.Win32.SafeHandles;
+
+namespace MxGateway.Worker.Sta;
+
+public sealed class StaMessagePump
+{
+    private const uint Infinite = 0xFFFFFFFF;
+    private const uint MsgWaitFailed = 0xFFFFFFFF;
+    private const uint MwmoInputAvailable = 0x0004;
+    private const uint PmRemove = 0x0001;
+    private const uint QsAllInput = 0x04FF;
+
+    public void WaitForWorkOrMessages(WaitHandle commandWakeEvent, TimeSpan timeout)
+    {
+        if (commandWakeEvent is null)
+        {
+            throw new ArgumentNullException(nameof(commandWakeEvent));
+        }
+
+        uint timeoutMilliseconds = ToTimeoutMilliseconds(timeout);
+
+        SafeWaitHandle safeHandle = commandWakeEvent.SafeWaitHandle;
+        IntPtr[] handles = [safeHandle.DangerousGetHandle()];
+        uint result = MsgWaitForMultipleObjectsEx(
+            (uint)handles.Length,
+            handles,
+            timeoutMilliseconds,
+            QsAllInput,
+            MwmoInputAvailable);
+
+        if (result == MsgWaitFailed)
+        {
+            throw new InvalidOperationException(
+                "The worker STA message pump failed while waiting for command work or Windows messages.");
+        }
+    }
+
+    public int PumpPendingMessages()
+    {
+        int pumpedMessages = 0;
+
+        while (PeekMessage(out NativeMessage message, IntPtr.Zero, 0, 0, PmRemove))
+        {
+            TranslateMessage(ref message);
+            DispatchMessage(ref message);
+            pumpedMessages++;
+        }
+
+        return pumpedMessages;
+    }
+
+    private static uint ToTimeoutMilliseconds(TimeSpan timeout)
+    {
+        if (timeout == Timeout.InfiniteTimeSpan)
+        {
+            return Infinite;
+        }
+
+        if (timeout <= TimeSpan.Zero)
+        {
+            return 0;
+        }
+
+        return timeout.TotalMilliseconds >= uint.MaxValue
+            ? uint.MaxValue - 1
+            : (uint)Math.Ceiling(timeout.TotalMilliseconds);
+    }
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern uint MsgWaitForMultipleObjectsEx(
+        uint count,
+        IntPtr[] handles,
+        uint milliseconds,
+        uint wakeMask,
+        uint flags);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool PeekMessage(
+        out NativeMessage message,
+        IntPtr windowHandle,
+        uint messageFilterMin,
+        uint messageFilterMax,
+        uint removeMessage);
+
+    [DllImport("user32.dll")]
+    private static extern bool TranslateMessage(ref NativeMessage message);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr DispatchMessage(ref NativeMessage message);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct NativeMessage
+    {
+        public IntPtr WindowHandle;
+        public uint Message;
+        public UIntPtr WParam;
+        public IntPtr LParam;
+        public uint Time;
+        public NativePoint Point;
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct NativePoint
+    {
+        public int X;
+        public int Y;
+    }
+}
diff --git a/src/MxGateway.Worker/Sta/StaRuntime.cs b/src/MxGateway.Worker/Sta/StaRuntime.cs
new file mode 100644
index 0000000..7d88401
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/StaRuntime.cs
@@ -0,0 +1,267 @@
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MxGateway.Worker.Sta;
+
+public sealed class StaRuntime : IDisposable
+{
+    private readonly IStaComApartmentInitializer comApartmentInitializer;
+    private readonly StaMessagePump messagePump;
+    private readonly ConcurrentQueue<IStaWorkItem> commandQueue = new();
+    private readonly AutoResetEvent commandWakeEvent = new(false);
+    private readonly ManualResetEventSlim startedEvent = new(false);
+    private readonly ManualResetEventSlim stoppedEvent = new(false);
+    private readonly object gate = new();
+    private readonly Thread staThread;
+    private readonly TimeSpan idlePumpInterval;
+    private bool disposed;
+    private bool startRequested;
+    private bool shutdownRequested;
+    private Exception? startupException;
+    private long lastActivityUtcTicks;
+    private bool comInitialized;
+
+    public StaRuntime()
+        : this(new StaComApartmentInitializer(), new StaMessagePump(), TimeSpan.FromMilliseconds(50))
+    {
+    }
+
+    public StaRuntime(
+        IStaComApartmentInitializer comApartmentInitializer,
+        StaMessagePump messagePump,
+        TimeSpan idlePumpInterval)
+    {
+        this.comApartmentInitializer = comApartmentInitializer
+            ?? throw new ArgumentNullException(nameof(comApartmentInitializer));
+        this.messagePump = messagePump ?? throw new ArgumentNullException(nameof(messagePump));
+
+        if (idlePumpInterval <= TimeSpan.Zero)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(idlePumpInterval),
+                "The idle pump interval must be greater than zero.");
+        }
+
+        this.idlePumpInterval = idlePumpInterval;
+        lastActivityUtcTicks = DateTimeOffset.UtcNow.UtcTicks;
+        staThread = new Thread(ThreadMain)
+        {
+            IsBackground = true,
+            Name = "MxGateway.Worker.STA"
+        };
+        staThread.SetApartmentState(ApartmentState.STA);
+    }
+
+    public int? StaThreadId { get; private set; }
+
+    public DateTimeOffset LastActivityUtc =>
+        new(new DateTime(Volatile.Read(ref lastActivityUtcTicks), DateTimeKind.Utc));
+
+    public bool IsRunning => startedEvent.IsSet && !stoppedEvent.IsSet;
+
+    public void Start()
+    {
+        ThrowIfDisposed();
+
+        lock (gate)
+        {
+            if (shutdownRequested)
+            {
+                throw new InvalidOperationException("The worker STA runtime is shutting down.");
+            }
+
+            if (!startRequested)
+            {
+                startRequested = true;
+                staThread.Start();
+            }
+        }
+
+        startedEvent.Wait();
+        if (startupException is not null)
+        {
+            throw new InvalidOperationException(
+                "The worker STA runtime failed to initialize.",
+                startupException);
+        }
+    }
+
+    public Task InvokeAsync(Action command, CancellationToken cancellationToken = default)
+    {
+        if (command is null)
+        {
+            throw new ArgumentNullException(nameof(command));
+        }
+
+        return InvokeAsync(
+            () =>
+            {
+                command();
+                return true;
+            },
+            cancellationToken);
+    }
+
+    public Task<T> InvokeAsync<T>(Func<T> command, CancellationToken cancellationToken = default)
+    {
+        if (command is null)
+        {
+            throw new ArgumentNullException(nameof(command));
+        }
+
+        ThrowIfDisposed();
+
+        if (cancellationToken.IsCancellationRequested)
+        {
+            return Task.FromCanceled<T>(cancellationToken);
+        }
+
+        StaWorkItem<T> workItem = new(command, cancellationToken);
+
+        lock (gate)
+        {
+            if (shutdownRequested)
+            {
+                return Task.FromException<T>(
+                    new InvalidOperationException("The worker STA runtime is shutting down."));
+            }
+
+            commandQueue.Enqueue(workItem);
+        }
+
+        commandWakeEvent.Set();
+        return workItem.Task;
+    }
+
+    public bool Shutdown(TimeSpan timeout)
+    {
+        if (timeout < TimeSpan.Zero && timeout != Timeout.InfiniteTimeSpan)
+        {
+            throw new ArgumentOutOfRangeException(nameof(timeout));
+        }
+
+        lock (gate)
+        {
+            shutdownRequested = true;
+        }
+
+        commandWakeEvent.Set();
+
+        if (!startedEvent.IsSet && !staThread.IsAlive)
+        {
+            CancelQueuedCommands();
+            stoppedEvent.Set();
+            return true;
+        }
+
+        bool stopped = stoppedEvent.Wait(timeout);
+        if (stopped)
+        {
+            CancelQueuedCommands();
+        }
+
+        return stopped;
+    }
+
+    public void Dispose()
+    {
+        if (disposed)
+        {
+            return;
+        }
+
+        bool stopped = Shutdown(TimeSpan.FromSeconds(5));
+        if (stopped)
+        {
+            commandWakeEvent.Dispose();
+            startedEvent.Dispose();
+            stoppedEvent.Dispose();
+        }
+
+        disposed = true;
+    }
+
+    private void ThreadMain()
+    {
+        try
+        {
+            StaThreadId = Thread.CurrentThread.ManagedThreadId;
+            comApartmentInitializer.Initialize();
+            comInitialized = true;
+            MarkActivity();
+            startedEvent.Set();
+
+            while (!IsShutdownRequested())
+            {
+                ProcessQueuedCommands();
+                messagePump.WaitForWorkOrMessages(commandWakeEvent, idlePumpInterval);
+                messagePump.PumpPendingMessages();
+                MarkActivity();
+            }
+
+            ProcessQueuedCommands();
+        }
+        catch (Exception exception)
+        {
+            startupException = exception;
+            startedEvent.Set();
+        }
+        finally
+        {
+            CancelQueuedCommands();
+            try
+            {
+                if (comInitialized)
+                {
+                    comApartmentInitializer.Uninitialize();
+                }
+            }
+            finally
+            {
+                MarkActivity();
+                stoppedEvent.Set();
+            }
+        }
+    }
+
+    private void ProcessQueuedCommands()
+    {
+        while (commandQueue.TryDequeue(out IStaWorkItem? workItem))
+        {
+            MarkActivity();
+            workItem.Execute();
+            MarkActivity();
+        }
+    }
+
+    private void CancelQueuedCommands()
+    {
+        while (commandQueue.TryDequeue(out IStaWorkItem? workItem))
+        {
+            workItem.CancelBeforeExecution();
+        }
+    }
+
+    private bool IsShutdownRequested()
+    {
+        lock (gate)
+        {
+            return shutdownRequested;
+        }
+    }
+
+    private void MarkActivity()
+    {
+        Volatile.Write(ref lastActivityUtcTicks, DateTimeOffset.UtcNow.UtcTicks);
+    }
+
+    private void ThrowIfDisposed()
+    {
+        if (disposed)
+        {
+            throw new ObjectDisposedException(nameof(StaRuntime));
+        }
+    }
+}
diff --git a/src/MxGateway.Worker/Sta/StaWorkItem.cs b/src/MxGateway.Worker/Sta/StaWorkItem.cs
new file mode 100644
index 0000000..3a7f8e0
--- /dev/null
+++ b/src/MxGateway.Worker/Sta/StaWorkItem.cs
@@ -0,0 +1,71 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MxGateway.Worker.Sta;
+
+internal sealed class StaWorkItem<T> : IStaWorkItem
+{
+    private readonly Func<T> command;
+    private readonly CancellationToken cancellationToken;
+    private readonly CancellationTokenRegistration cancellationRegistration;
+    private int started;
+
+    public StaWorkItem(Func<T> command, CancellationToken cancellationToken)
+    {
+        this.command = command ?? throw new ArgumentNullException(nameof(command));
+        this.cancellationToken = cancellationToken;
+        Completion = new TaskCompletionSource<T>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        if (cancellationToken.CanBeCanceled)
+        {
+            cancellationRegistration = cancellationToken.Register(
+                () =>
+                {
+                    if (Interlocked.CompareExchange(ref started, 1, 0) == 0)
+                    {
+                        Completion.TrySetCanceled(cancellationToken);
+                    }
+                });
+        }
+    }
+
+    public Task<T> Task => Completion.Task;
+
+    private TaskCompletionSource<T> Completion { get; }
+
+    public void CancelBeforeExecution()
+    {
+        if (Interlocked.CompareExchange(ref started, 1, 0) == 0)
+        {
+            Completion.TrySetCanceled(cancellationToken);
+            cancellationRegistration.Dispose();
+        }
+    }
+
+    public void Execute()
+    {
+        if (Interlocked.CompareExchange(ref started, 1, 0) != 0)
+        {
+            cancellationRegistration.Dispose();
+            return;
+        }
+
+        cancellationRegistration.Dispose();
+
+        if (cancellationToken.IsCancellationRequested)
+        {
+            Completion.TrySetCanceled(cancellationToken);
+            return;
+        }
+
+        try
+        {
+            Completion.TrySetResult(command());
+        }
+        catch (Exception exception)
+        {
+            Completion.TrySetException(exception);
+        }
+    }
+}