diff --git a/src/Netclaw.Actors.Tests/Channels/MattermostInitialConnectionGateTests.cs b/src/Netclaw.Actors.Tests/Channels/MattermostInitialConnectionGateTests.cs
new file mode 100644
index 000000000..a4cf396a8
--- /dev/null
+++ b/src/Netclaw.Actors.Tests/Channels/MattermostInitialConnectionGateTests.cs
@@ -0,0 +1,130 @@
+// -----------------------------------------------------------------------
+// <copyright file="MattermostInitialConnectionGateTests.cs" company="Petabridge, LLC">
+//      Copyright (C) 2026 - 2026 Petabridge, LLC <https://petabridge.com>
+// </copyright>
+// -----------------------------------------------------------------------
+using Mattermost.Events;
+using Microsoft.Extensions.Time.Testing;
+using Netclaw.Channels.Mattermost.Transport;
+using Xunit;
+
+namespace Netclaw.Actors.Tests.Channels;
+
+public sealed class MattermostInitialConnectionGateTests
+{
+    [Fact]
+    public async Task StartWaitsForConnectedAndRemovesHandler()
+    {
+        var timeProvider = new FakeTimeProvider();
+        var subscription = new ConnectionSubscription();
+        var startCount = 0;
+
+        var start = MattermostInitialConnectionGate.StartAndWaitAsync(
+            cancellationToken =>
+            {
+                Assert.False(cancellationToken.IsCancellationRequested);
+                Assert.NotNull(subscription.Handler);
+                startCount++;
+                return Task.CompletedTask;
+            },
+            subscription.Subscribe,
+            subscription.Unsubscribe,
+            timeProvider,
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(1, startCount);
+        Assert.False(start.IsCompleted);
+
+        subscription.RaiseConnected(timeProvider);
+        await start;
+
+        Assert.Equal(1, subscription.UnsubscribeCount);
+        Assert.Equal(subscription.Handler, subscription.RemovedHandler);
+    }
+
+    [Fact]
+    public async Task TimeoutUsesTimeProviderAndRemovesHandler()
+    {
+        var timeProvider = new FakeTimeProvider();
+        var subscription = new ConnectionSubscription();
+        var start = MattermostInitialConnectionGate.StartAndWaitAsync(
+            _ => Task.CompletedTask,
+            subscription.Subscribe,
+            subscription.Unsubscribe,
+            timeProvider,
+            TestContext.Current.CancellationToken);
+
+        timeProvider.Advance(MattermostInitialConnectionGate.Timeout);
+
+        await Assert.ThrowsAsync<TimeoutException>(() => start);
+        Assert.Equal(1, subscription.UnsubscribeCount);
+        Assert.Equal(subscription.Handler, subscription.RemovedHandler);
+    }
+
+    [Fact]
+    public async Task CancellationRemovesHandler()
+    {
+        var timeProvider = new FakeTimeProvider();
+        var subscription = new ConnectionSubscription();
+        using var cancellation = CancellationTokenSource.CreateLinkedTokenSource(
+            TestContext.Current.CancellationToken);
+        var start = MattermostInitialConnectionGate.StartAndWaitAsync(
+            _ => Task.CompletedTask,
+            subscription.Subscribe,
+            subscription.Unsubscribe,
+            timeProvider,
+            cancellation.Token);
+
+        await cancellation.CancelAsync();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(() => start);
+        Assert.Equal(1, subscription.UnsubscribeCount);
+        Assert.Equal(subscription.Handler, subscription.RemovedHandler);
+    }
+
+    [Fact]
+    public async Task StartFailureRemovesHandler()
+    {
+        var timeProvider = new FakeTimeProvider();
+        var subscription = new ConnectionSubscription();
+        var start = MattermostInitialConnectionGate.StartAndWaitAsync(
+            _ => throw new InvalidOperationException("Start failed."),
+            subscription.Subscribe,
+            subscription.Unsubscribe,
+            timeProvider,
+            TestContext.Current.CancellationToken);
+
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(() => start);
+
+        Assert.Equal("Start failed.", exception.Message);
+        Assert.Equal(1, subscription.UnsubscribeCount);
+        Assert.Equal(subscription.Handler, subscription.RemovedHandler);
+    }
+
+    private sealed class ConnectionSubscription
+    {
+        public EventHandler<ConnectionEventArgs>? Handler { get; private set; }
+
+        public EventHandler<ConnectionEventArgs>? RemovedHandler { get; private set; }
+
+        public int UnsubscribeCount { get; private set; }
+
+        public void Subscribe(EventHandler<ConnectionEventArgs> handler) => Handler = handler;
+
+        public void Unsubscribe(EventHandler<ConnectionEventArgs> handler)
+        {
+            RemovedHandler = handler;
+            UnsubscribeCount++;
+        }
+
+        public void RaiseConnected(TimeProvider timeProvider)
+        {
+            Assert.NotNull(Handler);
+            Handler(
+                this,
+                new ConnectionEventArgs(
+                    new Uri("wss://mattermost.test/api/v4/websocket"),
+                    timeProvider.GetUtcNow().UtcDateTime));
+        }
+    }
+}
diff --git a/src/Netclaw.Channels.Mattermost/Transport/MattermostNetGatewayClient.cs b/src/Netclaw.Channels.Mattermost/Transport/MattermostNetGatewayClient.cs
index 88a0cd895..f55dd86d0 100644
--- a/src/Netclaw.Channels.Mattermost/Transport/MattermostNetGatewayClient.cs
+++ b/src/Netclaw.Channels.Mattermost/Transport/MattermostNetGatewayClient.cs
@@ -195,7 +195,15 @@ public async Task<MattermostBotIdentity> StartAsync(string serverUrl, string bot
         _logger.LogInformation("Bot identity resolved: {BotUserId} (@{Username})",
             me.Id, me.Username);
 
-        await _client.StartReceivingAsync(cancellationToken);
+        // Mattermost.NET starts its receiver before the authenticated WebSocket
+        // exists. Preserve the transport contract by waiting for OnConnected.
+        await MattermostInitialConnectionGate.StartAndWaitAsync(
+            _client.StartReceivingAsync,
+            handler => _client.OnConnected += handler,
+            handler => _client.OnConnected -= handler,
+            _timeProvider,
+            cancellationToken);
+
         return new MattermostBotIdentity(me.Id, me.Username);
     }
 
@@ -400,3 +408,30 @@ private async Task AwaitDispatchAsync(string operation, Task task)
         }
     }
 }
+
+internal static class MattermostInitialConnectionGate
+{
+    internal static readonly TimeSpan Timeout = TimeSpan.FromSeconds(30);
+
+    public static async Task StartAndWaitAsync(
+        Func<CancellationToken, Task> startReceivingAsync,
+        Action<EventHandler<ConnectionEventArgs>> subscribe,
+        Action<EventHandler<ConnectionEventArgs>> unsubscribe,
+        TimeProvider timeProvider,
+        CancellationToken cancellationToken)
+    {
+        var connected = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        void HandleConnected(object? sender, ConnectionEventArgs e) => connected.TrySetResult();
+
+        subscribe(HandleConnected);
+        try
+        {
+            await startReceivingAsync(cancellationToken);
+            await connected.Task.WaitAsync(Timeout, timeProvider, cancellationToken);
+        }
+        finally
+        {
+            unsubscribe(HandleConnected);
+        }
+    }
+}