diff --git a/test/Finos.Fdc3.Backplane.Tests/Hubs/DesktopAgentsHubTests.cs b/test/Finos.Fdc3.Backplane.Tests/Hubs/DesktopAgentsHubTests.cs
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+++ b/test/Finos.Fdc3.Backplane.Tests/Hubs/DesktopAgentsHubTests.cs
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+/*
+	* SPDX-License-Identifier: Apache-2.0
+	* Copyright 2022 FINOS FDC3 contributors - see NOTICE file
+	*/
+
+using AutoFixture;
+using Finos.Fdc3.Backplane.Config;
+using Finos.Fdc3.Backplane.DTO;
+using Finos.Fdc3.Backplane.DTO.Envelope;
+using Finos.Fdc3.Backplane.DTO.FDC3;
+using Finos.Fdc3.Backplane.Hubs;
+using Finos.Fdc3.Backplane.MultiHost;
+using Microsoft.AspNetCore.SignalR;
+using Microsoft.Extensions.Logging;
+using Newtonsoft.Json.Linq;
+using NSubstitute;
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Finos.Fdc3.Backplane.Tests.Hubs
+{
+    [TestFixture]
+    public class DesktopAgentsHubTests
+    {
+        private const string ConnectionId = "test-connection-id";
+        private const string BroadcastEndpoint = "api/backplane/broadcast/context";
+
+        private Fixture _fixture;
+        private ILogger<DesktopAgentsHub> _logger;
+        private IHubContext<DesktopAgentsHub> _hubContext;
+        private IHubClients _hubClients;
+        private IClientProxy _allExceptProxy;
+        private IClientProxy _allProxy;
+        private IHttpClientFactory _httpClientFactory;
+        private INodesRepository _nodesRepository;
+        private IConfigRepository _configRepository;
+        private HubCallerContext _callerContext;
+        private TestHttpMessageHandler _httpMessageHandler;
+        private DesktopAgentsHub _sut;
+
+        [SetUp]
+        public void Setup()
+        {
+            _fixture = AutoFixtures.Create();
+
+            _logger = Substitute.For<ILogger<DesktopAgentsHub>>();
+
+            _allExceptProxy = Substitute.For<IClientProxy>();
+            _allProxy = Substitute.For<IClientProxy>();
+            _hubClients = Substitute.For<IHubClients>();
+            _hubClients.All.Returns(_allProxy);
+            _hubClients.AllExcept(Arg.Any<IReadOnlyList<string>>()).Returns(_allExceptProxy);
+            _hubContext = Substitute.For<IHubContext<DesktopAgentsHub>>();
+            _hubContext.Clients.Returns(_hubClients);
+
+            _httpMessageHandler = new TestHttpMessageHandler();
+            _httpClientFactory = Substitute.For<IHttpClientFactory>();
+            _httpClientFactory.CreateClient("Backplane").Returns(_ => new HttpClient(_httpMessageHandler, false));
+
+            _nodesRepository = Substitute.For<INodesRepository>();
+            _nodesRepository.MemberNodes.Returns(Enumerable.Empty<Uri>());
+
+            _configRepository = Substitute.For<IConfigRepository>();
+            _configRepository.HttpRequestTimeoutInMilliseconds.Returns(TimeSpan.FromMilliseconds(5000));
+            _configRepository.BroadcastEndpoint.Returns(BroadcastEndpoint);
+            _configRepository.Channels.Returns(_fixture.CreateMany<Channel>(8).ToList());
+
+            _callerContext = Substitute.For<HubCallerContext>();
+            _callerContext.ConnectionId.Returns(ConnectionId);
+
+            _sut = new DesktopAgentsHub(_logger, _hubContext, _httpClientFactory, _nodesRepository, _configRepository)
+            {
+                Context = _callerContext
+            };
+        }
+
+        [Test]
+        public void Broadcast_ShouldThrowHubException_WhenPayloadIsNull()
+        {
+            //Arrange
+            //Act
+            HubException ex = Assert.ThrowsAsync<HubException>(() => _sut.Broadcast(null));
+            //Assert
+            Assert.That(ex.HResult, Is.EqualTo((int)ResponseCodes.BroadcastPayloadInvalid));
+            Assert.That(_httpMessageHandler.Requests, Is.Empty);
+        }
+
+        [Test]
+        public void BroadcastToLocalClients_ShouldThrowHubException_WhenPayloadIsNull()
+        {
+            //Arrange
+            //Act
+            HubException ex = Assert.ThrowsAsync<HubException>(() => _sut.BroadcastToLocalClients(null));
+            //Assert
+            Assert.That(ex.HResult, Is.EqualTo((int)ResponseCodes.BroadcastPayloadInvalid));
+            Assert.That(_httpMessageHandler.Requests, Is.Empty);
+        }
+
+        [Test]
+        public async Task Broadcast_ShouldSendToAllClientsExceptSender_AndPostToEachMemberNode()
+        {
+            //Arrange
+            MessageEnvelope envelope = CreateEnvelope();
+            Uri[] memberNodes = { new Uri("http://node1:4475/"), new Uri("http://node2:4476/"), new Uri("http://node3:4477/") };
+            _nodesRepository.MemberNodes.Returns(memberNodes);
+            //Act
+            await _sut.Broadcast(envelope);
+            //Assert
+            _hubClients.Received(1).AllExcept(Arg.Is<IReadOnlyList<string>>(ids => ids.Count == 1 && ids[0] == ConnectionId));
+            await _allExceptProxy.Received(1).SendCoreAsync("OnMessage", Arg.Is<object[]>(args => args.Length == 1 && ReferenceEquals(args[0], envelope)), Arg.Any<CancellationToken>());
+            _ = _hubClients.DidNotReceive().All;
+            await _allProxy.DidNotReceiveWithAnyArgs().SendCoreAsync(default, default, default);
+
+            Assert.That(_httpMessageHandler.Requests, Has.Count.EqualTo(memberNodes.Length));
+            foreach (Uri node in memberNodes)
+            {
+                Uri expected = new Uri(node, BroadcastEndpoint);
+                Assert.That(_httpMessageHandler.Requests.Count(r => r.Method == HttpMethod.Post && r.RequestUri == expected), Is.EqualTo(1));
+            }
+        }
+
+        [Test]
+        public async Task BroadcastToLocalClients_ShouldSendToAllClients_AndNotPostToMemberNodes()
+        {
+            //Arrange
+            MessageEnvelope envelope = CreateEnvelope();
+            _nodesRepository.MemberNodes.Returns(new[] { new Uri("http://node1:4475/"), new Uri("http://node2:4476/") });
+            //Act
+            await _sut.BroadcastToLocalClients(envelope);
+            //Assert
+            _ = _hubClients.Received(1).All;
+            await _allProxy.Received(1).SendCoreAsync("OnMessage", Arg.Is<object[]>(args => args.Length == 1 && ReferenceEquals(args[0], envelope)), Arg.Any<CancellationToken>());
+            _hubClients.DidNotReceiveWithAnyArgs().AllExcept(default);
+            await _allExceptProxy.DidNotReceiveWithAnyArgs().SendCoreAsync(default, default, default);
+            Assert.That(_httpMessageHandler.Requests, Is.Empty);
+        }
+
+        [Test]
+        public async Task Broadcast_ShouldContinueToNextNode_AndLogError_WhenPostToOneNodeFails()
+        {
+            //Arrange
+            MessageEnvelope envelope = CreateEnvelope();
+            Uri failingNode = new Uri("http://failing-node:4475/");
+            Uri healthyNode = new Uri("http://healthy-node:4476/");
+            Uri failingUri = new Uri(failingNode, BroadcastEndpoint);
+            Uri healthyUri = new Uri(healthyNode, BroadcastEndpoint);
+            _nodesRepository.MemberNodes.Returns(new[] { failingNode, healthyNode });
+            _httpMessageHandler.Responder = request =>
+            {
+                if (request.RequestUri == failingUri)
+                {
+                    throw new HttpRequestException("Connection refused");
+                }
+                return new HttpResponseMessage(HttpStatusCode.OK);
+            };
+            //Act
+            //Assert
+            Assert.DoesNotThrowAsync(() => _sut.Broadcast(envelope));
+            Assert.That(_httpMessageHandler.Requests.Count(r => r.RequestUri == failingUri), Is.EqualTo(1));
+            Assert.That(_httpMessageHandler.Requests.Count(r => r.RequestUri == healthyUri), Is.EqualTo(1));
+
+            // Known limitation: LogError is a static extension, so the underlying ILogger.Log<TState> call is
+            // verified instead by matching the formatted state string against the failing node URI. Wording
+            // changes to the LogError call in DesktopAgentsHub.PostRequestAsync require updating this matcher.
+            Assert.That(CountErrorLogsContaining(failingUri.ToString()), Is.EqualTo(1));
+            Assert.That(CountErrorLogsContaining(healthyUri.ToString()), Is.EqualTo(0));
+            await Task.CompletedTask;
+        }
+
+        [Test]
+        public async Task Broadcast_ShouldLogError_WhenNodeReturnsNonSuccessStatusCode()
+        {
+            //Arrange
+            MessageEnvelope envelope = CreateEnvelope();
+            Uri failingNode = new Uri("http://failing-node:4475/");
+            Uri failingUri = new Uri(failingNode, BroadcastEndpoint);
+            _nodesRepository.MemberNodes.Returns(new[] { failingNode });
+            _httpMessageHandler.Responder = _ => new HttpResponseMessage(HttpStatusCode.InternalServerError);
+            //Act
+            await _sut.Broadcast(envelope);
+            //Assert
+            Assert.That(_httpMessageHandler.Requests, Has.Count.EqualTo(1));
+            Assert.That(CountErrorLogsContaining(failingUri.ToString()), Is.EqualTo(1));
+        }
+
+        [Test]
+        public async Task GetUserChannels_ShouldReturnChannelsFromConfigRepository()
+        {
+            //Arrange
+            List<Channel> channels = _fixture.CreateMany<Channel>(8).ToList();
+            _configRepository.Channels.Returns(channels);
+            //Act
+            IEnumerable<Channel> result = await _sut.GetUserChannels();
+            //Assert
+            Assert.That(result, Is.SameAs(channels));
+            Assert.That(result.Count(), Is.EqualTo(8));
+            Assert.That(result, Is.EqualTo(channels));
+        }
+
+        /// <summary>
+        /// Counts ILogger.Log&lt;TState&gt; calls at Error level whose formatted state contains <paramref name="substring"/>.
+        /// TState is the framework's internal FormattedLogValues, so the call cannot be matched via
+        /// <c>Received().Log&lt;TState&gt;(...)</c> with a public TState; the recorded calls are inspected instead.
+        /// </summary>
+        private int CountErrorLogsContaining(string substring)
+        {
+            return _logger.ReceivedCalls()
+                .Where(call => call.GetMethodInfo().Name == nameof(ILogger.Log))
+                .Select(call => call.GetArguments())
+                .Count(args => (LogLevel)args[0] == LogLevel.Error && args[2]?.ToString()?.Contains(substring) == true);
+        }
+
+        private static MessageEnvelope CreateEnvelope()
+        {
+            JObject context = JObject.Parse(@"{'type': 'fdc3.Instrument'}");
+            return new MessageEnvelope()
+            {
+                Payload = new EnvelopeData() { ChannelId = "fdc3.channel.1", Context = context },
+                Meta = new EnvelopeMeta() { RequestGuid = Guid.NewGuid().ToString(), Source = new AppIdentifier() { AppId = "Test" } }
+            };
+        }
+
+        private sealed class TestHttpMessageHandler : HttpMessageHandler
+        {
+            public List<HttpRequestMessage> Requests { get; } = new List<HttpRequestMessage>();
+
+            public Func<HttpRequestMessage, HttpResponseMessage> Responder { get; set; } = _ => new HttpResponseMessage(HttpStatusCode.OK);
+
+            protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+            {
+                Requests.Add(request);
+                return Task.FromResult(Responder(request));
+            }
+        }
+    }
+}