.NET: Add background agents support to HarnessAgent (microsoft#5977)

* Add background agents support to HarnessAgent

* Add unit tests

* Address PR comments

Author: westey-m

dotnet/samples/02-agents/Harness/Harness_Step02_Research_WithBackgroundAgents/Program.cs

@@ -102,10 +102,7 @@ 5. Clear all completed tasks to free memory.
         DisableFileAccess = true,   // If enabled, this would allow the agent to read/write files in a working directory
         DisableToolApproval = true, // If enabled, this allows don't-ask-again approval functionality.
         DisableWebSearch = true,
-        AIContextProviders =
-        [
-            new BackgroundAgentsProvider([webSearchAgent]),
-        ],
+        BackgroundAgents = [webSearchAgent],
         ChatOptions = new ChatOptions
         {
             Instructions = parentInstructions,

dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgent.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
+using System.Linq;
 using Microsoft.Agents.AI.Compaction;
 using Microsoft.Extensions.AI;
 using Microsoft.Shared.DiagnosticIds;
@@ -249,6 +250,15 @@ private static List<AIContextProvider> BuildContextProviders(HarnessAgentOptions
             providers.Add(skillsProvider);
         }
 
+        if (options?.BackgroundAgents is IEnumerable<AIAgent> backgroundAgents)
+        {
+            var materializedAgents = backgroundAgents.ToList();
+            if (materializedAgents.Count > 0)
+            {
+                providers.Add(new BackgroundAgentsProvider(materializedAgents, options.BackgroundAgentsProviderOptions));
+            }
+        }
+
         if (options?.AIContextProviders is IEnumerable<AIContextProvider> userProviders)
         {
             providers.AddRange(userProviders);

dotnet/src/Microsoft.Agents.AI.Harness/HarnessAgentOptions.cs

@@ -218,4 +218,26 @@ public sealed class HarnessAgentOptions
     /// This property is ignored when <see cref="DisableOpenTelemetry"/> is <see langword="true"/>.
     /// </remarks>
     public string? OpenTelemetrySourceName { get; set; }
+
+    /// <summary>
+    /// Gets or sets the collection of background agents available for delegation via <see cref="BackgroundAgentsProvider"/>.
+    /// </summary>
+    /// <remarks>
+    /// When non-null and non-empty, a <see cref="BackgroundAgentsProvider"/> is automatically included in the
+    /// agent's context providers, enabling the agent to start, monitor, and retrieve results from background tasks.
+    /// When <see langword="null"/> or empty, no <see cref="BackgroundAgentsProvider"/> is configured.
+    /// Each agent in the collection must have a non-empty <see cref="AIAgent.Name"/> and names must be unique
+    /// (case-insensitive). If these requirements are not met, <see cref="BackgroundAgentsProvider"/> will throw
+    /// an <see cref="System.ArgumentException"/> during construction.
+    /// </remarks>
+    public IEnumerable<AIAgent>? BackgroundAgents { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional configuration for the <see cref="BackgroundAgentsProvider"/>.
+    /// </summary>
+    /// <remarks>
+    /// Use this to customize instructions or agent list formatting for the background agents feature.
+    /// This property is ignored when <see cref="BackgroundAgents"/> is <see langword="null"/> or empty.
+    /// </remarks>
+    public BackgroundAgentsProviderOptions? BackgroundAgentsProviderOptions { get; set; }
 }

dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentOptionsTests.cs

@@ -37,6 +37,8 @@ public void DefaultPropertyValues()
         Assert.Null(options.FileAccessStore);
         Assert.Null(options.AgentModeProviderOptions);
         Assert.Null(options.AgentSkillsSource);
+        Assert.Null(options.BackgroundAgents);
+        Assert.Null(options.BackgroundAgentsProviderOptions);
     }
 
     /// <summary>
@@ -52,6 +54,8 @@ public void PropertiesCanBeSetAndRetrieved()
         var fileAccessStore = new Mock<AgentFileStore>().Object;
         var agentModeOptions = new AgentModeProviderOptions();
         var skillsSource = new Mock<AgentSkillsSource>().Object;
+        var backgroundAgents = new AIAgent[] { new Mock<AIAgent>().Object };
+        var backgroundAgentsOptions = new BackgroundAgentsProviderOptions();
 
         // Act
         var options = new HarnessAgentOptions
@@ -77,6 +81,8 @@ public void PropertiesCanBeSetAndRetrieved()
             AgentSkillsSource = skillsSource,
             DisableOpenTelemetry = true,
             OpenTelemetrySourceName = "custom-source",
+            BackgroundAgents = backgroundAgents,
+            BackgroundAgentsProviderOptions = backgroundAgentsOptions,
         };
 
         // Assert
@@ -103,5 +109,7 @@ public void PropertiesCanBeSetAndRetrieved()
         Assert.Same(skillsSource, options.AgentSkillsSource);
         Assert.True(options.DisableOpenTelemetry);
         Assert.Equal("custom-source", options.OpenTelemetrySourceName);
+        Assert.Same(backgroundAgents, options.BackgroundAgents);
+        Assert.Same(backgroundAgentsOptions, options.BackgroundAgentsProviderOptions);
     }
 }

dotnet/tests/Microsoft.Agents.AI.Harness.UnitTests/HarnessAgentTests.cs

@@ -1197,4 +1197,154 @@ public async Task AllDefaults_AllFeaturesEnabledAsync()
     }
 
     #endregion
+
+    #region Feature: BackgroundAgentsProvider
+
+    /// <summary>
+    /// Verify that BackgroundAgentsProvider is included when BackgroundAgents are specified.
+    /// </summary>
+    [Fact]
+    public void BackgroundAgentsProvider_IncludedWhenAgentsSpecified()
+    {
+        // Arrange
+        var chatClient = new Mock<IChatClient>().Object;
+        var bgAgentMock = new Mock<AIAgent>();
+        bgAgentMock.Setup(a => a.Name).Returns("TestBackgroundAgent");
+        var options = CreateAllDisabledOptions();
+        options.BackgroundAgents = [bgAgentMock.Object];
+
+        // Act
+        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
+        var innerAgent = agent.GetService<ChatClientAgent>();
+
+        // Assert
+        Assert.NotNull(innerAgent?.AIContextProviders);
+        Assert.Contains(innerAgent!.AIContextProviders!, p => p is BackgroundAgentsProvider);
+    }
+
+    /// <summary>
+    /// Verify that BackgroundAgentsProvider is not included when BackgroundAgents is null.
+    /// </summary>
+    [Fact]
+    public void BackgroundAgentsProvider_ExcludedWhenAgentsNull()
+    {
+        // Arrange
+        var chatClient = new Mock<IChatClient>().Object;
+        var options = CreateAllDisabledOptions();
+        options.BackgroundAgents = null;
+
+        // Act
+        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
+        var innerAgent = agent.GetService<ChatClientAgent>();
+
+        // Assert
+        Assert.NotNull(innerAgent);
+        if (innerAgent!.AIContextProviders != null)
+        {
+            Assert.DoesNotContain(innerAgent.AIContextProviders, p => p is BackgroundAgentsProvider);
+        }
+    }
+
+    /// <summary>
+    /// Verify that BackgroundAgentsProvider is not included when BackgroundAgents is an empty collection.
+    /// </summary>
+    [Fact]
+    public void BackgroundAgentsProvider_ExcludedWhenAgentsEmpty()
+    {
+        // Arrange
+        var chatClient = new Mock<IChatClient>().Object;
+        var options = CreateAllDisabledOptions();
+        options.BackgroundAgents = Array.Empty<AIAgent>();
+
+        // Act
+        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
+        var innerAgent = agent.GetService<ChatClientAgent>();
+
+        // Assert
+        Assert.NotNull(innerAgent);
+        if (innerAgent!.AIContextProviders != null)
+        {
+            Assert.DoesNotContain(innerAgent.AIContextProviders, p => p is BackgroundAgentsProvider);
+        }
+    }
+
+    /// <summary>
+    /// Verify that BackgroundAgentsProviderOptions is passed through when specified.
+    /// </summary>
+    [Fact]
+    public async Task BackgroundAgentsProvider_UsesProvidedOptionsAsync()
+    {
+        // Arrange
+        var chatClient = new Mock<IChatClient>().Object;
+        var bgAgentMock = new Mock<AIAgent>();
+        bgAgentMock.Setup(a => a.Name).Returns("TestBackgroundAgent");
+        bgAgentMock.Setup(a => a.Description).Returns("A test background agent");
+        var providerOptions = new BackgroundAgentsProviderOptions
+        {
+            Instructions = "Custom instructions with {background_agents} list.",
+        };
+        var options = CreateAllDisabledOptions();
+        options.BackgroundAgents = [bgAgentMock.Object];
+        options.BackgroundAgentsProviderOptions = providerOptions;
+
+        // Act
+        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
+        var innerAgent = agent.GetService<ChatClientAgent>();
+        var bgProvider = innerAgent!.AIContextProviders!.OfType<BackgroundAgentsProvider>().Single();
+
+#pragma warning disable MAAI001
+        var invokingContext = new AIContextProvider.InvokingContext(
+            new Mock<AIAgent>().Object,
+            new Mock<AgentSession>().Object,
+            new AIContext());
+#pragma warning restore MAAI001
+
+        AIContext result = await bgProvider.InvokingAsync(invokingContext);
+
+        // Assert — custom instructions template is used and agent info is included
+        Assert.NotNull(result.Instructions);
+        Assert.Contains("Custom instructions with", result.Instructions);
+        Assert.Contains("TestBackgroundAgent", result.Instructions);
+    }
+
+    /// <summary>
+    /// Verify that multiple background agents are all passed to the provider.
+    /// </summary>
+    [Fact]
+    public async Task BackgroundAgentsProvider_IncludesMultipleAgentsAsync()
+    {
+        // Arrange
+        var chatClient = new Mock<IChatClient>().Object;
+        var agent1Mock = new Mock<AIAgent>();
+        agent1Mock.Setup(a => a.Name).Returns("Agent1");
+        agent1Mock.Setup(a => a.Description).Returns("First agent");
+        var agent2Mock = new Mock<AIAgent>();
+        agent2Mock.Setup(a => a.Name).Returns("Agent2");
+        agent2Mock.Setup(a => a.Description).Returns("Second agent");
+        var options = CreateAllDisabledOptions();
+        options.BackgroundAgents = [agent1Mock.Object, agent2Mock.Object];
+
+        // Act
+        var agent = new HarnessAgent(chatClient, TestMaxContextWindowTokens, TestMaxOutputTokens, options);
+        var innerAgent = agent.GetService<ChatClientAgent>();
+        var bgProvider = innerAgent!.AIContextProviders!.OfType<BackgroundAgentsProvider>().Single();
+
+#pragma warning disable MAAI001
+        var invokingContext = new AIContextProvider.InvokingContext(
+            new Mock<AIAgent>().Object,
+            new Mock<AgentSession>().Object,
+            new AIContext());
+#pragma warning restore MAAI001
+
+        AIContext result = await bgProvider.InvokingAsync(invokingContext);
+
+        // Assert — both agents appear in the provider's instructions
+        Assert.NotNull(result.Instructions);
+        Assert.Contains("Agent1", result.Instructions);
+        Assert.Contains("First agent", result.Instructions);
+        Assert.Contains("Agent2", result.Instructions);
+        Assert.Contains("Second agent", result.Instructions);
+    }
+
+    #endregion
 }