.NET: Implement Task support for A2A Hosting package

dotnet/src/Microsoft.Agents.AI.Hosting.A2A.AspNetCore/Microsoft.Agents.AI.Hosting.A2A.AspNetCore.csproj

@@ -8,6 +8,12 @@
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />
 
+  <PropertyGroup>
+    <InjectSharedThrow>true</InjectSharedThrow>
+    <InjectSharedDiagnosticIds>true</InjectSharedDiagnosticIds>
+    <InjectExperimentalAttributeOnLegacy>true</InjectExperimentalAttributeOnLegacy>
+  </PropertyGroup>
+
   <ItemGroup>
     <PackageReference Include="A2A.AspNetCore" />
   </ItemGroup>

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AHostingJsonUtilities.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Text.Json;
+
+namespace Microsoft.Agents.AI.Hosting.A2A;
+
+/// <summary>
+/// Provides JSON serialization options for A2A Hosting APIs to support AOT and trimming.
+/// </summary>
+public static class A2AHostingJsonUtilities
+{
+    /// <summary>
+    /// Gets the default <see cref="JsonSerializerOptions"/> instance used for A2A Hosting serialization.
+    /// </summary>
+    public static JsonSerializerOptions DefaultOptions { get; } = CreateDefaultOptions();
+
+    private static JsonSerializerOptions CreateDefaultOptions()
+    {
+        JsonSerializerOptions options = new(global::A2A.A2AJsonUtilities.DefaultOptions);
+
+        // Chain in the resolvers from both AgentAbstractionsJsonUtilities and the A2A SDK context.
+        // AgentAbstractionsJsonUtilities is first to ensure M.E.AI types (e.g. ResponseContinuationToken)
+        // are handled via its resolver, followed by the A2A SDK resolver for protocol types.
+        options.TypeInfoResolverChain.Clear();
+        options.TypeInfoResolverChain.Add(AgentAbstractionsJsonUtilities.DefaultOptions.TypeInfoResolver!);
+        options.TypeInfoResolverChain.Add(global::A2A.A2AJsonUtilities.DefaultOptions.TypeInfoResolver!);
+
+        options.MakeReadOnly();
+        return options;
+    }
+}

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2ARunDecisionContext.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using A2A;
+
+namespace Microsoft.Agents.AI.Hosting.A2A;
+
+/// <summary>
+/// Provides context for a custom A2A run mode decision.
+/// </summary>
+public sealed class A2ARunDecisionContext
+{
+    internal A2ARunDecisionContext(MessageSendParams messageSendParams)
+    {
+        this.MessageSendParams = messageSendParams;
+    }
+
+    /// <summary>
+    /// Gets the parameters of the incoming A2A message that triggered this run.
+    /// </summary>
+    public MessageSendParams MessageSendParams { get; }
+}

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/AgentRunMode.cs

@@ -0,0 +1,105 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Agents.AI.Hosting.A2A;
+
+/// <summary>
+/// Specifies how the A2A hosting layer determines whether to run <see cref="AIAgent"/> in background or not.
+/// </summary>
+[Experimental(DiagnosticIds.Experiments.AIResponseContinuations)]
+public sealed class AgentRunMode : IEquatable<AgentRunMode>
+{
+    private const string MessageValue = "message";
+    private const string TaskValue = "task";
+    private const string DynamicValue = "dynamic";
+
+    private readonly string _value;
+    private readonly Func<A2ARunDecisionContext, CancellationToken, ValueTask<bool>>? _runInBackground;
+
+    private AgentRunMode(string value, Func<A2ARunDecisionContext, CancellationToken, ValueTask<bool>>? runInBackground = null)
+    {
+        this._value = value;
+        this._runInBackground = runInBackground;
+    }
+
+    /// <summary>
+    /// Dissallows the background responses from the agent. Is equivalent to configuring <see cref="AgentRunOptions.AllowBackgroundResponses"/> as <c>false</c>.
+    /// In the A2A protocol terminology will make responses be returned as <c>AgentMessage</c>.
+    /// </summary>
+    public static AgentRunMode DisallowBackground => new(MessageValue);
+
+    /// <summary>
+    /// Allows the background responses from the agent. Is equivalent to configuring <see cref="AgentRunOptions.AllowBackgroundResponses"/> as <c>true</c>.
+    /// In the A2A protocol terminology will make responses be returned as <c>AgentTask</c> if the agent supports background responses, and as <c>AgentMessage</c> otherwise.
+    /// </summary>
+    public static AgentRunMode AllowBackgroundIfSupported => new(TaskValue);
+
+    /// <summary>
+    /// The agent run mode is decided by the supplied <paramref name="runInBackground"/> delegate.
+    /// The delegate receives an <see cref="A2ARunDecisionContext"/> with the incoming
+    /// message and returns a boolean specifying whether to run the agent in background mode.
+    /// <see langword="true"/> indicates that the agent should run in background mode and return an
+    /// <c>AgentTask</c> if the agent supports background mode; otherwise, it returns an <c>AgentMessage</c>
+    /// if the mode is not supported. <see langword="false"/> indicates that the agent should run in
+    /// non-background mode and return an <c>AgentMessage</c>.
+    /// </summary>
+    /// <param name="runInBackground">
+    /// An async delegate that decides whether the response should be wrapped in an <c>AgentTask</c>.
+    /// </param>
+    public static AgentRunMode AllowBackgroundWhen(Func<A2ARunDecisionContext, CancellationToken, ValueTask<bool>> runInBackground)
+    {
+        ArgumentNullException.ThrowIfNull(runInBackground);
+        return new(DynamicValue, runInBackground);
+    }
+
+    /// <summary>
+    /// Determines whether the agent response should be returned as an <c>AgentTask</c>.
+    /// </summary>
+    internal ValueTask<bool> ShouldRunInBackgroundAsync(A2ARunDecisionContext context, CancellationToken cancellationToken)
+    {
+        if (string.Equals(this._value, MessageValue, StringComparison.OrdinalIgnoreCase))
+        {
+            return ValueTask.FromResult(false);
+        }
+
+        if (string.Equals(this._value, TaskValue, StringComparison.OrdinalIgnoreCase))
+        {
+            return ValueTask.FromResult(true);
+        }
+
+        // Dynamic: delegate to custom callback.
+        if (this._runInBackground is not null)
+        {
+            return this._runInBackground(context, cancellationToken);
+        }
+
+        // No delegate provided — fall back to "message" behavior.
+        return ValueTask.FromResult(true);
+    }
+
+    /// <inheritdoc/>
+    public bool Equals(AgentRunMode? other) =>
+        other is not null && string.Equals(this._value, other._value, StringComparison.OrdinalIgnoreCase);
+
+    /// <inheritdoc/>
+    public override bool Equals(object? obj) => this.Equals(obj as AgentRunMode);
+
+    /// <inheritdoc/>
+    public override int GetHashCode() => StringComparer.OrdinalIgnoreCase.GetHashCode(this._value);
+
+    /// <inheritdoc/>
+    public override string ToString() => this._value;
+
+    /// <summary>Determines whether two <see cref="AgentRunMode"/> instances are equal.</summary>
+    public static bool operator ==(AgentRunMode? left, AgentRunMode? right) =>
+        left?.Equals(right) ?? right is null;
+
+    /// <summary>Determines whether two <see cref="AgentRunMode"/> instances are not equal.</summary>
+    public static bool operator !=(AgentRunMode? left, AgentRunMode? right) =>
+        !(left == right);
+}

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/Converters/AdditionalPropertiesDictionaryExtensions.cs

@@ -2,7 +2,6 @@
 
 using System.Collections.Generic;
 using System.Text.Json;
-using A2A;
 using Microsoft.Extensions.AI;
 
 namespace Microsoft.Agents.AI.Hosting.A2A.Converters;
@@ -37,7 +36,7 @@ internal static class AdditionalPropertiesDictionaryExtensions
                 continue;
             }
 
-            metadata[kvp.Key] = JsonSerializer.SerializeToElement(kvp.Value, A2AJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object)));
+            metadata[kvp.Key] = JsonSerializer.SerializeToElement(kvp.Value, A2AHostingJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object)));
         }
 
         return metadata;

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/Microsoft.Agents.AI.Hosting.A2A.csproj

@@ -10,6 +10,8 @@
 
   <PropertyGroup>
     <InjectSharedThrow>true</InjectSharedThrow>
+    <InjectSharedDiagnosticIds>true</InjectSharedDiagnosticIds>
+    <InjectExperimentalAttributeOnLegacy>true</InjectExperimentalAttributeOnLegacy>
   </PropertyGroup>
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />