feat: add a new project used to handle the a2ui extension

A2A.slnx

@@ -31,6 +31,7 @@
     <Project Path="src/A2A.Client/A2A.Client.csproj" Id="1314fd10-cebf-4b91-b9cf-56cae0205d50" />
     <Project Path="src/A2A.Core.JsonRpc/A2A.Core.JsonRpc.csproj" />
     <Project Path="src/A2A.Core/A2A.Core.csproj" />
+    <Project Path="src/A2A.Extensions.A2UI/A2A.Extensions.A2UI.csproj" Id="02619520-5a85-47a8-ac72-d6c2e68f2504" />
     <Project Path="src/A2A.Server.Abstractions/A2A.Server.Abstractions.csproj" Id="9d4c091b-82aa-4d82-b77b-80f4b03a842a" />
     <Project Path="src/A2A.Server.AspNetCore/A2A.Server.csproj" />
     <Project Path="src/A2A.Server.Persistence.Memory/A2A.Server.Persistence.Memory.csproj" Id="42f60b02-fa31-4431-8735-f1e56f738f27" />

src/A2A.Client.Abstractions/A2A.Client.Abstractions.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A.Client</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Client.Abstractions/IA2AClient.cs

@@ -20,6 +20,18 @@ public interface IA2AClient
     : IA2AProtocolApi
 {
 
+    /// <summary>
+    /// Activates the A2A extension identified by the given URI.
+    /// </summary>
+    /// <param name="uri">The URI of the A2A extension to activate.</param>
+    /// <returns>The configured <see cref="IA2AClient"/>.</returns>
+    IA2AClient ActivateExtension(Uri uri);
 
+    /// <summary>
+    /// Deactivates the A2A extension identified by the given URI.
+    /// </summary>
+    /// <param name="uri">The URI of the A2A extension to deactivate.</param>
+    /// <returns>The configured <see cref="IA2AClient"/>.</returns>
+    IA2AClient DeactivateExtension(Uri uri);
 
 }

src/A2A.Client.Abstractions/IA2AClientBuilder.cs

@@ -35,6 +35,7 @@ IA2AClientBuilder UseTransport<TTransport>()
     /// <summary>
     /// Builds the configured <see cref="IA2AClient"/>.
     /// </summary>
-    void Build();
+    /// <returns>A new <see cref="IA2AClient"/>.</returns>
+    IA2AClient Build();
 
 }

src/A2A.Client.Abstractions/IA2AClientTransport.cs

@@ -20,6 +20,16 @@ public interface IA2AClientTransport
     : IA2AProtocolApi
 {
 
+    /// <summary>
+    /// Activates the A2A extension identified by the given URI.
+    /// </summary>
+    /// <param name="uri">The URI of the A2A extension to activate.</param>
+    void ActivateExtension(Uri uri);
 
+    /// <summary>
+    /// Deactivates the A2A extension identified by the given URI.
+    /// </summary>
+    /// <param name="uri">The URI of the A2A extension to deactivate.</param>
+    void DeactivateExtension(Uri uri);
 
 }

src/A2A.Client.Transports.Grpc/A2A.Client.Transports.Grpc.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A.Client.Transports</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Client.Transports.Grpc/A2AGrpcClientTransport.cs

@@ -12,6 +12,7 @@
 // limitations under the License.
 
 using Grpc.Core;
+using System.Collections.Generic;
 using Task = System.Threading.Tasks.Task;
 
 namespace A2A.Client.Transports;
@@ -24,18 +25,27 @@ public sealed class A2AGrpcClientTransport(A2a.V1.A2AService.A2AServiceClient gr
     : IA2AClientTransport
 {
 
+    const string VersionMetadataKey = "A2A-Version";
+    const string ExtensionMetadataKey = "A2A-Extensions";
+
+    ListValue? extensions;
+
     /// <inheritdoc/>
     public async Task<Models.Response> SendMessageAsync(Models.SendMessageRequest request, CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(request);
-        return A2AGrpcMapper.MapFromGrpc(await grpcClient.SendMessageAsync(A2AGrpcMapper.MapToGrpc(request), cancellationToken: cancellationToken).ConfigureAwait(false));
+        var grpcRequest = A2AGrpcMapper.MapToGrpc(request);
+        SetRequestMetadata(grpcRequest.Metadata);
+        return A2AGrpcMapper.MapFromGrpc(await grpcClient.SendMessageAsync(grpcRequest, cancellationToken: cancellationToken).ConfigureAwait(false));
     }
 
     /// <inheritdoc/>
     public async IAsyncEnumerable<Models.StreamResponse> SendStreamingMessageAsync(Models.SendMessageRequest request, [EnumeratorCancellation] CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(request);
-        var result = grpcClient.SendStreamingMessage(A2AGrpcMapper.MapToGrpc(request), cancellationToken: cancellationToken);
+        var grpcRequest = A2AGrpcMapper.MapToGrpc(request);
+        SetRequestMetadata(grpcRequest.Metadata);
+        var result = grpcClient.SendStreamingMessage(grpcRequest, cancellationToken: cancellationToken);
         await foreach (var streamResponse in result.ResponseStream.ReadAllAsync(cancellationToken).ConfigureAwait(false))
         {
             if (streamResponse is null) continue;
@@ -142,4 +152,37 @@ await grpcClient.DeleteTaskPushNotificationConfigAsync(new()
         }, cancellationToken: cancellationToken).ConfigureAwait(false);
     }
 
+    void SetRequestMetadata(Struct metadata)
+    {
+        metadata.Fields[VersionMetadataKey] = Value.ForString(A2AProtocolVersion.Latest);
+        if (extensions is not null) metadata.Fields[ExtensionMetadataKey] = new()
+        {
+            ListValue = extensions
+        };
+    }
+
+    /// <inheritdoc/>
+    public void ActivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        var value = uri.OriginalString;
+        extensions ??= new();
+        if (extensions.Values.Any(v => v.StringValue.Equals(value, StringComparison.OrdinalIgnoreCase))) return;
+        extensions.Values.Add(new Value() 
+        { 
+            StringValue = value 
+        });
+    }
+
+    /// <inheritdoc/>
+    public void DeactivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        if (extensions is null) return;
+        var value = uri.OriginalString;
+        var toRemove = extensions.Values.FirstOrDefault(v => v.StringValue.Equals(value, StringComparison.OrdinalIgnoreCase));
+        if (toRemove is null) return;
+        extensions.Values.Remove(toRemove);
+    }
+
 }

src/A2A.Client.Transports.Http/A2A.Client.Transports.Http.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A.Client.Transports</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Client.Transports.Http/A2AHttpClientTransport.cs

@@ -24,6 +24,8 @@ public sealed class A2AHttpClientTransport(HttpClient httpClient)
     : IA2AClientTransport
 {
 
+    const string ExtensionHeaderName = "A2A-Extensions";
+
     /// <inheritdoc/>
     public async Task<Response> SendMessageAsync(SendMessageRequest request, CancellationToken cancellationToken = default)
     {
@@ -169,4 +171,25 @@ public async Task DeletePushNotificationConfigAsync(string taskId, string config
 
     }
 
+    /// <inheritdoc/>
+    public void ActivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        var value = uri.OriginalString;
+        if (httpClient.DefaultRequestHeaders.TryGetValues(ExtensionHeaderName, out var existing) && existing.Contains(value, StringComparer.OrdinalIgnoreCase)) return;
+        httpClient.DefaultRequestHeaders.TryAddWithoutValidation(ExtensionHeaderName, value);
+    }
+
+    /// <inheritdoc/>
+    public void DeactivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        var value = uri.OriginalString;
+        if (!httpClient.DefaultRequestHeaders.TryGetValues(ExtensionHeaderName, out var existing)) return;
+        var extensions = existing.Where(v => !string.Equals(v, value, StringComparison.OrdinalIgnoreCase)).ToArray();
+        httpClient.DefaultRequestHeaders.Remove(ExtensionHeaderName);
+        if (extensions.Length == 0) return;
+        foreach (var extension in extensions) httpClient.DefaultRequestHeaders.TryAddWithoutValidation(ExtensionHeaderName, extension);
+    }
+
 }

src/A2A.Client.Transports.Http/Extensions/A2AClientBuilderExtensions.cs

@@ -33,6 +33,7 @@ public static IA2AClientBuilder UseHttpTransport(this IA2AClientBuilder builder,
         builder.Services.AddHttpClient<A2AHttpClientTransport>(httpClient =>
         {
             httpClient.BaseAddress = baseAddress;
+            httpClient.DefaultRequestHeaders.Add("A2A-Version", A2AProtocolVersion.Latest);
         });
         builder.Services.AddSingleton<IA2AClientTransport>(provider => provider.GetRequiredService<A2AHttpClientTransport>());
         return builder.UseTransport<A2AHttpClientTransport>();

src/A2A.Client.Transports.JsonRpc/A2A.Client.Transports.JsonRpc.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A.Client.Transports</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Client.Transports.JsonRpc/A2AJsonRpcClientTransport.cs

@@ -24,6 +24,8 @@ public sealed class A2AJsonRpcClientTransport(HttpClient httpClient)
     : IA2AClientTransport
 {
 
+    const string ExtensionHeaderName = "A2A-Extensions";
+
     /// <inheritdoc/>
     public async Task<Response> SendMessageAsync(SendMessageRequest request, CancellationToken cancellationToken = default)
     {
@@ -257,4 +259,25 @@ public async Task DeletePushNotificationConfigAsync(string taskId, string config
         if (rpcResponse.Error is not null) throw new Exception($"An error occurred while processing the JSON-RPC request (error code: {rpcResponse.Error.Code}): {rpcResponse.Error.Message}).");
     }
 
+    /// <inheritdoc/>
+    public void ActivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        var value = uri.OriginalString;
+        if (httpClient.DefaultRequestHeaders.TryGetValues(ExtensionHeaderName, out var existing) && existing.Contains(value, StringComparer.OrdinalIgnoreCase)) return;
+        httpClient.DefaultRequestHeaders.TryAddWithoutValidation(ExtensionHeaderName, value);
+    }
+
+    /// <inheritdoc/>
+    public void DeactivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        var value = uri.OriginalString;
+        if (!httpClient.DefaultRequestHeaders.TryGetValues(ExtensionHeaderName, out var existing)) return;
+        var extensions = existing.Where(v => !string.Equals(v, value, StringComparison.OrdinalIgnoreCase)).ToArray();
+        httpClient.DefaultRequestHeaders.Remove(ExtensionHeaderName);
+        if (extensions.Length == 0) return;
+        foreach (var extension in extensions) httpClient.DefaultRequestHeaders.TryAddWithoutValidation(ExtensionHeaderName, extension);
+    }
+
 }

src/A2A.Client.Transports.JsonRpc/Extensions/A2AClientBuilderExtensions.cs

@@ -33,6 +33,7 @@ public static IA2AClientBuilder UseJsonRpcTransport(this IA2AClientBuilder build
         builder.Services.AddHttpClient<A2AJsonRpcClientTransport>(httpClient =>
         {
             httpClient.BaseAddress = baseAddress;
+            httpClient.DefaultRequestHeaders.Add("A2A-Version", A2AProtocolVersion.Latest);
         });
         builder.Services.AddSingleton<IA2AClientTransport>(provider => provider.GetRequiredService<A2AJsonRpcClientTransport>());
         return builder.UseTransport<A2AJsonRpcClientTransport>();

src/A2A.Client/A2A.Client.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A.Client</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Client/A2AClient.cs

@@ -11,7 +11,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-using A2A.Models;
 using Task = System.Threading.Tasks.Task;
 
 namespace A2A.Client;
@@ -54,4 +53,20 @@ public sealed class A2AClient(IA2AClientTransport transport)
     /// <inheritdoc/>
     public Task DeletePushNotificationConfigAsync(string taskId, string configId, string? tenant = null, CancellationToken cancellationToken = default) => transport.DeletePushNotificationConfigAsync(taskId, configId, tenant, cancellationToken);
 
+    /// <inheritdoc/>
+    public IA2AClient ActivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        transport.ActivateExtension(uri);
+        return this;
+    }
+
+    /// <inheritdoc/>
+    public IA2AClient DeactivateExtension(Uri uri)
+    {
+        ArgumentNullException.ThrowIfNull(uri);
+        transport.DeactivateExtension(uri);
+        return this;
+    }
+
 }

src/A2A.Client/A2AClientBuilder.cs

@@ -16,15 +16,20 @@ namespace A2A.Client;
 /// <summary>
 /// Represents the default implementation of the <see cref="IA2AClientBuilder"/> interface.
 /// </summary>
-/// <param name="services">The <see cref="IServiceCollection"/> to configure.</param>
-public sealed class A2AClientBuilder(IServiceCollection services)
+public sealed class A2AClientBuilder 
     : IA2AClientBuilder
 {
 
     Type? transportType;
 
+    internal A2AClientBuilder(IServiceCollection services)
+    {
+        Services = services;
+        Services.AddSingleton<IA2AClient, A2AClient>();
+    }
+
     /// <inheritdoc/>
-    public IServiceCollection Services { get; } = services;
+    public IServiceCollection Services { get; }
 
     /// <inheritdoc/>
     public IA2AClientBuilder UseTransport<TTransport>()
@@ -35,10 +40,16 @@ public IA2AClientBuilder UseTransport<TTransport>()
     }
 
     /// <inheritdoc/>
-    public void Build()
+    public IA2AClient Build()
     {
         if (transportType is null) throw new InvalidOperationException("The transport type must be specified before building the client.");
-        Services.AddSingleton<IA2AClient, A2AClient>();
+        return Services.BuildServiceProvider().GetRequiredService<IA2AClient>();
     }
 
+    /// <summary>
+    /// Creates a new <see cref="IA2AClientBuilder"/>.
+    /// </summary>
+    /// <returns>A new <see cref="IA2AClientBuilder"/>.</returns>
+    public static IA2AClientBuilder Create() => new A2AClientBuilder(new ServiceCollection());
+
 }

src/A2A.Client/Extensions/A2AClientServiceCollectionExtensions.cs

@@ -30,8 +30,6 @@ public static IServiceCollection AddA2AClient(this IServiceCollection services,
     {
         var builder = new A2AClientBuilder(services);
         setup.Invoke(builder);
-        builder.Build();
-        services.AddSingleton<IA2AClient, A2AClient>();
         return services;
     }
 

src/A2A.Core.JsonRpc/A2A.Core.JsonRpc.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Core/A2A.Core.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
 	<RootNamespace>A2A</RootNamespace>
-    <VersionPrefix>0.15.0</VersionPrefix>
+    <VersionPrefix>0.16.0</VersionPrefix>
     <AssemblyVersion>$(VersionPrefix)</AssemblyVersion>
     <FileVersion>$(VersionPrefix)</FileVersion>
     <NeutralLanguage>en</NeutralLanguage>

src/A2A.Core/A2AProtocolVersion.cs

@@ -0,0 +1,27 @@
+// Copyright © 2025-Present the a2a-net Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+namespace A2A;
+
+/// <summary>
+/// Enumerates the supported A2A protocol versions.
+/// </summary>
+public static class A2AProtocolVersion
+{
+
+    /// <summary>
+    /// Gets the latest supported A2A protocol version.
+    /// </summary>
+    public const string Latest = "0.3";
+
+}