feat: add AdditionalInterfaces and PreferredTransport props

src/A2A/Models/AgentCard.cs

@@ -118,4 +118,22 @@ public class AgentCard
     /// </remarks>
     [JsonPropertyName("supportsAuthenticatedExtendedCard")]
     public bool SupportsAuthenticatedExtendedCard { get; set; } = false;
+
+    /// <summary>
+    /// Announcement of additional supported transports.
+    /// </summary>
+    /// <remarks>
+    /// The client can use any of the supported transports.
+    /// </remarks>
+    [JsonPropertyName("additionalInterfaces")]
+    public List<AgentInterface>? AdditionalInterfaces { get; set; }
+
+    /// <summary>
+    /// The transport of the preferred endpoint.
+    /// </summary>
+    /// <remarks>
+    /// If empty, defaults to JSONRPC.
+    /// </remarks>
+    [JsonPropertyName("preferredTransport")]
+    public AgentTransport? PreferredTransport { get; set; }
 }

src/A2A/Models/AgentInterface.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel.DataAnnotations;
+using System.Text.Json.Serialization;
+
+namespace A2A;
+
+/// <summary>
+/// Provides a declaration of a combination of target URL and supported transport to interact with an agent.
+/// </summary>
+public class AgentInterface
+{
+    /// <summary>
+    /// The transport supported by this URL.
+    /// </summary>
+    /// <remarks>
+    /// This is an open form string, to be easily extended for many transport protocols.
+    /// The core ones officially supported are JSONRPC, GRPC, and HTTP+JSON.
+    /// </remarks>
+    [JsonPropertyName("transport")]
+    [Required]
+    public required AgentTransport Transport { get; set; }
+
+    /// <summary>
+    /// The target URL for the agent interface.
+    /// </summary>
+    [JsonPropertyName("url")]
+    [Required]
+    public required string Url { get; set; }
+}

src/A2A/Models/AgentTransport.cs

@@ -0,0 +1,120 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace A2A;
+
+/// <summary>
+/// Represents the transport protocol for an AgentInterface.
+/// </summary>
+[JsonConverter(typeof(AgentTransportConverter))]
+public readonly struct AgentTransport : IEquatable<AgentTransport>
+{
+    /// <summary>
+    /// JSON-RPC transport.
+    /// </summary>
+    public static AgentTransport JsonRpc { get; } = new("JSONRPC");
+
+    /// <summary>
+    /// Gets the label associated with this <see cref="AgentTransport"/>.
+    /// </summary>
+    public string Label { get; }
+
+    /// <summary>
+    /// Creates a new <see cref="AgentTransport"/> instance with the provided label.
+    /// </summary>
+    /// <param name="label">The label to associate with this <see cref="AgentTransport"/>.</param>
+    [JsonConstructor]
+    public AgentTransport(string label)
+    {
+        if (string.IsNullOrWhiteSpace(label))
+            throw new ArgumentException("Transport label cannot be null or whitespace.", nameof(label));
+        this.Label = label;
+    }
+
+    /// <summary>
+    /// Determines whether two <see cref="AgentTransport"/> instances are equal.
+    /// </summary>
+    /// <param name="left">The first <see cref="AgentTransport"/> to compare.</param>
+    /// <param name="right">The second <see cref="AgentTransport"/> to compare.</param>
+    /// <returns><c>true</c> if the instances are equal; otherwise, <c>false</c>.</returns>
+    public static bool operator ==(AgentTransport left, AgentTransport right)
+        => left.Equals(right);
+
+    /// <summary>
+    /// Determines whether two <see cref="AgentTransport"/> instances are not equal.
+    /// </summary>
+    /// <param name="left">The first <see cref="AgentTransport"/> to compare.</param>
+    /// <param name="right">The second <see cref="AgentTransport"/> to compare.</param>
+    /// <returns><c>true</c> if the instances are not equal; otherwise, <c>false</c>.</returns>
+    public static bool operator !=(AgentTransport left, AgentTransport right)
+        => !(left == right);
+
+    /// <summary>
+    /// Determines whether the specified object is equal to the current <see cref="AgentTransport"/>.
+    /// </summary>
+    /// <param name="obj">The object to compare with the current <see cref="AgentTransport"/>.</param>
+    /// <returns><c>true</c> if the specified object is equal to the current <see cref="AgentTransport"/>; otherwise, <c>false</c>.</returns>
+    public override bool Equals(object? obj)
+        => obj is AgentTransport other && this == other;
+
+    /// <summary>
+    /// Determines whether the specified <see cref="AgentTransport"/> is equal to the current <see cref="AgentTransport"/>.
+    /// </summary>
+    /// <param name="other">The <see cref="AgentTransport"/> to compare with the current <see cref="AgentTransport"/>.</param>
+    /// <returns><c>true</c> if the specified <see cref="AgentTransport"/> is equal to the current <see cref="AgentTransport"/>; otherwise, <c>false</c>.</returns>
+    public bool Equals(AgentTransport other)
+        => string.Equals(this.Label, other.Label, StringComparison.OrdinalIgnoreCase);
+
+    /// <summary>
+    /// Returns the hash code for this <see cref="AgentTransport"/>.
+    /// </summary>
+    /// <returns>A hash code for the current <see cref="AgentTransport"/>.</returns>
+    public override int GetHashCode()
+        => StringComparer.OrdinalIgnoreCase.GetHashCode(this.Label);
+
+    /// <summary>
+    /// Returns the string representation of this <see cref="AgentTransport"/>.
+    /// </summary>
+    /// <returns>The label of this <see cref="AgentTransport"/>.</returns>
+    public override string ToString() => this.Label;
+
+    /// <summary>
+    /// Custom JSON converter for <see cref="AgentTransport"/> that serializes it as a simple string value.
+    /// </summary>
+    internal sealed class AgentTransportConverter : JsonConverter<AgentTransport>
+    {
+        /// <summary>
+        /// Reads and converts the JSON to <see cref="AgentTransport"/>.
+        /// </summary>
+        /// <param name="reader">The reader to read from.</param>
+        /// <param name="typeToConvert">The type to convert.</param>
+        /// <param name="options">Serializer options.</param>
+        /// <returns>The converted <see cref="AgentTransport"/>.</returns>
+        public override AgentTransport Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType != JsonTokenType.String)
+            {
+                throw new JsonException("Expected a string for AgentTransport.");
+            }
+
+            var label = reader.GetString();
+            if (string.IsNullOrWhiteSpace(label))
+            {
+                throw new JsonException("AgentTransport string value cannot be null or whitespace.");
+            }
+
+            return new AgentTransport(label!);
+        }
+
+        /// <summary>
+        /// Writes the <see cref="AgentTransport"/> as a JSON string.
+        /// </summary>
+        /// <param name="writer">The writer to write to.</param>
+        /// <param name="value">The value to convert.</param>
+        /// <param name="options">Serializer options.</param>
+        public override void Write(Utf8JsonWriter writer, AgentTransport value, JsonSerializerOptions options)
+        {
+            writer.WriteStringValue(value.Label);
+        }
+    }
+}

src/A2A/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs

@@ -0,0 +1,35 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NETCOREAPP
+namespace System.Runtime.CompilerServices;
+
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+internal sealed class CompilerFeatureRequiredAttribute : Attribute
+{
+    public CompilerFeatureRequiredAttribute(string featureName)
+    {
+        this.FeatureName = featureName;
+    }
+
+    /// <summary>
+    /// The name of the compiler feature.
+    /// </summary>
+    public string FeatureName { get; }
+
+    /// <summary>
+    /// If true, the compiler can choose to allow access to the location where this attribute is applied if it does not understand <see cref="FeatureName"/>.
+    /// </summary>
+    public bool IsOptional { get; init; }
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the ref structs C# feature.
+    /// </summary>
+    public const string RefStructs = nameof(RefStructs);
+
+    /// <summary>
+    /// The <see cref="FeatureName"/> used for the required members C# feature.
+    /// </summary>
+    public const string RequiredMembers = nameof(RequiredMembers);
+}
+#endif

src/A2A/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NETCOREAPP
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Reserved to be used by the compiler for tracking metadata.
+/// This class should not be used by developers in source code.
+/// </summary>
+internal static class IsExternalInit;
+#endif

src/A2A/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NETCOREAPP
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Specifies that a type has required members or that a member is required.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
+internal sealed class RequiredMemberAttribute : Attribute;
+#endif