fix: relax A2A.V0_3.AgentMessage kind/messageId requirements

src/A2A.V0_3/Models/A2AResponse.cs

@@ -19,7 +19,15 @@ public abstract class A2AEvent(string kind)
     /// <summary>
     /// The 'kind' discriminator value
     /// </summary>
-    [JsonRequired, JsonPropertyName(BaseKindDiscriminatorConverter<A2AEvent>.DiscriminatorPropertyName), JsonInclude, JsonPropertyOrder(int.MinValue)]
+    /// <remarks>
+    /// Not <c>[JsonRequired]</c>: presence of the discriminator is already enforced by
+    /// <see cref="BaseKindDiscriminatorConverter{TBase}.Read"/> for polymorphic deserialization
+    /// (through <see cref="A2AEvent"/> or <see cref="A2AResponse"/>). Requiring it here as well
+    /// broke deserialization of a concretely-typed <see cref="AgentMessage"/>, such as
+    /// <see cref="AgentTaskStatus.Message"/>, where the converter never runs and some producers
+    /// omit the redundant discriminator.
+    /// </remarks>
+    [JsonPropertyName(BaseKindDiscriminatorConverter<A2AEvent>.DiscriminatorPropertyName), JsonInclude, JsonPropertyOrder(int.MinValue)]
     public string Kind { get; internal set; } = kind;
 }
 

src/A2A.V0_3/Models/AgentMessage.cs

@@ -50,12 +50,24 @@ public sealed class AgentMessage() : A2AResponse(A2AEventKind.Message)
     [JsonPropertyName("referenceTaskIds")]
     public List<string>? ReferenceTaskIds { get; set; }
 
+    private string? _messageId;
+
     /// <summary>
     /// Identifier created by the message creator.
     /// </summary>
+    /// <remarks>
+    /// Not <c>[JsonRequired]</c>: some v0.3 producers omit it (a discrepancy between the
+    /// <c>.proto</c> and JSON Schema definitions of v0.3). Falls back to a lazily generated id
+    /// rather than an empty string, so a message deserialized without one still has a usable,
+    /// unique identifier. Lazy so a message whose JSON already carries a messageId never
+    /// allocates a discarded id on construction.
+    /// </remarks>
     [JsonPropertyName("messageId")]
-    [JsonRequired]
-    public string MessageId { get; set; } = string.Empty;
+    public string MessageId
+    {
+        get => _messageId ??= Guid.NewGuid().ToString();
+        set => _messageId = value;
+    }
 
     /// <summary>
     /// Identifier of task the message is related to.

tests/A2A.V0_3.UnitTests/GitHubIssues/Issue396.cs

@@ -0,0 +1,80 @@
+using System.Text.Json;
+
+namespace A2A.V0_3.UnitTests.GitHubIssues
+{
+    public sealed class Issue396
+    {
+        [Fact]
+        public void Issue_396_Passes()
+        {
+            var json = """
+            {
+              "jsonrpc": "2.0",
+              "id": "1",
+              "result": {
+                "id": "63b2861e-1234-4a1a-9a3a-000000000001",
+                "contextId": "2c727c41-1234-4a1a-9a3a-000000000002",
+                "kind": "task",
+                "status": {
+                  "state": "completed",
+                  "message": {
+                    "role": "agent",
+                    "parts": [ { "kind": "text", "text": "Skill completed." } ]
+                  },
+                  "timestamp": "2026-05-17T15:59:43.401Z"
+                }
+              }
+            }
+            """;
+
+            var deserializedResponseObj = JsonSerializer.Deserialize<JsonRpcResponse>(json, A2AJsonUtilities.DefaultOptions);
+            Assert.NotNull(deserializedResponseObj);
+
+            var task = deserializedResponseObj.Result.Deserialize<AgentTask>(A2AJsonUtilities.DefaultOptions);
+            Assert.NotNull(task);
+
+            Assert.Equal("63b2861e-1234-4a1a-9a3a-000000000001", task.Id);
+            Assert.NotNull(task.Status.Message);
+
+            // kind and messageId were both omitted from status.message; both fall back to
+            // their defaults instead of failing deserialization.
+            Assert.Equal("message", task.Status.Message!.Kind);
+            Assert.False(string.IsNullOrEmpty(task.Status.Message.MessageId));
+            Assert.Equal(MessageRole.Agent, task.Status.Message.Role);
+            Assert.Single(task.Status.Message.Parts);
+        }
+
+        [Fact]
+        public void MessageId_WhenNotSet_IsLazyAndUniquePerInstance()
+        {
+            var first = new AgentMessage { Role = MessageRole.Agent };
+            var second = new AgentMessage { Role = MessageRole.Agent };
+
+            // Each unset instance gets its own generated id, not a shared/cached one.
+            Assert.False(string.IsNullOrEmpty(first.MessageId));
+            Assert.False(string.IsNullOrEmpty(second.MessageId));
+            Assert.NotEqual(first.MessageId, second.MessageId);
+
+            // Reading it twice on the same instance returns the same cached value.
+            Assert.Equal(first.MessageId, first.MessageId);
+        }
+
+        [Fact]
+        public void MessageId_WhenSetFromJson_IsPreservedExactly()
+        {
+            const string json = """
+            {
+                "kind": "message",
+                "role": "user",
+                "messageId": "explicit-id",
+                "parts": []
+            }
+            """;
+
+            var message = JsonSerializer.Deserialize<AgentMessage>(json, A2AJsonUtilities.DefaultOptions);
+
+            Assert.NotNull(message);
+            Assert.Equal("explicit-id", message.MessageId);
+        }
+    }
+}