refactor: use IgnoreUnrecognizedTypeDiscriminators per review feedback

Author: Ron537

dotnet/src/Client.cs

@@ -1303,8 +1303,11 @@ public void OnSessionEvent(string sessionId, JsonElement? @event)
             var session = client.GetSession(sessionId);
             if (session != null && @event != null)
             {
-                var evt = SessionEvent.TryFromJson(@event.Value.GetRawText(), client._logger);
-                session.DispatchEvent(evt);
+                var evt = SessionEvent.FromJson(@event.Value.GetRawText());
+                if (evt != null)
+                {
+                    session.DispatchEvent(evt);
+                }
             }
         }
 

dotnet/src/Generated/SessionEvents.cs

@@ -17,7 +17,7 @@ namespace GitHub.Copilot.SDK;
 [DebuggerDisplay("{DebuggerDisplay,nq}")]
 [JsonPolymorphic(
     TypeDiscriminatorPropertyName = "type",
-    UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization)]
+    IgnoreUnrecognizedTypeDiscriminators = true)]
 [JsonDerivedType(typeof(AbortEvent), "abort")]
 [JsonDerivedType(typeof(AssistantIntentEvent), "assistant.intent")]
 [JsonDerivedType(typeof(AssistantMessageEvent), "assistant.message")]
@@ -79,7 +79,7 @@ namespace GitHub.Copilot.SDK;
 [JsonDerivedType(typeof(UserInputCompletedEvent), "user_input.completed")]
 [JsonDerivedType(typeof(UserInputRequestedEvent), "user_input.requested")]
 [JsonDerivedType(typeof(UserMessageEvent), "user.message")]
-public abstract partial class SessionEvent
+public partial class SessionEvent
 {
     /// <summary>Unique event identifier (UUID v4), generated when the event is emitted.</summary>
     [JsonPropertyName("id")]
@@ -102,7 +102,7 @@ public abstract partial class SessionEvent
     /// The event type discriminator.
     /// </summary>
     [JsonIgnore]
-    public abstract string Type { get; }
+    public virtual string Type => "unknown";
 
     /// <summary>Deserializes a JSON string into a <see cref="SessionEvent"/>.</summary>
     public static SessionEvent FromJson(string json) =>

dotnet/src/Session.cs

@@ -681,7 +681,8 @@ public async Task<IReadOnlyList<SessionEvent>> GetMessagesAsync(CancellationToke
             "session.getMessages", [new GetMessagesRequest { SessionId = SessionId }], cancellationToken);
 
         return response.Events
-            .Select(e => SessionEvent.TryFromJson(e.ToJsonString(), _logger))
+            .Select(e => SessionEvent.FromJson(e.ToJsonString()))
+            .OfType<SessionEvent>()
             .ToList();
     }
 

dotnet/src/SessionEventExtensions.cs

@@ -0,0 +1,100 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------------------------------------------*/
+
+using Xunit;
+
+namespace GitHub.Copilot.SDK.Test;
+
+/// <summary>
+/// Tests for forward-compatible handling of unknown session event types.
+/// Verifies that the SDK gracefully handles event types introduced by newer CLI versions.
+/// </summary>
+public class ForwardCompatibilityTests
+{
+    [Fact]
+    public void FromJson_KnownEventType_DeserializesNormally()
+    {
+        var json = """
+        {
+            "id": "00000000-0000-0000-0000-000000000001",
+            "timestamp": "2026-01-01T00:00:00Z",
+            "parentId": null,
+            "type": "user.message",
+            "data": {
+                "content": "Hello"
+            }
+        }
+        """;
+
+        var result = SessionEvent.FromJson(json);
+
+        Assert.IsType<UserMessageEvent>(result);
+        Assert.Equal("user.message", result.Type);
+    }
+
+    [Fact]
+    public void FromJson_UnknownEventType_ReturnsBaseSessionEvent()
+    {
+        var json = """
+        {
+            "id": "12345678-1234-1234-1234-123456789abc",
+            "timestamp": "2026-06-15T10:30:00Z",
+            "parentId": "abcdefab-abcd-abcd-abcd-abcdefabcdef",
+            "type": "future.feature_from_server",
+            "data": { "key": "value" }
+        }
+        """;
+
+        var result = SessionEvent.FromJson(json);
+
+        Assert.IsType<SessionEvent>(result);
+        Assert.Equal("unknown", result.Type);
+    }
+
+    [Fact]
+    public void FromJson_UnknownEventType_PreservesBaseMetadata()
+    {
+        var json = """
+        {
+            "id": "12345678-1234-1234-1234-123456789abc",
+            "timestamp": "2026-06-15T10:30:00Z",
+            "parentId": "abcdefab-abcd-abcd-abcd-abcdefabcdef",
+            "type": "future.feature_from_server",
+            "data": {}
+        }
+        """;
+
+        var result = SessionEvent.FromJson(json);
+
+        Assert.Equal(Guid.Parse("12345678-1234-1234-1234-123456789abc"), result.Id);
+        Assert.Equal(DateTimeOffset.Parse("2026-06-15T10:30:00Z"), result.Timestamp);
+        Assert.Equal(Guid.Parse("abcdefab-abcd-abcd-abcd-abcdefabcdef"), result.ParentId);
+    }
+
+    [Fact]
+    public void FromJson_MultipleEvents_MixedKnownAndUnknown()
+    {
+        var events = new[]
+        {
+            """{"id":"00000000-0000-0000-0000-000000000001","timestamp":"2026-01-01T00:00:00Z","parentId":null,"type":"user.message","data":{"content":"Hi"}}""",
+            """{"id":"00000000-0000-0000-0000-000000000002","timestamp":"2026-01-01T00:00:00Z","parentId":null,"type":"future.unknown_type","data":{}}""",
+            """{"id":"00000000-0000-0000-0000-000000000003","timestamp":"2026-01-01T00:00:00Z","parentId":null,"type":"user.message","data":{"content":"Bye"}}""",
+        };
+
+        var results = events.Select(SessionEvent.FromJson).ToList();
+
+        Assert.Equal(3, results.Count);
+        Assert.IsType<UserMessageEvent>(results[0]);
+        Assert.IsType<SessionEvent>(results[1]);
+        Assert.IsType<UserMessageEvent>(results[2]);
+    }
+
+    [Fact]
+    public void SessionEvent_Type_DefaultsToUnknown()
+    {
+        var evt = new SessionEvent();
+
+        Assert.Equal("unknown", evt.Type);
+    }
+}