fix(dotnet): handle unknown session event types gracefully

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/test/ForwardCompatibilityTests.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);
+    }
+}

scripts/codegen/csharp.ts

@@ -521,11 +521,11 @@ namespace GitHub.Copilot.SDK;
     lines.push(`/// Provides the base class from which all session events derive.`);
     lines.push(`/// </summary>`);
     lines.push(`[DebuggerDisplay("{DebuggerDisplay,nq}")]`);
-    lines.push(`[JsonPolymorphic(`, `    TypeDiscriminatorPropertyName = "type",`, `    UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization)]`);
+    lines.push(`[JsonPolymorphic(`, `    TypeDiscriminatorPropertyName = "type",`, `    IgnoreUnrecognizedTypeDiscriminators = true)]`);
     for (const variant of [...variants].sort((a, b) => a.typeName.localeCompare(b.typeName))) {
         lines.push(`[JsonDerivedType(typeof(${variant.className}), "${variant.typeName}")]`);
     }
-    lines.push(`public abstract partial class SessionEvent`, `{`);
+    lines.push(`public partial class SessionEvent`, `{`);
     lines.push(...xmlDocComment(baseDesc("id"), "    "));
     lines.push(`    [JsonPropertyName("id")]`, `    public Guid Id { get; set; }`, "");
     lines.push(...xmlDocComment(baseDesc("timestamp"), "    "));
@@ -535,7 +535,7 @@ namespace GitHub.Copilot.SDK;
     lines.push(...xmlDocComment(baseDesc("ephemeral"), "    "));
     lines.push(`    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]`, `    [JsonPropertyName("ephemeral")]`, `    public bool? Ephemeral { get; set; }`, "");
     lines.push(`    /// <summary>`, `    /// The event type discriminator.`, `    /// </summary>`);
-    lines.push(`    [JsonIgnore]`, `    public abstract string Type { get; }`, "");
+    lines.push(`    [JsonIgnore]`, `    public virtual string Type => "unknown";`, "");
     lines.push(`    /// <summary>Deserializes a JSON string into a <see cref="SessionEvent"/>.</summary>`);
     lines.push(`    public static SessionEvent FromJson(string json) =>`, `        JsonSerializer.Deserialize(json, SessionEventsJsonContext.Default.SessionEvent)!;`, "");
     lines.push(`    /// <summary>Serializes this event to a JSON string.</summary>`);