Fix(dotnet): serialization of SessionEvent

[xoofx]

Summary

This fixes a .NET SDK serialization gap where SessionEvent.ToJson() could fail for valid session events containing payloads that are stored through object-typed members and materialize as JsonElement after deserialization.

Affected event shapes include:

• assistant.message with toolRequests[].arguments
• tool.execution_start with arguments
• tool.execution_complete with nested toolTelemetry
• session.shutdown with nested modelMetrics

Problem

SessionEvent.FromJson(...) could successfully deserialize these events, but SessionEvent.ToJson() could then throw System.NotSupportedException because the source-generated SessionEventsJsonContext did not include metadata for JsonElement.

This is a real issue for SDK consumers that subscribe to events and re-serialize them for logging, capture, persistence, or forwarding.

A concrete symptom is that richer runtime events can appear to be “missing” from application-side captures even though the Copilot session emitted and persisted them correctly.

Root cause

Some generated session event models contain members such as:

• object? Arguments
• Dictionary<string, object>? ToolTelemetry
• Dictionary<string, object> ModelMetrics

When those are populated from JSON, their runtime values are often JsonElement. The generated SessionEventsJsonContext did not include JsonElement / JsonElement?, so re-serialization failed.

Fix

Update the C# code generator to include:

• JsonElement
• JsonElement?

in the generated SessionEventsJsonContext.

This keeps the fix source-generated and AOT/trimming-friendly, instead of relying on reflection-based serialization.

Generator change

The fix was made in the generator, not only in generated output:

• csharp.ts

and then regenerated into:

• SessionEvents.cs

Tests

Added a regression test that builds typed C# session event objects and verifies ToJson() round-trips event payloads that contain JsonElement-backed dynamic members.

Covered cases:

• assistant.message
• tool.execution_start
• tool.execution_complete
• session.shutdown

The test intentionally constructs real SDK event/data types so it will fail if the generated C# model shape changes.

Validation

Verified locally with:

dotnet build C:\code\github\copilot-sdk\dotnet\src\GitHub.Copilot.SDK.csproj -c Release
dotnet test C:\code\github\copilot-sdk\dotnet\test\GitHub.Copilot.SDK.Test.csproj -c Release --filter SessionEventSerializationTests

The targeted regression tests pass.

[Copilot]

Pull request overview

Fixes a .NET SDK gap where SessionEvent.ToJson() could throw when event payloads include object/Dictionary<string, object> members that materialize as JsonElement, by ensuring the source-generated serializer context includes JsonElement metadata.

Changes:

• Updated the C# session-events code generator to add JsonElement and JsonElement? to SessionEventsJsonContext.
• Regenerated dotnet/src/Generated/SessionEvents.cs with the updated source-gen context annotations.
• Added a regression test covering round-trip serialization for multiple JsonElement-backed event shapes.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
scripts/codegen/csharp.ts	Extends generated SessionEventsJsonContext to include JsonElement/JsonElement? metadata for AOT-safe serialization.
dotnet/src/Generated/SessionEvents.cs	Regenerated output reflecting the updated source-gen context.
dotnet/test/SessionEventSerializationTests.cs	Adds regression coverage validating ToJson() round-trips dynamic JsonElement payloads.

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