[Bug] `AssistantReasoningEvent` no longer delivered to `session.On()` handlers after SDK 0.1.24 upgrade?

[pdeligia]

Observed Behavior

After upgrading the .NET Copilot SDK from 0.1.23 to 0.1.24, AssistantReasoningEvent events are no longer delivered to session.On() handlers. AssistantMessageEvent continues to work fine. The handler case for AssistantReasoningEvent is never hit (confirmed via logging inside the case branch).

I tried to investigate the SDK code for some regression, but seems it might be something on the copilot CLI side?

Reproduction

var session = await client.CreateSessionAsync(new SessionConfig { Streaming = true });

session.On(evt =>
{
    switch (evt)
    {
        case AssistantReasoningEvent reasoning:
            // Never hit on 0.1.24, works on 0.1.23
            Console.WriteLine($"[reasoning: {reasoning.Data.Content}]");
            break;
        case AssistantMessageEvent message:
            // Still works
            Console.WriteLine(message.Data.Content);
            break;
    }
});

await session.SendAndWaitAsync(new MessageOptions { Prompt = "Hello" });

Expected Behavior

AssistantReasoningEvent should be delivered to event handlers, as it was in 0.1.23 and as documented in the README.

Environment

• .NET Copilot SDK 0.1.24 (previously working on 0.1.23)
• Streaming = true
• macOS v26.3

[github-actions]

Thank you for reporting this issue! This appears to be a regression in the .NET SDK between versions 0.1.23 and 0.1.24 where AssistantReasoningEvent events are no longer being delivered to session.On() handlers.

I've labeled this as:

• bug — regression in event delivery
• sdk/dotnet — .NET SDK specific
• priority/high — breaking change affecting existing functionality

Your issue includes excellent detail (reproduction code, expected vs. actual behavior, version info, and environment). The team will investigate the changes between 0.1.23 and 0.1.24 to identify the root cause.

AI generated by Issue Triage Agent