SkillInvokedEvent (skill.invoked) is never emitted when skills are loaded and used

[danmoseley]

Describe the bug

SkillInvokedEvent (type skill.invoked) is defined in the SDK's generated SessionEvents.cs with a full data model (Name, Path, Content, AllowedTools), but it is never emitted during a session — even when the agent successfully loads a skill via SkillDirectories and calls the skill tool.

Impact

The dotnet/skills eval harness relies on this event for reliable skill activation detection (MetricsCollector.ExtractSkillActivation). Without it, activation detection falls back to an unreliable "extra tools" heuristic — checking whether the skilled run used tools the baseline didn't. This heuristic produces false negatives for knowledge-only skills that don't add new tools.

Reproduction

1. Create a session with SkillDirectories pointing to a directory with a valid SKILL.md
2. Subscribe to all events via session.On(...)
3. Send a prompt that triggers the skill (e.g., "Use the X skill to help me")
4. Observe: ToolExecutionStartEvent fires for tool "skill", but SkillInvokedEvent never fires

Tested on SDK versions 0.1.26 and 0.1.30 — skillEventCount is 0 across hundreds of agent runs in both versions.

Expected behavior

SkillInvokedEvent should fire when the agent loads/invokes a skill, providing the skill's Name, Path, and Content.

Environment

• GitHub.Copilot.SDK 0.1.26 and 0.1.30
• .NET 10
• Windows 11

Minimal repro code

skills/SKILL.md:

---
name: my-skill
description: A test skill for demonstrating SkillInvokedEvent.
---
# My Skill
When invoked, respond with "Hello from my-skill!"

Program.cs:

using GitHub.Copilot.SDK;

await using var client = new CopilotClient();
await using var session = await client.CreateSessionAsync(new SessionConfig
{
    Model = "claude-sonnet-4.5",
    SkillDirectories = ["./skills"]
});

var skillInvokedCount = 0;
var done = new TaskCompletionSource();

session.On(evt =>
{
    if (evt is SkillInvokedEvent si)
    {
        skillInvokedCount++;
        Console.WriteLine($"Skill invoked: {si.Data.Name}");
    }
    else if (evt is ToolExecutionStartEvent ts && ts.Data.ToolName == "skill")
    {
        Console.WriteLine("Skill tool called (but no SkillInvokedEvent)");
    }
    else if (evt is SessionIdleEvent)
    {
        done.SetResult();
    }
});

await session.SendAsync(new MessageOptions
{
    Prompt = "Use the my-skill skill to help me."
});
await done.Task;

Console.WriteLine($"SkillInvokedEvent count: {skillInvokedCount}");
// Expected: >= 1
// Actual: 0