CustomTool.cs

using GitHub.Copilot.SDK;
using Microsoft.Extensions.AI;

// Define the tool
[AIFunction("get_current_weather", "Gets the current weather for a given city")]
static string GetCurrentWeather(string city)
{
    // In a real app, call a weather API here
    return $"It's 18°C and partly cloudy in {city}.";
}

await using var client = new CopilotClient();
await client.StartAsync();

var session = await client.CreateSessionAsync(new SessionConfig
{
    Model = "gpt-4.1",
    OnPermissionRequest = PermissionHandler.ApproveAll,
    Tools = [AIFunctionFactory.Create(GetCurrentWeather)]
});

var response = await session.SendAndWaitAsync(new MessageOptions
{
    Prompt = "What's the weather like in Amsterdam and Berlin?"
});

Console.WriteLine(response?.Data.Content);

await client.StopAsync();