Tool Harnesses

A tool harness groups related C# capabilities and registers them with one builder call.

Use a harness when the agent should see a small tool surface that belongs together, such as support tools, file tools, account tools, or project tools.

Add Program.cs

using HPD.Agent;
using HPD.Agent.Providers.OpenAI;
using Microsoft.Extensions.AI;

var agent = await new AgentBuilder()
    .WithOpenAI(model: "gpt-5-mini")
    .WithInstructions("Use the support tools when they help answer the user.")
    .WithToolHarness<SupportToolHarness>()
    .BuildAsync();

var result = await agent.RunAsync("Look up ticket 4815 and summarize the next action.");
Console.WriteLine(result.Text);

public class SupportToolHarness
{
    [AIFunction(Name = "lookup_ticket")]
    [AIDescription("Looks up a support ticket by id.")]
    public string LookupTicket(
        [AIDescription("The support ticket id.")] string ticketId)
        => $"Ticket {ticketId}: customer cannot sign in after changing email.";

    [AIFunction(Name = "suggest_next_action")]
    [AIDescription("Suggests the next support action for a ticket summary.")]
    public string SuggestNextAction(
        [AIDescription("The ticket summary.")] string summary)
        => "Verify the new email address, reset active sessions, and ask the customer to sign in again.";
}

Run it:

dotnet run

What Happens

SupportToolHarness is the unit of registration.

WithToolHarness<SupportToolHarness>() exposes all generated functions from that harness.

Each [AIFunction] method becomes a tool the model can call. The method name, descriptions, and parameter descriptions shape how the model chooses and fills the tool call.

Use WithTool<T>("function_name") when the agent should see one function. Use WithToolHarness<T>() when the agent should see the whole harness.

Keep The First Harness Small

Start with two or three functions that clearly belong together. A smaller tool surface usually gives the model a cleaner choice.

When the harness grows, split it by workflow or audience. For example, expose SupportToolHarness to support agents and BillingToolHarness to billing agents instead of one large general harness.

Next

Next: return to the primary path with Multi-Turn Sessions.

Go deeper: for selective registration, configured harnesses, collapsed tool containers, permissions, subagents, and source generation behavior, see Author A Tool Harness and Tools, Functions, And Harnesses.