Write and call your first agent

Overview

An echo turn isolates transport and provisioning from model configuration. FabrCoreAgentProxy already owns routing, status and lifecycle integration. Use message.Response() so the reply carries the correct recipient and correlation rather than constructing a disconnected message.

Creation and messaging are separate operations

Discovery answers “which implementations are installed?” Creating assistant answers “which instance belongs to dev?” The creation endpoint accepts a batch, so an HTTP success can still contain a failed item. Inspect the result before sending chat. Otherwise a later missing-agent error hides the earlier provisioning failure.

Why the response helper matters

OnMessage receives an AgentMessage envelope, not just a string. message.Response() constructs a reply tied to that incoming message's routing and correlation. Assigning response.Message changes the content without losing that relationship. Echoing the incoming text is a controlled baseline: no model, plugin, business record, or network service can alter the answer.

A model lookup log during the echo exercise

The current 2.0 runtime can inspect the default model configuration while initializing context and run-safety policy, even before the deterministic echo handler runs. Without a model catalog, this can log “Model configuration default was not found.” In the verified echo exercise, creation still succeeded and the response contained the exact input.

Use failureCount and the returned message to judge this particular echo check. A missing default alias must be resolved before the AI exercise in lesson 1.4, but this log by itself does not mean the echo handler made an inference call or that the transport failed.

Implement and call the echo agent

Download the Operations Desk source. The README lists project setup, package prerequisites and local ports.

1. Add EchoAgent.cs to OperationsDesk.Agents and rebuild/restart the Host. Keep the Host at http://localhost:5098.
2. Create the caller with dotnet new console -n OperationsDesk.Console -f net10.0 and replace its Program.cs with the client example below. The raw HTTP caller needs no SDK package.
3. Run dotnet run --project OperationsDesk.Console. The x-user-handle header is only for this trusted localhost exercise; deployed clients require authenticated principal mapping.

using FabrCore.Core;
using FabrCore.Sdk;

[AgentAlias("ops-echo")]
public sealed class EchoAgent(
    AgentConfiguration config,
    IServiceProvider services,
    IFabrCoreAgentHost host) : FabrCoreAgentProxy(config, services, host)
{
    public override Task OnInitialize() => Task.CompletedTask;

    public override Task<AgentMessage> OnMessage(AgentMessage message)
    {
        var response = message.Response();
        response.Message = message.Message;
        return Task.FromResult(response);
    }
}

using System.Net.Http.Json;
using System.Text.Json;
// Trusted localhost exercise only. External callers require authenticated principal mapping.
using var http = new HttpClient { BaseAddress = new Uri("http://localhost:5098") };
http.DefaultRequestHeaders.Add("x-user-handle", "dev");
var useTools = args.Contains("--tools");
var useAi = args.Contains("--ai") || useTools;
var alias = useAi ? "ops-assistant" : "ops-echo";
var handle = useTools ? "tool-assistant" : useAi ? "ai-assistant" : "assistant";
var prompt = args.FirstOrDefault(value => !value.StartsWith("--", StringComparison.Ordinal))
    ?? "Investigate request SR-1042";
using var created = await http.PostAsJsonAsync("/fabrcoreapi/agent/create", new[] {
    new { Handle = handle, AgentType = alias, Models = useAi ? "default" : null,
        Plugins = useTools ? new[] { "requests" } : Array.Empty<string>() }
});
created.EnsureSuccessStatusCode();
var creation = await created.Content.ReadAsStringAsync();
Console.WriteLine(creation);
using (var document = JsonDocument.Parse(creation))
{
    var failures = document.RootElement.EnumerateObject().FirstOrDefault(p =>
        p.Name.Equals("failureCount", StringComparison.OrdinalIgnoreCase));
    if (failures.Value.ValueKind == JsonValueKind.Number && failures.Value.GetInt32() != 0)
        throw new InvalidOperationException("Agent creation failed; inspect the result above.");
}
using var reply = await http.PostAsJsonAsync("/fabrcoreapi/agent/chat/" + handle, prompt);
reply.EnsureSuccessStatusCode();
Console.WriteLine(await reply.Content.ReadAsStringAsync());

Go deeper

Explore the related documentation.

• FabrCore Server
• FabrCore Configuration
• FabrCore Client