Streaming Events

This page prints assistant text as it arrives during a run.

The public first-reader path uses direct in-process typed event subscriptions with RunAsync(...).

Continue in the same HpdAgentQuickstart folder from Hello Agent. This page uses the same agent, but adds event subscriptions before the run.

Add Program.cs

using HPD.Agent;
using HPD.Agent.Providers.OpenAI;

var agent = await new AgentBuilder()
    .WithOpenAI(model: "gpt-5-mini")
    .WithInstructions("You are a concise helpful assistant.")
    .BuildAsync();

using var output = agent.Subscribe<TextDeltaEvent>(evt => Console.Write(evt.Text));
using var finished = agent.Subscribe<MessageTurnFinishedEvent>(evt =>
    Console.WriteLine($"\nFinished in {evt.Duration.TotalMilliseconds:N0} ms"));

_ = await agent.RunAsync("Write a three-line welcome for someone learning HPD Agent.");

Run it:

dotnet run

What Happens

agent.Subscribe<TextDeltaEvent>(...) registers a handler for progressive assistant text. The sample writes each text delta to the console as the turn runs.

agent.Subscribe<MessageTurnFinishedEvent>(...) registers a handler for turn completion. The sample prints the elapsed duration when the run finishes.

The using var declarations keep each subscription active until the end of the program and dispose the subscriptions afterward.

RunAsync(...) still performs the turn. Event subscriptions observe what happens during the turn; they do not replace the run call.

This page shows the direct Agent API. ASP.NET Core hosted clients observe the same live activity through SSE or WebSocket and submit input through hosted routes instead of calling agent.Subscribe(...) or RunAsync(...).

result.Text is still available after the run. In this sample, the text has already been written by the TextDeltaEvent handler, so the code does not print result.Text again.

When To Use Events

Use typed subscriptions when in-process code such as a console app, local UI, TUI runtime, hosted server implementation, or bot adapter needs to react while an agent run is happening.

Text streaming is the simplest event projection. Richer clients can group the same event stream into transcripts, tool timelines, permission prompts, workflow nodes, subagent activity, or trace views.

Use SubscribeAny(...) when you want to inspect or route every event:

using var all = agent.SubscribeAny(evt =>
{
    Console.WriteLine($"{evt.GetType().Name} {evt.Metadata?.AgentChain.LastOrDefault()}");
});

This page keeps to two event types. More event families exist for sessions, branches, tools, middleware, permissions, retries, workflows, and model calls. See Events Overview, Event Streams And Hierarchies, and Events.

Middleware can also emit or respond to lifecycle behavior around turns and functions. See Middleware Lifecycle.

Common Mistakes

If no streaming text appears, make sure the subscription is registered before RunAsync(...).

If you subscribed but nothing happens, remember that subscriptions only observe a run. RunAsync(...) still starts the turn.

Next

Next: register one local function in Add A Tool.