[ApiController]
[Route("api/orders")]
public sealed class OrdersController : ControllerBase
{
    [HttpGet("updates")]
    public IActionResult Updates(CancellationToken cancellationToken)
        => Sse.Stream(OrderUpdates.Read(cancellationToken));
}

Sse.Stream(...) accepts any IAsyncEnumerable<T>:

typed values become compact Newtonsoft JSON;
strings remain unquoted text; and
SseEnvelope values preserve explicit event name, id, retry, comment, and unnamed control frames.

The returned SseResult implements both MVC IActionResult and ASP.NET IResult, so Koan transports and application controllers use the same wire engine without two helper vocabularies.

Configure only the fallback event name when needed:

{
  "Koan": {
    "Web": {
      "Sse": {
        "DefaultEvent": "message"
      }
    }
  }
}

Guarantees and boundaries

Responses use text/event-stream, add ordinary proxy buffering/caching guidance when absent, flush every frame, and stop on request cancellation. Existing stronger cache or application-owned transport directives survive.
Typed and text values receive the explicit Sse.Stream(..., eventName) value or configured default. An explicit envelope remains unnamed unless the caller supplies eventName.
An envelope with empty data plus a comment, retry, or id is a control frame. It emits only those control fields, even when the stream has a fallback event name; comment-only heartbeats are exactly : text plus the blank frame terminator.
Empty text chunks are skipped; multiline data is emitted as valid repeated data: lines.
The package does not promise delivery, replay, resume storage, heartbeat generation, backpressure persistence, authentication, or proxy-specific buffering behavior. Compose those concerns at their owning transport or host.

See TECHNICAL.md for projection and execution ownership.