Report progress over MCP

A tool call of an MCP server can take long, such as a call that works on many hosts. When the call came with a progress token, the client can show how far it got. mcpprogress.Notifier is a sink that sends the progress notifications of the Model Context Protocol.

Procedure#

1. When a call comes with a progress token, make a Bus for that call alone, with a Notifier among its sinks.
2. Give the Notifier a Send that puts the token in the notification and sends it on the call’s session. The kit imports no MCP SDK (decision 3).
3. Run the call under the Bus.
4. When the handler returned, close the Bus, then close the Notifier with a short deadline. Then answer the call. So nothing is sent after the result.

var last string
n := mcpprogress.New(mcpprogress.Options{
	Send: func(_ context.Context, progress, total float64, message string) error {
		last = fmt.Sprintf("%v/%v %s", progress, total, message)
		return nil
	},
})
bus := progress.NewBus(progress.BusOptions{Sinks: []progress.Sink{n}, Program: "prog"})
ctx := progress.WithBus(context.Background(), bus)

fanout.Map(ctx, []string{"exe1", "exe2", "exe3"}, fanout.MapOptions[string]{Step: "probe the nodes"},
	func(context.Context, string) (struct{}, error) { return struct{}{}, nil })

bus.Close()
ctx, cancel := context.WithTimeout(context.Background(), time.Second)
defer cancel()
if err := n.Close(ctx); err != nil {
	fmt.Println(err)
}
fmt.Println(last)

3/3 probe the nodes: 3/3 done

What the client sees#

• The progress counts the targets that ended, of how many the call expects. It never goes back, and never passes the total.
• A notification goes out at most once per interval, and at once when a step or a batch ends. It goes only when more targets ended.
• The message is a line such as read the groups: 3/16 done, 1 failed.

The rate, the rule that the progress only grows, and the words of the line are an interface (decisions 27 and 32).