subscriptions

subscriptions

One producing session, three Observer clients, three different sinks. None of them ever issue a command.

Before ARCP

Most teams sidecar the agent with a tee: agent emits to stdout, a shipper tails the log, a second tail re-parses for metrics, a third process writes to SQLite for replay. Three pipelines diverge over time, none of them know about each other, and adding a fourth consumer means another sidecar.

With ARCP

const client = new ARCPClient({ /* ... observer client ... */ });
const sub = await client.subscribe({ filter: { session_id: [target], types: ["metric"] } });
for (;;) {
  const next = await sub.feed.next();
  if (next.done) break;
  await sink.handle(next.value);
}

Three observers. One transport each. Filters declared inline. The agent never knows they exist.

ARCP primitives

• Subscriptions, filters, Observer role — RFC §13, §5.
• since.after_message_id backfill + the synthetic subscription.backfill_complete marker — §13.3.
• Standard metrics + trace spans — §17.
• Stream-kind filtering for kind: thought redaction — §11.4.

File tour

• main.ts — boots three clients in parallel.
• sinks/stdout_sink.ts — log-summarizer.
• sinks/sqlite_sink.ts — uses the SDK's EventLog schema.
• sinks/otlp_sink.ts — metric and trace.span → OTLP.

Variations

• Replace SQLite with ClickHouse for fleet-wide replay.
• Tee stdout into Slack via a min_priority: critical filter.
• A fourth subscriber on kind: thought only, gated by stricter access control.