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

let client: ARCPClient = .placeholder            // observer client
let subId = try await subscribe(client, sessionId: target, types: ["metric"])
for await env in client.unhandled {
    if let inner = unwrapEvent(env) {
        await sink.handle(inner)
    }
}

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

• Sources/Subscriptions/main.swift — boots three clients in a TaskGroup.
• Sources/Subscriptions/Sinks.swift — StdoutSink, SQLiteSink, OTLPSink, plus the ARCPClient.placeholder setup elision.

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.