fix(actor-plugin): live sessionEvent streaming over the RivetKit actor path

[NathanFlurry]

The published 0.2.1 quickstart streamed ZERO sessionEvents (conn.on("sessionEvent")).
Root-caused three distinct bugs in the native actor plugin, each blocking the next:

1. actor_loop starvation (WS-setup timeout). The single event loop ran
   ensure_vm().await inline on the first action; a cold VM boot (>5s) starved the
   queued ConnOpen, so RivetKit-core failed connection setup at 5000ms
   ("actor websocket connection setup timed out after 5000 ms") and no events
   could flow. Fix: move VM-touching work (Action/Http/Sleep/Destroy) onto a
   dedicated serial worker task; the loop now answers connection-lifecycle events
   inline and never blocks on the VM.

2. Subscribe event rejected. RivetKit delivers an AbiEventTag::Subscribe to the
   plugin when a client subscribes to an event name; the loop had no arm, so it
   hit the generic "event not supported" reply, REJECTING every subscription. With
   no subscription registered, broadcast() (which filters on is_subscribed) routed
   zero events to the connection. Fix: accept Subscribe inline (reply_ok).

3. sessionEvent broadcast missing + mis-encoded. spawn_event_capture only persisted
   each event to SQLite and never broadcast it. Added the broadcast, encoded as the
   RivetKit event wire expects: a CBOR array of handler arguments
   ([{ "event": }]) so the client (data) => data.event listener
   receives it. Also fixed vmBooted/vmShutdown, which had the same JSON-instead-of-
   CBOR encoding bug.

Verified locally against the published 0.2.1 npm packages with AGENTOS_PLUGIN_BIN
override + a real Anthropic key: 13 live session/update events stream end-to-end.

Known residual: a timing race remains between subscription registration and the
event burst (reliable under normal/slower timing, ~50% under rapid back-to-back
connections). The action reaches the plugin ~before the subscribe, and the engine
spawns actions without blocking, so this appears to live in the RivetKit
client/engine subscribe-vs-action ordering (preview dep), not the plugin.

[NathanFlurry]

Stack for rivet-dev/agentos

• fix(actor-plugin): live sessionEvent streaming over the RivetKit actor path #1541 ntmzqmwv · 2026-06-26 04:19:16 👈
• main

Get stack: forklift get 1541
Push local edits: forklift submit
Merge when ready: forklift merge 1541

[railway-app]

🚅 Deployed to the agentos-pr-1541 environment in agentos

Service	Status	Web	Updated (UTC)
agentos	😴 Sleeping (View Logs)	Web	Jun 26, 2026 at 4:25 am

---

🚅 Deployed to the agentos-pr-1541 environment in rivet-frontend

Service	Status	Web	Updated (UTC)
agent-os	😴 Sleeping (View Logs)		Jun 26, 2026 at 4:27 am