Actor Invocation

This section is the formal reference for how a caller invokes an op on an actor. Read these pages when you are writing a transport, a gateway, an SDK, or a contract spec; the Overview section (Request/reply) covers the same material conceptually.

Page	What it covers
Request envelope	The MxEnvelope shape with every field, its purpose, and what is required vs optional.
Response envelope	Success replies — shape, op: '$reply', payload semantics.
Error envelope	Errors — shape, classification, the error field convention.
Correlation IDs	How CIDs are generated and scoped; relationship to NATS native _INBOX.*.
Timeouts	Default 5000 ms, per-call override, classification of timeout errors.
Streaming / sessions	Multi-message exchanges layered over request/reply + events.

The two interaction shapes

Every actor invocation reduces to one of two shapes:

1. Request/reply — one envelope in, one envelope out. Default timeout 5 s. The caller awaits a single response on a reply subject.
2. Streaming session — one initiating envelope, then a stream of events (and optional follow-up requests) tagged by session ID. The actor decides when the session ends.

Pub/sub events without a request to anchor them are documented under Events, not here.

See also

• Subjects — $inbox and $reply.{cid} wire format.
• Subject grammar — exact transformation.
• Reserved subjects — the facet list.
• Envelope schemas — JSON-Schema-style reference for the envelope and its standard payloads.
• Error codes — the standardized error code reference.