# Messaging

The substrate everything else rides on: durable, addressed, reply-routable
messages between endpoints — live when the target listens, spooled when it
doesn't, across machines once nodes are paired.

You've probably already run the
[quickstart](../quickstart/messaging.md); this page is the model.

## Semantics

- **Live-first, spool-fallback.** `spt send <id>` connects directly to a
  listening target (`SENT`); if the perch exists but nothing is listening,
  the message lands in the target's durable spool (`QUEUED`) and drains on
  its next `ready`. A target with *no* perch is an error (`NO_PERCH`)
  — identity is never invented on someone else's behalf.
- **Reply routing.** Every message carries its sender id structurally;
  the arriving `<EVENT from="…">` envelope surfaces it, and
  `spt send --reply-to <sender>` answers without knowing anything else.
- **The blocking ask.** `spt ring <id>` sends and waits for the reply (with
  a timeout) — the synchronous question between agents.
- **Deferred delivery.** `--deferred` spools without waking a live listener:
  for context that should reach the agent at its next natural boundary
  rather than interrupting now. Deferred messages are also held for resting
  (dormant/suspended) instances and released exactly once on wake.
- **Typed payloads.** Message bodies carry typed operations and file blobs,
  not just text — file transfers are addressable and progress-queryable
  mid-flight.

## Addressing

Bare ids (`sergey`) resolve locally first, then across the subnet; when the
same id is live on several nodes, resolution **refuses and asks you to
qualify** (`sergey@desktop` — node labels and key prefixes both work) rather
than guessing. The full form is `[subnet:]id[@node]`.

## Commands

`send` · `ring` · `ready` (blocks; `--once` drains and exits) · `list` ·
`stop` · `whoami` — every flag in the [CLI reference](../cli/reference.md).
Agents get the task-oriented version from the binary itself: `spt how-to
ready` / `spt how-to send`.