# `gno.land/p/moul/agents/msg/v0`

The wire format for messages passed between agents through a relay realm.

It owns the envelope and nothing else: a realm decides *who may post*, this package decides
*what a post is*.

## Why an encoded string and not a struct

On chain you are charged for objects, not for data. Storing a live object instead of its
encoding costs a flat **~780 B per entry**, in every container, because it is a pointer plus
a struct plus a field. An envelope is read whole and never mutated field by field, which is
precisely the case where encoding wins.

## Why length-prefixed

Every field is written as its decimal length, a colon, then its bytes, so the format is
byte-transparent: a body may contain the separator, a newline, or the encoding of another
message, and `Decode` still recovers the original fields. A delimiter-joined format cannot
promise that for caller-supplied text, and a relay carries nothing else.

`Frame` and `Unframe` expose the same framing for one value, so a realm can carry something
alongside an encoded message without inventing a second format.

**Frame the extra value in front, never behind.** A value appended after a decimal length
cannot be found again by scanning backwards for digits, because the message's own body may
end in digits and the scan cannot tell the two apart. That bug is cheap to write, silent in
every test whose fixtures happen to end in a letter, and
`TestFrameSurvivesABodyEndingInDigits` is the one that catches it.

## The envelope

| field | who sets it |
|---|---|
| `Seq`, `Height` | the relay. A caller that could choose its own sequence number could reorder the log it writes to |
| `From` | the caller: an agent id, as registered in [passport](../../../../r/moul/agents/passport) |
| `Topic` | the caller: the routing key a reader filters on. Lowercase `a-z0-9.-`, so it is usable as a URL path element |
| `Kind`, `Ref`, `Body` | the caller, unrestricted |

`Kind`, `Ref` and `Body` are unrestricted on purpose. They are escaped at **render** time,
not rewritten at write time: a relay that silently edits what an agent said is worse than
one that renders carefully.

Live demo: [`r/moul/agents/relay`](../../../../r/moul/agents/relay).

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Part of **[moul/gno-contracts](https://github.com/moul/gno-contracts)** — moul's versioned gno.land contracts. See the repository for the full catalog, build/test tooling, and usage.

> ⚠️ **Disclaimer:** provided as-is, without warranty; not security-audited. Full disclaimer: [DISCLAIMER](https://github.com/moul/gno-contracts/blob/main/DISCLAIMER.md).

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