Versioning

version.mm is the single source of truth for the shared core's version. Every library in the core loads it, so any packet that links the core carries exactly one version stamp.

Source: version.mm.

The version type

metadef version_t: (
    $MAJ -> int,
    $MIN -> int,
    $PATCH -> int
).

The current version is stored in the hidden core_version field and exposed via get_core_version. The source comment states:

This MUST be updated every time we update ANY code in the shared core.

Because every library loads version, a single edit to any .mm file in the core requires a version bump before the change ships.

Runtime observability

Each deployed packet exposes its compiled-in version through the read-only get_version transaction described in the README. An integrator can query any running node to confirm which core version it is running — no out-of-band coordination needed.

What changes mean for integrators

Change	What to do
$PATCH bump	Wire format is unchanged. Re-compile against the updated submodule; re-run integration tests to confirm nothing regressed.
$MIN bump	New features added; no existing behaviour removed. Re-compile, re-run integration tests, and read the diff to find new transactions or capability verbs you may want to use.
$MAJ bump	Breaking changes. Re-compile, re-run all integration tests, and read the diff carefully — wire shapes, transaction names, or type contracts may have changed.

The docs on this site track main; always check the current core_version in version.mm to confirm which version the docs describe.

The versioned type registry (core 0.5.0)

Since 0.5.0, wire-level backward compatibility is an explicit mechanism, not a convention: for every wire input surface whose shape ever changed, a2a_versions.mm declares one frozen metadef per shipped wire version of its payload, the accepted union (the handler's visible contract), the ordered version vector, a discriminator, and a dispatch function:

metadef sir_payload_t: sir_payload_v5_t || sir_payload_v3_t || sir_payload_v2_t.

fn try_narrow_sir (raw: any) -> sir_narrowed_t   // ($ok, $payload, $err)

try_narrow_* reads the discriminator off the RAW value and exact-casts to the matched version's type (dispatch-then-narrow — never cast-to-union as the selector, because disjunction casts pick alternatives in canonical order and rebuild/strip; those toolchain behaviors are pinned by tests/mufl_semantics/). Handlers then branch per version: the v2 branch handles the payload the 0.2.0 way, the v5 branch the new way — backward compatibility is visible in the types and the branches, not buried in NIL defaults.

$pv — the wire dialect id

Every 0.5.0+ core-originated send stamps $pv -> 5 (minor-version ints) on its $targ and inside the boxed identity-bundle payloads. Absence means a pre-0.5 peer; the registry infers the shape (e.g. leg-1 bundle: $name present ⇒ v3, else v2).

$pv	core	notes
(absent)	0.2.0 / 0.3.0 / 0.4.x	shape-inferred per registry
2	0.2.0 dialect	synthetic (0.2.0 never stamps)
3–4	0.3.0 dialect	0.4.x never shipped; wire-identical to 0.3
5	0.5.0	first stamped dialect
> 5	future	narrows as the newest registered version (class-A additions strip safely)

Peers' dialects are learned passively into a2a_messaging::contact_pv (and their advertised capability ids into contact_caps, from the $caps piggyback on the invite/ restore bundles) — no handshake round-trip. Both gate feature selection and diagnostics only, never authorization.

Errors as data — old peers never crash

A payload from below the version floor (or matching no registered shape) is converted into a first-class error value — a2a_versions::version_error_t($code, $surface, $message, $peer_version, $min_supported, $max_supported) — delivered to the local client as a $protocol_error notify event, while the transaction completes successfully with zero state writes. A version-incompatible invite redeem therefore does not consume the invite: the peer can update and redeem the very same invite. Crypto/tamper and identity-verification failures remain hard aborts.

The four-class change taxonomy

Class	Change	Cost
A	add an optional field	new registered version + one dispatch branch (MINOR)
B	new transaction	new single-version registry (MINOR)
C	change/remove a field or semantics	breaking — parallel transaction or dual-accept window (MAJOR)
D	evolve a signed artifact	new versioned metadef, verifiers fail-closed on unknown versions

The binding rules (REG-1…6), the OSP declaration, the full registry index, and the wire- change PR checklist live in COMPATIBILITY.md.

Message receipts (core 0.7.0)

Delivery + read confirmations, capability-gated and fail-closed: a recipient emits receive_receipt pings ($kind "delivered" on arrival, "read" on its get/mark-read path) only when it advertises core.receipts.emit AND the sender positively advertises core.receipts.receive — so old clients exchange no receipt traffic at all and the sender's per-peer state is simply unknown, never failed. Details in COMPATIBILITY.md.