Reserved subjects

The protocol reserves a small set of facet names and system command names. Packages MUST NOT use these for application ops or events.

Reserved facets (transport-handled)

These appear in the wire grammar as {root}.{mount}.<facet>:

Facet	Direction	Use	Source
$inbox	inbound	actor receives ops	NatsTransport.semanticToNats lines 308–330
$events	outbound	pub/sub events fan-out	same
$reply (with suffix .<cid>)	actor → caller	request/reply correlation	line 297–305
$exit	child → parent	lifecycle exit signal for supervision	line 316–319
$state	actor → watchers	KV last-value updates (handled by JetStream)	spec

These are reserved at the transport layer. Custom mounts MUST NOT name themselves with leading $ (e.g., a mount named $mything would collide with facet handling).

Reserved root-level subjects

Subject	Use
{root}.$inbox	root-level actor inbox (the runtime root)
{root}.$events	root-level events
{root}.$exit	root-level exit
{root}.$reply.<cid>	reply subjects for any caller in this root
{root}.> (wildcard)	match-all under root

Reserved system commands

Auto-subscribed by MatrixActor. Verbatim from projects/matrix-3/packages/core/src/core/MatrixActor.ts:111:

const SYSTEM_COMMANDS = [
  '$join',
  '$join_ack',
  '$leave',
  'introspect',
  '$introspect',
  '$ping',
  '$reply',
  '$getState',
  '$getHistory',
  '$createControl',
  '$activity',
  '$preferences',
  '$config',
  '$membrane-inherit',
  '$membrane',
  '$skill',
  '$cognitive.set',
  '$cognitive.get',
  '$triggers.add',
  '$triggers.remove',
  '$triggers.list',
  '$skills.add',
  '$skills.remove',
] as const;

Each is auto-handled; no actor needs to declare them in static accepts.

Command	What it does
$join / $join_ack / $leave	child registers/unregisters with parent
$introspect (and introspect)	returns the actor's full declared surface
$ping	health check, returns $pong
$reply	response envelope op
$getState / $getHistory	state read / event history read
$createControl	spawn a control child
$activity	activity tracking (oracle loop)
$preferences / $config	actor configuration
$membrane / $membrane-inherit	CESK membrane program management
$skill / $skills.add / $skills.remove	runtime skill management
$cognitive.set / $cognitive.get	cognitive overrides
$triggers.add / $triggers.remove / $triggers.list	runtime prompt triggers

Excluded system commands

$prompt and $source-code are NOT in SYSTEM_COMMANDS. The source comment (MatrixActor.ts:135):

"NOTE: $prompt and $source-code are intentionally NOT here. They live in headlessFrameworkOps (see _subscribeToInbox) so that shell-managed actors defer to the shell handler, avoiding the dual-dispatch race (actor + shell both fire for the same op)."

These two commands route through a separate framework-ops layer to preserve shell ownership of conversational/source surfaces.

Aspirational system commands (target)

WORKSTREAMS/core-and-packaging/ACTOR-COMMUNICATION-CONTRACT.md § "System Commands" documents Plan 9-aligned commands not yet implemented:

Command	Plan 9 analog	Status
$ctl	/proc/<pid>/ctl	target
$status	/proc/<pid>/status	target
$children	ls /proc/<pid>/	target
$ns	/proc/<pid>/ns	target
$env	/proc/<pid>/env	target
$health	(Matrix-only)	target

$ctl would unify stop/restart/reconfigure/gc/debug under one op. The spec is settled; the implementation is workstream material.

Reserved op name prefix

$<verb> and $<verb>.<modifier> are reserved for system commands. Application ops MUST NOT use a leading $. Instead use <area>.<verb>:

Application op	Reserved (forbidden)
chat.send	$send
registry.resolve	$resolve
runtimes.start	$start

NATS protocol-internal subjects

These are NATS server internals and not Matrix subjects:

Subject	Use
_INBOX.*	NATS native request/reply reply subject
_R_.*	NATS service routing
$SYS.*	NATS system account subjects

Matrix code does NOT publish to these. The transport rejects publishes that fall outside the Matrix root prefix; NATS rejects publishes to $SYS.* from non-system accounts.

See also

• Subjects — conceptual.
• Subject grammar — wire format.
• Subject naming reference — conventions.
• Actors — system commands the base class handles.