Boundaries

arca is one organism in a stack — it owns secret materialization and nothing else. If a feature needs another kind of memory or authority, it belongs to a neighbor.

neighbor	owns	how it meets arca
myco	work memory — tasks, issues, proposals, plans, ADRs, handoffs	this repo's project/ ledger is myco's; arca has no PM surface
sturn	ephemeral coordination — live claims/ops/topics	not secrets: sturn state is transient, arca state is sealed
kuda	code memory — symbol/index knowledge	arca refs name secrets, not code entities
peregrin	orchestration — campaigns, multi-actor arbitration, policy	campaigns consume secrets via arca run; audit aggregation + real policy are peregrin's (ARCA_ACTOR is self-claimed inside arca); arca serve — the ndjson IPC wire + [serve.pdp] exec consult (ADR-0003) — is its embedding point
bupha	CI gate	asserts no plaintext secrets land in pipelines; arca check in pipeline steps
mimus	critic	secrets resolve env → arca → legacy (keyring, ~/.config/mimus/env) via mimus-arca shelling arca; vaults are canonical, writes land in them
sepia	session lifecycle	session wrappers can capture arca's stderr audit stream

What that means in practice

• A hosted vault service is in scope — arca-vault-cf is a store impl behind the same port, ciphertext-only, self-hosted.
• A daemon is out of scope — persistent coordination, multi-machine views, arbitration between actors → peregrin. arca stays a sync, embeddable binary: env vars are the interface, no SDK.
• Policy is out of scope — [guardrails] are advisory accident prevention. Enforcement, identity, and unforgeable attribution are orchestration concerns.
• The audit wire stops at stderr — emitting ndjson is arca's; collecting, storing, and alerting on it is peregrin's (ADR-0001 deliberately removed the local audit.jsonl).