Feature Flags

Feature flags

Permanent switches, and temporary ones that cannot be forgotten. Package @ultimat3/flags (tier 1) — the full reference is packages/flags/README.md.

N flags are 2^N states nothing tested. This package takes that trade only on terms that bound the exponent: every flag declares one of two kinds, and a temporary flag cannot be declared without an expiry.

Kind	Meaning	Lifecycle
permanent	a real product or ops switch — a plan capability, a kill switch	none
temporary	scaffolding around an in-progress change	expiresAt and owner are required by the type; past the expiry every evaluation reports X_FLAG_EXPIRED to the app's error monitor

defineFlag() is a define* helper, not a ninth primitive: a flag has no handler, no input schema and no surface of its own (The eight primitives).

import { defineFlag, isEnabled } from '@ultimat3/flags';

export const newTaxEngine = defineFlag({
  kind: 'temporary',
  key: 'checkout.new-tax-engine',
  description: 'routes checkout through the rewritten tax engine',
  owner: 'payments',
  expiresAt: '2026-12-01',
  targeting: { default: false, rollout: 10, roles: ['staff'] },
});

if (isEnabled('checkout.new-tax-engine', actor)) {
  // …
}

isEnabled is synchronous, for the reason can() is: it runs inside policy predicates and render passes. An undeclared key throws X_FLAG_UNKNOWN rather than answering false — a typo read as "off" is a branch that never runs in production.

Targeting

Allow lists first, then the rollout, then default:

Field	Meaning
actors, roles, orgs	always on for these ids, roles or tenants
subjects	allow lists for the app's own record kinds — { bank: ['bank_integration:bbva'] }, read from the call site's third argument
rollout	whole percentage 0–100, stable per subject: fnv1a(key + ':' + id) % 100, never Math.random()
bucketBy	what the rollout divides — 'actor' (default), 'org', or any subject kind, so a tenant is on one side whole

Targeting by a subject kind the call does not carry throws X_FLAG_SUBJECT_REQUIRED — never a fallback to the actor or to default.

Overrides and reporting

applyFlagSnapshot({ key: targeting }) lands targeting from outside — a poller, a job, a channel — so evaluation never loads anything. Keys this build does not declare come back in unknown rather than throwing: a control plane is often ahead of a deploy. An overdue flag is reported through core's one error-reporting seam (configureErrorReporting), at most once an hour per flag.

flagsReport() projects every flag with its kind, owner and expiry. Nothing reads it yet: no CLI command prints it and no manifest section carries it.

Codes: X_FLAG_DUPLICATE, X_FLAG_EXPIRED, X_FLAG_EXPIRY_INVALID, X_FLAG_SUBJECT_REQUIRED, X_FLAG_TARGETING_INVALID, X_FLAG_UNKNOWN — Error codes.