diff --git a/__tests__/stripe-logic.test.ts b/__tests__/stripe-logic.test.ts
new file mode 100644
index 0000000..48fc1db
--- /dev/null
+++ b/__tests__/stripe-logic.test.ts
@@ -0,0 +1,78 @@
+import { describe, it, expect } from 'vitest'
+import {
+  STRIPE_API_VERSION,
+  stripeStatusIsPaid,
+  duesMemberStatus,
+  graceExceeded,
+  priceIdForTier,
+  isStripeConfigured,
+} from '@/lib/stripe-logic'
+
+describe('STRIPE_API_VERSION', () => {
+  it('is pinned (conscious upgrades only)', () => {
+    expect(STRIPE_API_VERSION).toBe('2026-04-22.dahlia')
+  })
+})
+
+describe('stripeStatusIsPaid', () => {
+  it('active/trialing are paid; everything else is not', () => {
+    expect(stripeStatusIsPaid('active')).toBe(true)
+    expect(stripeStatusIsPaid('trialing')).toBe(true)
+    for (const s of ['past_due', 'canceled', 'unpaid', 'incomplete', 'incomplete_expired', '', null, undefined]) {
+      expect(stripeStatusIsPaid(s as string)).toBe(false)
+    }
+  })
+})
+
+describe('duesMemberStatus', () => {
+  it('paid statuses -> current', () => {
+    expect(duesMemberStatus('active', false)).toBe('current')
+    expect(duesMemberStatus('trialing', true)).toBe('current')
+  })
+  it('past_due respects the grace window', () => {
+    expect(duesMemberStatus('past_due', false)).toBe('current')
+    expect(duesMemberStatus('past_due', true)).toBe('late')
+  })
+  it('terminal/failed statuses -> late, NEVER inactive', () => {
+    for (const s of ['canceled', 'unpaid', 'incomplete', 'incomplete_expired']) {
+      expect(duesMemberStatus(s, false)).toBe('late')
+    }
+  })
+  it('unknown/empty -> null (no change, never blind overwrite)', () => {
+    expect(duesMemberStatus(undefined, true)).toBeNull()
+    expect(duesMemberStatus('something_new', true)).toBeNull()
+  })
+})
+
+describe('graceExceeded', () => {
+  const end = '2026-05-01T00:00:00Z'
+  it('false within the window, true past it', () => {
+    expect(graceExceeded(end, 7, '2026-05-05T00:00:00Z')).toBe(false)
+    expect(graceExceeded(end, 7, '2026-05-08T00:01:00Z')).toBe(true)
+    expect(graceExceeded(end, 0, '2026-05-01T00:00:01Z')).toBe(true)
+  })
+  it('missing/invalid period end is never "exceeded" (fail safe)', () => {
+    expect(graceExceeded(null, 7, '2030-01-01T00:00:00Z')).toBe(false)
+    expect(graceExceeded('nope', 7, '2030-01-01T00:00:00Z')).toBe(false)
+  })
+})
+
+describe('priceIdForTier', () => {
+  const cfg = { prices: { plus: 'price_1', basic: ' ', associate: undefined } }
+  it('returns the mapped non-empty price or null', () => {
+    expect(priceIdForTier(cfg, 'plus')).toBe('price_1')
+    expect(priceIdForTier(cfg, 'basic')).toBeNull()
+    expect(priceIdForTier(cfg, 'associate')).toBeNull()
+    expect(priceIdForTier(cfg, 'missing')).toBeNull()
+    expect(priceIdForTier(null, 'plus')).toBeNull()
+  })
+})
+
+describe('isStripeConfigured', () => {
+  it('needs publishable key + secret ref + >=1 mapped price', () => {
+    expect(isStripeConfigured(null)).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', secret_key_ref: 's', prices: {} })).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', prices: { plus: 'price_1' } })).toBe(false)
+    expect(isStripeConfigured({ publishable_key: 'pk', secret_key_ref: 's', prices: { plus: 'price_1' } })).toBe(true)
+  })
+})
diff --git a/lib/stripe-logic.ts b/lib/stripe-logic.ts
new file mode 100644
index 0000000..63b539b
--- /dev/null
+++ b/lib/stripe-logic.ts
@@ -0,0 +1,73 @@
+// Pure, dependency-free decision logic for Stripe recurring dues. No Stripe
+// SDK, no Supabase, no React. Unit-tested directly so the money-critical
+// status mapping cannot silently drift. The locked rules (owner, 2026-05):
+// per-space OWN keys; subscriptions; lapse = grace -> `late`, NEVER auto-
+// inactive.
+
+// Pin deliberately so an SDK/API bump is a conscious change (see
+// integration-api-facts memory). Consumed by the server Stripe client.
+export const STRIPE_API_VERSION = '2026-04-22.dahlia'
+
+// Stripe subscription.status values that mean "the member has paid access".
+const PAID = new Set(['active', 'trialing'])
+// Terminal/failed states: access is gone. `past_due` is handled separately
+// (grace window) and is deliberately NOT in either set here.
+const LAPSED = new Set(['canceled', 'unpaid', 'incomplete', 'incomplete_expired'])
+
+export function stripeStatusIsPaid(status: string | null | undefined): boolean {
+  return !!status && PAID.has(status)
+}
+
+// Map a Stripe subscription status to the member status we should set.
+// Returns `null` when no change is warranted (unknown/irrelevant status), so
+// the caller never blindly overwrites. NEVER returns 'inactive' — lapse tops
+// out at 'late' and an admin decides inactivation manually (locked decision).
+// `graceExceeded` is computed by the caller (now past current_period_end +
+// grace days); only relevant while `past_due`.
+export function duesMemberStatus(
+  stripeStatus: string | null | undefined,
+  graceExceeded: boolean,
+): 'current' | 'late' | null {
+  if (!stripeStatus) return null
+  if (PAID.has(stripeStatus)) return 'current'
+  if (stripeStatus === 'past_due') return graceExceeded ? 'late' : 'current'
+  if (LAPSED.has(stripeStatus)) return 'late'
+  return null
+}
+
+// Whether `now` is past the grace window after the period end.
+export function graceExceeded(
+  currentPeriodEnd: string | null | undefined,
+  graceDays: number,
+  now?: Date | string,
+): boolean {
+  if (!currentPeriodEnd) return false
+  const end = new Date(currentPeriodEnd).getTime()
+  if (Number.isNaN(end)) return false
+  const t = now == null ? Date.now() : new Date(now).getTime()
+  return t > end + Math.max(0, graceDays) * 86_400_000
+}
+
+type StripeConfig = {
+  mode?: string
+  publishable_key?: string
+  secret_key_ref?: string
+  webhook_secret_ref?: string
+  prices?: Record<string, string | null | undefined>
+} | null | undefined
+
+// The Stripe Price id mapped to a membership tier, or null if unmapped.
+export function priceIdForTier(config: StripeConfig, tier: string | null | undefined): string | null {
+  const p = config?.prices
+  if (!p || !tier) return null
+  const v = p[tier]
+  return typeof v === 'string' && v.trim() !== '' ? v : null
+}
+
+// Enough config to actually run Checkout: a publishable key, a secret-key
+// vault ref, and at least one tier price mapped.
+export function isStripeConfigured(config: StripeConfig): boolean {
+  if (!config?.publishable_key || !config?.secret_key_ref) return false
+  const prices = config.prices ?? {}
+  return Object.values(prices).some(v => typeof v === 'string' && v.trim() !== '')
+}