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diff --git a/tools/nwc-budget-test.js b/tools/nwc-budget-test.js
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+// The NWC budget window: a UTC day, week (Monday) or month; what a
+// connection spent in the window on now, counting the app's counter and the
+// worker's unabsorbed spends; when it renews. Run: bun tools/nwc-budget-test.js
+import { budgetWindow, budgetRenewsAt, renewalPeriod, spentInWindow } from '../src/nwc-budget.js';
+
+let fails = 0;
+const check = (name, ok, detail = '') => { console.log(` ${ok ? '✓' : '✗'} ${name}${detail ? ' — ' + detail : ''}`); if (!ok) fails++; };
+
+// Wednesday 2026-10-14 23:30 UTC
+const T = Date.UTC(2026, 9, 14, 23, 30);
+check('day window is the UTC date', budgetWindow('day', T) === '2026-10-14', budgetWindow('day', T));
+check('day: half an hour later is the next day', budgetWindow('day', T + 3600_000) === '2026-10-15');
+check('week window is keyed by its Monday', budgetWindow('week', T) === 'W2026-10-12', budgetWindow('week', T));
+check('week: Sunday night is still that week', budgetWindow('week', Date.UTC(2026, 9, 18, 23, 59)) === 'W2026-10-12');
+check('week: Monday 00:00 starts the next', budgetWindow('week', Date.UTC(2026, 9, 19, 0, 0)) === 'W2026-10-19');
+check('month window is the UTC month', budgetWindow('month', T) === '2026-10');
+check('month: Nov 1 00:00 is the next', budgetWindow('month', Date.UTC(2026, 10, 1)) === '2026-11');
+check('unknown period counts as a day', budgetWindow(undefined, T) === '2026-10-14');
+
+check('day renews at next midnight', budgetRenewsAt('day', T) === Date.UTC(2026, 9, 15) / 1000);
+check('week renews next Monday', budgetRenewsAt('week', T) === Date.UTC(2026, 9, 19) / 1000);
+check('month renews on the 1st', budgetRenewsAt('month', T) === Date.UTC(2026, 10, 1) / 1000);
+check('December rolls into January', budgetRenewsAt('month', Date.UTC(2026, 11, 20)) === Date.UTC(2027, 0, 1) / 1000);
+check('NIP-47 period words', renewalPeriod('day') === 'daily' && renewalPeriod('week') === 'weekly' && renewalPeriod('month') === 'monthly' && renewalPeriod(undefined) === 'daily');
+
+console.log('\n[spent in the window]');
+const conn = { id: 'c1', dailySat: 10000, period: 'month', spentDate: '2026-10', spentToday: 3000 };
+{
+  const r = spentInWindow(conn, [], T);
+  check('the app counter counts when stamped with this window', r.spent === 3000 && r.left === 7000, JSON.stringify(r));
+}
+{
+  const r = spentInWindow({ ...conn, spentDate: '2026-09' }, [], T);
+  check('a counter from last month does not', r.spent === 0 && r.left === 10000);
+}
+{
+  const spends = [
+    { connId: 'c1', amountSat: 500, feeSat: 2, ts: Date.UTC(2026, 9, 2) },          // this month, unabsorbed
+    { connId: 'c1', amountSat: 400, feeSat: 0, ts: Date.UTC(2026, 9, 3), absorbed: true }, // already in the counter
+    { connId: 'c1', amountSat: 900, feeSat: 0, ts: Date.UTC(2026, 8, 30) },         // last month
+    { connId: 'c2', amountSat: 800, feeSat: 0, ts: Date.UTC(2026, 9, 5) },          // another connection
+  ];
+  const r = spentInWindow(conn, spends, T);
+  check('the worker adds only its own, unabsorbed spends from this window', r.spent === 3502, String(r.spent));
+}
+{
+  const r = spentInWindow({ id: 'c1', dailySat: 100, spentDate: '2026-10-14', spentToday: 80 }, [{ connId: 'c1', amountSat: 30, ts: T - 60_000 }], T);
+  check('a day budget, over: left is 0 not negative', r.spent === 110 && r.left === 0);
+}
+
+console.log(fails ? `\n❌ ${fails} failure(s)` : '\n✅ budget windows behave');
+process.exit(fails ? 1 : 0);