A zap answered over NWC waits on far fewer round trips, and a phone with Coinos behind another app answers from its worker. "I'm on V3 … I find nwc zaps a lot more slower than usual" (398ja, 2026-11-10). Measured from the notifier's wakes and the ASP's log over four days: a closed app took about six seconds from the push to its cosign (the Lightning part after that is one and a half), an Android phone fifteen at the median, up to thirty. Where it went: every call to the ASP and the notifier was two round trips (no Access-Control-Max-Age, so Chrome re-asked the CORS preflight after five seconds; a Cloudflare response-header rule now sets 7200 on OPTIONS for ark.coinos.io, ark-staging and nwcpush), and the worker made them one after another: answered?, GetArkInfo, Handshake, claim, a save, the chain tip, then the payment, then status ("pending"), the tip again, the held status call, and status once more for the preimage it had just been handed. Now: what the mirror alone decides (invoice, network, budgets, coins, keys) comes first with no network; the claim (which now also says "already answered", coinosv3-server 3c5e9c3) goes out beside the manager's start, whose info call, handshake and tip run together (init, warmTip/_tipFresh); a send just initiated returns to its caller instead of asking at once, and awaitLnPay settles from the held call's own answer. A worker-style send on regtest: eight calls, six in sequence, where there were thirteen. The app's own answer no longer waits a second on a phone that is on screen (only one behind another app holds back for a computer).

Android freezes an app behind another one, and the worker's nudge to it sat unread until it was opened: on a phone the worker now hands a request only to a window on screen (as it already did on iOS), nudges a hidden one in case it is awake, and answers itself; the request's lock decides between them. For that the mirror is no longer written over the worker's hand: the worker stamps its writes (workerAt), the app writes under the worker's own lock (nwc-bg-work) and folds the worker's state in first (never mid-payment), carries the stamp, absorbs it when it comes back on screen (arkBgAbsorb), and refreshes the mirror as it goes behind another app.
Tests: ln-pay-double-wake (the one held call settles; a failed or mismatched answer doesn't), sw-push-quota (an Android phone's hidden window), nwc-sw-test's notifier stand-in answers /claim as the real one; tools/ark/ln-e2e-test's pay half against regtest captaind 0.8 + CLN

Co-Authored-By: Claude Fable 5.2 <[email protected]>

5 files changed, 155 insertions, 30 deletions

diff --git a/src/ark/manager.js b/src/ark/manager.js
index 0370a77..8aa8707 100644
--- a/src/ark/manager.js
+++ b/src/ark/manager.js
@@ -310,6 +310,9 @@ export class ArkManager {
       seen.add(m.vtxoId);
       return true;
     });
+    // (the handshake goes out beside the info call, not after it: a wake of
+    // the phone's worker pays for every round trip in the zap it answers)
+    const hsP = handshake(this.arkUrl).catch(() => null);
     try {
       this.info = await getArkInfo(this.arkUrl);
       this.offline = false;
@@ -323,7 +326,7 @@ export class ArkManager {
       this.info = this.infoHint;
       this.offline = true;
     }
-    const hs = await handshake(this.arkUrl).catch(() => null);
+    const hs = await hsP;
     this.psa = hs?.psa;
     this.serverPub = hex.decode(this.info.serverPubkey);
 
@@ -387,6 +390,20 @@ export class ArkManager {
     return Date.now() - age * 600_000;
   }
 
+  // The tip for a payment about to be built: one asked for moments ago (a
+  // worker warms it while the manager starts) is as good as asking again.
+  warmTip() {
+    const p = this.chain.tipHeight().then((h) => { this._tipH = h; this._tipAt = Date.now(); this._tipNow = Date.now(); return h; });
+    this._tipWarm = p;
+    p.catch(() => {}).finally(() => { if (this._tipWarm === p) this._tipWarm = null; });
+    return p;
+  }
+  async _tipFresh() {
+    if (this._tipNow && Date.now() - this._tipNow < 5000 && this._tipH) return this._tipH;
+    if (this._tipWarm) { try { return await this._tipWarm; } catch { /* ask again */ } }
+    return this.chain.tipHeight();
+  }
+
   async _tipMemo() {
     if (this._tipAt && Date.now() - this._tipAt < 60_000) return this._tipH;
     try {
@@ -1421,10 +1438,17 @@ export class ArkManager {
   async awaitLnPay(id, timeoutMs = 90_000) {
     const a = this.lnAction(id);
     if (!a || a.step !== 'initiated') return this.driveLn(id);
-    await Promise.race([
+    const res = await Promise.race([
       checkLightningPayment(this.arkUrl, hex.decode(a.paymentHash), true).catch(() => null),
       new Promise((r) => setTimeout(r, timeoutMs)),
     ]);
+    // the held call's own answer settles it: asking once more for the same
+    // preimage was a round trip between the payment and its reply
+    if (res && res.status === 'success' && res.preimage && a.step === 'initiated'
+        && hex.encode(sha256(hex.decode(res.preimage))) === a.paymentHash) {
+      this._settleLnPay(a, res.preimage);
+      return a;
+    }
     return this.driveLn(id);
   }
 
@@ -1556,8 +1580,14 @@ export class ArkManager {
       // as paid, hours before the money moved)
       throw new Error(prior.step === 'done' ? 'this invoice was already paid' : 'a payment for this invoice is already in progress');
     }
-    const tip = await this.chain.tipHeight();
-    const routing = routingFeeSat != null ? routingFeeSat : await this.lnRouteFee(invoice, amountSat);
+    const [tip, routing] = await Promise.all([
+      this._tipFresh(),
+      routingFeeSat != null ? routingFeeSat : this.lnRouteFee(invoice, amountSat),
+    ]);
+    // the two awaits above let a second call for the same invoice in
+    const twin = this.state.actions.find((a) =>
+      a.type === 'ln-pay' && a.paymentHash === dec.paymentHash && a.step !== 'failed');
+    if (twin) throw new Error(twin.step === 'done' ? 'this invoice was already paid' : 'a payment for this invoice is already in progress');
     // smallest single vtxo that covers amount + routing + its expiry-dependent
     // fee — else gather largest-first: each input funds its own HTLC vtxo with
     // the same payment hash, and the server sums them into one payment.
@@ -1721,6 +1751,11 @@ export class ArkManager {
         });
         action.step = 'initiated';
         this._save();
+        // Just handed to the server: nothing to learn from asking this
+        // instant (it answered "pending", and the chain tip was fetched to
+        // go with it — two round trips before the caller's held call could
+        // even start). The caller waits on it (awaitLnPay) or polls.
+        return;
       } catch (e) {
         // the HTLC already went to this payment: another context (a second
         // wake of the phone's worker, seconds apart, over its own copy of the
diff --git a/src/features/ark.js b/src/features/ark.js
index f5ba9d8..075cf4b 100644
--- a/src/features/ark.js
+++ b/src/features/ark.js
@@ -1052,10 +1052,29 @@ export function arkFeature(ctx) {
       return v;
     };
   }
-  async function writeBgMirror(connections, offer, prefs) {
+  // The worker can now pay while this app is alive behind another one (a
+  // phone with Coinos in the background: the page is frozen or throttled,
+  // the worker is not). Its change coin and its spent coins then exist only
+  // in the mirror — so the mirror is never written over the worker's hand:
+  // one writer at a time (the lock the worker holds for a whole push), and
+  // whatever the worker did since we last looked is folded in first.
+  let bgAbsorbedAt = 0; // the worker stamp (rec.workerAt) this app has folded in
+  const withBgLock = (fn) => (typeof navigator !== 'undefined' && navigator.locks
+    ? navigator.locks.request('nwc-bg-work', fn) : fn());
+  const bgBusy = (mgr) => !!(mgr && mgr._lnDriving && mgr._lnDriving.size);
+  async function writeBgMirror(connections, offer, prefs) { return withBgLock(() => writeBgMirrorLocked(connections, offer, prefs)); }
+  async function writeBgMirrorLocked(connections, offer, prefs) {
     const cfg = getArkConfig();
     if (!cfg || !ark || !ark.info || !ark.state) return;
-    const prev = await loadBg(wallet._cacheKey());
+    let prev = await loadBg(wallet._cacheKey());
+    if (prev && prev.v >= 3 && prev.mgr && (prev.workerAt || 0) > bgAbsorbedAt) {
+      // a payment of ours in flight holds the very records a merge replaces:
+      // leave the worker's state standing and write at the next call
+      if (bgBusy(ark)) return;
+      await mergeBgWorkerState(ark).catch((e) => console.warn('ark: worker merge failed', e && e.message));
+      if ((prev.workerAt || 0) > bgAbsorbedAt) return; // not folded in: don't write over it
+      prev = await loadBg(wallet._cacheKey());
+    }
     if (connections === undefined) connections = (prev && prev.connections) || [];
     if (offer === undefined) offer = (prev && prev.offer) || null;
     if (prefs === undefined) prefs = (prev && prev.prefs) || undefined;
@@ -1068,7 +1087,7 @@ export function arkFeature(ctx) {
       offer,
       autowithdraw: await awMirror(prev),
     });
-    if (prev) { rec.spends = prev.spends; rec.pendingPays = prev.pendingPays; } // the worker's log survives rewrites
+    if (prev) { rec.spends = prev.spends; rec.pendingPays = prev.pendingPays; if (prev.workerAt) rec.workerAt = prev.workerAt; } // the worker's log survives rewrites
     if (prefs) rec.prefs = prefs; // notification choices the worker honours
     await saveBg(wallet._cacheKey(), rec);
     await disarmSiblingRecords(wallet._cacheKey(), rec).catch(() => {});
@@ -1088,6 +1107,7 @@ export function arkFeature(ctx) {
     }
     if (!rec.mgr) return;
     bootPhase('ark-merge');
+    const stamp = rec.workerAt || 0;
     const merged = mergeArkStates(mgr.state, rec.mgr);
     // the worker's view of OUR vtxos is newer for anything it spent
     const workerState = new Map((rec.mgr.vtxos || []).map((v) => [v.id, v.state]));
@@ -1097,6 +1117,7 @@ export function arkFeature(ctx) {
     merged.nextKeyIndex = Math.max(merged.nextKeyIndex || 1, rec.mgr.nextKeyIndex || 1);
     mgr.state = merged;
     mgr._save();
+    if (mgr === ark) bgAbsorbedAt = Math.max(bgAbsorbedAt, stamp);
     bootPhase('ark-merged');
     await mgr.reconcile().catch(() => {});
     bootPhase('ark-reconciled');
@@ -4166,6 +4187,18 @@ export function arkFeature(ctx) {
     // nwc.js calls this (with its connections) whenever the mirror should
     // refresh: on enable, on connection changes, after payments, on a timer.
     async arkBgWrite(connections, offer, prefs) { return writeBgMirror(connections, offer, prefs); },
+    // Back on screen: what the worker paid while this app sat behind another
+    // one is folded in now, not at the next start. true when there was any.
+    async arkBgAbsorb() {
+      if (!ark || !ark.state) return false;
+      return withBgLock(async () => {
+        const rec = await loadBg(wallet._cacheKey());
+        if (!rec || rec.v < 3 || !rec.mgr || (rec.workerAt || 0) <= bgAbsorbedAt || bgBusy(ark)) return false;
+        await mergeBgWorkerState(ark);
+        render();
+        return true;
+      });
+    },
     async arkBgSpendableSat() {
       const rec = await loadBg(wallet._cacheKey());
       return ((rec && rec.mgr && rec.mgr.vtxos) || [])
diff --git a/src/features/nwc.js b/src/features/nwc.js
index dc87388..9768740 100644
--- a/src/features/nwc.js
+++ b/src/features/nwc.js
@@ -388,7 +388,10 @@ export function nwcFeature(ctx) {
         return !!(prior && prior.length);
       };
       if (age > 5 && await lookBack()) return dlog('nwc: skip — request already answered elsewhere', ev.id.slice(0, 8));
-      if (mobile && age <= 5) await new Promise((r) => setTimeout(r, 1000));
+      // (a phone with this app on screen is as awake as any computer: only
+      // one behind another app holds back — every zap answered by an open
+      // phone used to wait this second for nobody)
+      if (mobile && age <= 5 && typeof document !== 'undefined' && document.hidden) await new Promise((r) => setTimeout(r, 1000));
       claimed = await claim(ev.id);
       if (claimed === false) return dlog('nwc: standing down — another device claimed', ev.id.slice(0, 8));
       if (claimed === null && age <= 5) {
@@ -981,7 +984,14 @@ export function nwcFeature(ctx) {
     if (typeof window !== 'undefined') {
       window.addEventListener('online', () => listen());
       document.addEventListener('visibilitychange', () => {
-        if (document.visibilityState === 'visible' && conns().length) { listen(); refreshRegistration(); }
+        if (document.visibilityState === 'visible' && conns().length) {
+          listen(); refreshRegistration();
+          // the worker may have paid while this app sat behind another one
+          Promise.resolve(hook('arkBgAbsorb')).then((had) => { if (had) return reconcileBg(); }).catch(() => {});
+        }
+        // going behind another app: the worker answers from the mirror now, so
+        // it gets this moment's coins, not the last minute-and-a-half's
+        if (document.visibilityState === 'hidden' && conns().length) writeBg();
       });
     }
   }
diff --git a/src/nwc-respond.js b/src/nwc-respond.js
index 98fd19c..311b928 100644
--- a/src/nwc-respond.js
+++ b/src/nwc-respond.js
@@ -85,15 +85,21 @@ export async function bgManager(rec, walletKey, saveFn, log = null) {
           log(`ln-pay ${String(a.paymentHash || '').slice(0, 8)} → ${a.step}${a.error ? ': ' + a.error : ''}`);
         }
       }
+      // the worker's hand on the mirror, for an app alive behind it: it
+      // folds this state in before it next writes its own over it
+      rec.workerAt = Date.now();
       Promise.resolve(saveFn(walletKey, rec)).catch(() => {});
     },
   };
-  return new ArkManager({
+  const m = new ArkManager({
     account: shimAccount(rec), storage,
     arkUrl: rec.ark.arkUrl, esploraUrl: rec.ark.esploraUrl, network: rec.ark.network,
     honorsExpired: !!rec.ark.honorsExpired,
     lnOldestFirst: () => !!rec.custody && rec.custody !== 'custodial',
-  }).init();
+  });
+  // the chain tip a payment needs, asked for while the manager starts
+  try { m.warmTip().catch(() => {}); } catch { /* asked again when needed */ }
+  return m.init();
 }
 
 // ---- sends in flight across wakes ------------------------------------------
@@ -300,6 +306,8 @@ export async function respondFromBg(data, {
   // ...and hands each pay request to the first device that claims it, so
   // two awake devices don't each lock an HTLC for one zap. Unreachable →
   // proceed: a lone worker must not go silent over a notifier hiccup.
+  // (the same call says when a reply already exists: one round trip where
+  // "answered?" and "mine?" used to be two, one after the other)
   const claim = async () => {
     try {
       const r = await fetchFn(`${notifier}/claim`, {
@@ -307,9 +315,25 @@ export async function respondFromBg(data, {
         body: JSON.stringify({ event: ev.id, by: `sw-${walletKey}`.slice(0, 64) }),
       });
       if (!r.ok) return true;
-      return (await r.json()).ok !== false;
+      const j = await r.json();
+      if (j.answered) return 'answered';
+      return j.ok !== false;
     } catch { return true; }
   };
+  // a claim this worker can't make good on (the wallet's server unreachable)
+  // is handed back, so the app the user opens isn't told to stand down
+  const unclaim = () => {
+    try {
+      Promise.resolve(fetchFn(`${notifier}/claim`, {
+        method: 'POST', headers: { 'content-type': 'application/json' },
+        body: JSON.stringify({ event: ev.id, by: `sw-${walletKey}`.slice(0, 64), release: true }),
+      })).catch(() => {});
+    } catch { /* it lapses by itself */ }
+  };
+  const lostClaim = (c) => {
+    log(c === 'answered' ? 'already answered elsewhere' : 'another device claimed this request — standing down');
+    return true;
+  };
 
   if (method === 'get_info') {
     await publish({
@@ -433,12 +457,21 @@ export async function respondFromBg(data, {
   }
 
   // ---- pay_invoice ------------------------------------------------------
-  if (await answered()) { log('already answered elsewhere'); return true; }
-
+  // What the mirror alone can decide comes first, with no network at all;
+  // then the claim (which also says if someone already answered) goes out
+  // beside the manager's start. Each of these used to wait for the one
+  // before it: a zap answered from a closed phone spent about six seconds
+  // getting to the payment (398ja, 2026-11-10: "a lot more slower").
+  // A refusal never goes out over another device's answer.
+  const refuse = async (code, message) => {
+    if (await answered()) { log('already answered elsewhere'); return true; }
+    await publish(errRes('pay_invoice', code, message));
+    return true;
+  };
   const invoice = params.invoice;
   const dec = invoice && maybeBolt11(invoice);
-  if (!dec) { await publish(errRes('pay_invoice', 'OTHER', 'not a bolt11 invoice')); return true; }
-  if (!dec.amountSat) { await publish(errRes('pay_invoice', 'OTHER', 'zero-amount invoices are not supported')); return true; }
+  if (!dec) return refuse('OTHER', 'not a bolt11 invoice');
+  if (!dec.amountSat) return refuse('OTHER', 'zero-amount invoices are not supported');
   // The bolt11 names its network; only a record on that network can pay it.
   // A mismatched copy fails in milliseconds with a nonsense error that beats
   // a sibling device's real payment to the client — so stand down instead,
@@ -451,17 +484,11 @@ export async function respondFromBg(data, {
     return answered();
   }
   ({ walletKey, rec, conn } = fit);
-  if (dec.amountSat > conn.maxSat) {
-    await publish(errRes('pay_invoice', 'QUOTA_EXCEEDED', `over the ${conn.maxSat} sat per-payment limit`));
-    return true;
-  }
+  if (dec.amountSat > conn.maxSat) return refuse('QUOTA_EXCEEDED', `over the ${conn.maxSat} sat per-payment limit`);
   // What the app spent this window rides on the mirrored connection; the
   // worker's own spends the app has not yet folded in are counted from its log.
   const { left } = spentInWindow(conn, rec.spends || []);
-  if (dec.amountSat > left) {
-    await publish(errRes('pay_invoice', 'QUOTA_EXCEEDED', `over the remaining ${renewalPeriod(conn.period)} budget (${left} sat)`));
-    return true;
-  }
+  if (dec.amountSat > left) return refuse('QUOTA_EXCEEDED', `over the remaining ${renewalPeriod(conn.period)} budget (${left} sat)`);
   // A send this worker began on an earlier wake and never finished (PullCap's
   // night of 2026-10-23: three hours of fifteen). Its coin sat reserved in
   // the mirror, so a fresh attempt at the SAME invoice was refused as
@@ -511,7 +538,8 @@ export async function respondFromBg(data, {
     return sentNotice(rec, a, c, dec.amountSat);
   };
   if (same) {
-    if (!(await claim())) { log('another device claimed this request — standing down'); return true; }
+    const c = await claim();
+    if (c !== true) return lostClaim(c);
     log(`a send for this invoice is already ${same.step} — finishing it`);
     const wasDone = same.step === 'done';
     let a = same;
@@ -530,6 +558,7 @@ export async function respondFromBg(data, {
   // Only when they hold the coins this payment needs: otherwise the answer
   // comes first and the wake's tending finishes them after.
   if (stuck.length && spendableSat(rec) < dec.amountSat) {
+    if (await answered()) { log('already answered elsewhere'); return true; }
     try {
       await ready();
       for (const a of stuck) {
@@ -558,12 +587,21 @@ export async function respondFromBg(data, {
   // outputs allocate from nextKeyIndex, which must stay inside the key
   // window the app pre-derived; if it ran out, wake the user instead.
   if (!rec.keys[String(rec.mgr.nextKeyIndex || 1)]) return declineLater('key window exhausted');
-  try { await ready(); } catch (e) { return declineLater('ark init failed: ' + e.message); }
-
-  if (!(await claim())) { log('another device claimed this request — standing down'); return true; }
+  // the claim and the manager's start (begun above), side by side
+  const claimP = claim();
+  try { await ready(); } catch (e) {
+    const c = await claimP;
+    if (c !== true) return lostClaim(c);
+    unclaim();
+    return declineLater('ark init failed: ' + e.message);
+  }
+  const mine = await claimP;
+  if (mine !== true) return lostClaim(mine);
   // whose send this is, for the wake that finishes it should this one not
   notePending(rec, dec.paymentHash, conn);
-  await saveFn(walletKey, rec);
+  // (not waited on: the manager's own save of the new send, a moment on,
+  // writes this same record)
+  Promise.resolve(saveFn(walletKey, rec)).catch(() => {});
   log(`paying ${dec.amountSat} sat`);
   let a = null;
   for (let attempt = 0; ; attempt++) {
diff --git a/src/sw-nwc.js b/src/sw-nwc.js
index a4e4e56..b0ef49f 100644
--- a/src/sw-nwc.js
+++ b/src/sw-nwc.js
@@ -187,14 +187,23 @@ self.addEventListener('push', (e) => {
     // An open window handles requests itself with full wallet state — nudge
     // it and stay out of the way. On iOS only one on screen counts: a
     // backgrounded app is suspended, so a nudge to it went unanswered.
+    // Android does the same to an app behind another one, minutes in: the
+    // page is frozen, its relay sockets dead, and the nudge sat unread until
+    // the app was opened — the zap spun for as long (wake to payment: 15 s
+    // at the median on Android phones against 6 for a closed app, 2026-11-10).
+    // So on any phone only a window on screen is handed the request; one
+    // behind another app is nudged too, in case it is still awake, and the
+    // request's lock decides between it and this worker.
+    const phone = apple || /Mobi|Android/i.test((self.navigator && self.navigator.userAgent) || '');
     const all = await self.clients.matchAll({ type: 'window', includeUncontrolled: true });
-    const clients = all.filter((c) => !apple || c.visibilityState === 'visible');
+    const clients = all.filter((c) => !phone || c.visibilityState === 'visible');
     if (clients.length) {
       for (const c of clients) c.postMessage({ type: 'nwc-wake', servicePubkey: data.servicePubkey });
       if (apple) await dropStandIn(); // the window on screen shows it live
       diary('window on screen — handed over');
       return;
     }
+    if (!apple) for (const c of all) { try { c.postMessage({ type: 'nwc-wake', servicePubkey: data.servicePubkey }); } catch (_) {} }
     const notice = (n) => show(n.title, {
       body: n.body, icon: 'icon-192.png', badge: 'badge-96.png',
       tag: n.tag || 'nwc-incoming', renotify: !apple, data: { url: './' },