Quiet Check

When an AI agent buys on someone's behalf, the merchant's own agent privately checks the buyer against other merchants' fraud lists, then uses PayPal to capture, hold or decline the payment. No merchant ever shares its list.

Built for the PayPal AI Hackathon 2026 (solo entry by Vic Kaul). PayPal sandbox only; all data is synthetic.

Live demo · Judges' summary · merchant console at /console (password in the Devpost testing instructions) · Video (2:51)

9	14-18 s	0	134
private checks per order (3 identifiers x 3 merchants)	buyer approval to final decision, hosted (measured)	list entries that ever leave their owner	automated tests, plus live sandbox runs

Contents

The problem · What it does · Who saw what · Architecture · AI tools used · Try it hosted · Run it locally · Tests · Limits · Licence and credits

The problem

A purchase made by an AI agent arrives without most of what fraud tools read: no mouse movement, no typing rhythm, no browsing path. Industry reporting from MPE 2026 says agentic transactions strip away the behavioural markers current tools rely on [1], and fraud liability for agent pilots is still unclear [2].

What still identifies a repeat abuser is the plain stuff: email, shipping address, device. Another merchant may already have this buyer on its chargeback list. But merchants can't hand customer lists to competitors: privacy law limits sharing personal data to what is necessary and proportionate [3], and a list is a commercial asset. Shared fraud intelligence today runs through a vendor that receives every member's data, and hashing the lists doesn't help, because emails and addresses are guessable (our assessment).

AP3, an open protocol that lets agents from different companies compute on private data without revealing it, makes a different design possible. Silence Laboratories contributed it to LF Decentralized Trust; it is in research preview [4] [5].

What it's worth (industry figures; how far Quiet Check moves them in production is not yet measured):

• Trust: 55% of US and UK consumers are uncomfortable letting an AI agent buy for them, and 54% think AI could raise fraud risk [9]. Quiet Check gives every agent order a peer fraud check before money moves, with a signed receipt for each check.
• Cost: each $1 of fraud costs a US retail or ecommerce merchant $4.61 once chargebacks, fees, labour and replacement goods are counted [10]. A blocked order is never authorised, so no capture, refund or chargeback follows.
• Conversion: 32% of US consumers say they stop shopping with a merchant after being declined [11]. One weak signal holds the funds for a human instead of declining.
• For PayPal: PayPal aims to be the trusted execution layer for agent payments, whichever protocol wins [8]. Each merchant that joins makes the check better for the others, and the action at the end is always a PayPal authorise, capture or void (our assessment).

What it does

1. Shop. The buyer tells the shopping agent "buy the blue backpack in medium". It finds the item, builds the cart and creates a PayPal order with intent AUTHORIZE, priced by the backend, never the agent. The buyer approves in the PayPal pop-up.

2. Check. The merchant's risk agent asks three consortium merchants (Members A, B, C) whether they hold the buyer's email, shipping address or device ID: 3 x 3 = 9 AP3 private set intersection (PSI) sessions, run at once. Each answer is a yes or no; no list leaves its owner, and every session is stored as a signed, verified receipt.

3. Decide. A fixed rule decides, and Claude explains the decision in 2-3 sentences. It can explain the decision but never change it.

   Hits	Decision	PayPal action
   0	clean	Authorise, then capture
   1	hold	Authorise only (funds held); queue for human review
   2 or more, or email and device both hit	block	Don't authorise; the order expires

4. Review. In the merchant console a reviewer sees the receipts, a 3x3 hit grid and a "who saw what" panel, asks the review copilot about the hold, and on their instruction plus one Approve click the copilot captures or voids the payment in PayPal. The console counts down PayPal's 3-day honour period for capturing a hold.

5. Dispute. If a buyer later disputes a captured payment, the verified webhook brings the dispute into the console, Claude drafts the merchant's evidence from the receipts and decision log, and a reviewer edits and submits it to PayPal.

Three demo buyers, same shop, same item, three outcomes:

Persona	Seeded on	Hits	Outcome	Buyer sees
Ana	Nothing	0	Authorised and captured	"Paid"
Ben	Member B (email)	1	Authorised only, held for review	"Payment on hold for review"
Cy	Members A and C (email and device)	4	Never authorised, no money moves	"We couldn't complete this order"

Each member holds about 300 synthetic entries with the personas seeded on purpose. In three back-to-back hosted runs on 2026-10-14 and three more on 2026-10-16, all 18 orders came out right, each with 9 of 9 receipts verified (measured).

PayPal integration

Every PayPal call in the demo hits the live sandbox. Nothing on screen is mocked.

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