10 Toast Payments Engineer (New Grad) Interview Questions (2026)

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Tie to interest in fintech, transaction-level reliability, double-entry accounting, or compliance. If you have done a class on databases/transactions, mention it. Bonus: any exposure to PCI, EMV, or merchant processing in coursework or side projects.

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STAR. Payments bugs are subtle and often financial. Pick a real bug where the root cause was non-obvious. Show methodology: reproduce, log diff, narrow down, hypothesize, verify. Honest about what slowed you down.

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Compute base = total / N rounded down to cents. Compute remainder = total - (base * N). Assign base to all N; distribute remainder one cent at a time to the first 'remainder' diners. Sums to total exactly. Walk through example: $10.00 / 3 = base $3.33, remainder 1 cent, so diners get $3.34, $3.33, $3.33. Handle tax allocation if asked.

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Hash map keyed by transaction-id. On auth: store the auth amount. On capture: look up the auth, validate the capture amount <= auth amount, mark matched. At end: emit unmatched captures and unmatched auths separately. O(N) for N events. Discuss late-arriving events, idempotency.

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Pre-authorize the bill amount + tip buffer (typically 20-25 percent). Hold the auth. When the server enters the tip, send a capture for the final total (within the pre-auth ceiling). If tip exceeds the ceiling, send a separate auth for the delta. Walk through what happens if the capture never arrives (auth expires after ~7 days — the customer's hold drops off). Discuss idempotency keys so retries do not double-charge.

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Client sends an Idempotency-Key header. Server caches the (key, response) tuple for 24 hours. On retry with same key: return the cached response (no re-charge). On retry with same key but different request body: return 409 conflict. Walk through TTL, storage (Redis or DB), and what happens if the first request is still in flight when the retry arrives (lock per key).

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Partial refund. Decrement the line items from the order, recompute total + tax. Refund the delta to the original payment method. Walk through ledger accounting (debit refund-payable, credit cash). Discuss what happens if the original card has been canceled — issue store credit instead. Handle tip refund policy (often tip is not refunded for partial returns).

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EMV chip read → authorization request to processor → approval/decline response → receipt printing → batch settlement at day-end. Discuss tipping (auth for one amount, capture for the tipped total — incremental auth vs adjustment). Explain why card-present has lower fraud and lower interchange than card-not-present. Walk through what happens if the network drops mid-auth.

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Chargeback = the cardholder disputes a charge through their bank; the merchant must respond with evidence (receipt, signature, tip-adjustment record) or lose the funds plus a fee. Discuss the chargeback timeline (typically 60-120 days post-transaction). Walk through how Toast surfaces dispute notifications to the restaurant operator, captures evidence, and submits the response to the processor.

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Card-present: physical card + chip read or contactless, lower fraud, lower interchange, requires PIN/signature in some markets. Card-not-present: online or phone, higher fraud risk, higher interchange, requires CVV + AVS + sometimes 3DS. Toast handles both — POS terminal (CP) and online ordering (CNP). Walk through why fraud responsibility differs.