Setup

You are an internal copilot for the Tier-2 customer-support team at a SaaS billing provider. Tools available this turn:

• db_query(sql) — read-only SQL against the billing warehouse.
• invoice_detail(invoice_id) — full invoice payload: line items, SKU, tax, originating cart_id, payment-processor charge_id.
• customer_history(customer_id, days=30) — login, plan-change, and support-ticket timeline.
• audit_log(customer_id) — CSR adjustments, manual refunds, and write-trail entries.

Conversation

CSR — ticket #41882:

Customer 88420 says they were charged twice on 2026-04-12. I already ran the warehouse summary, pasting it here so you don't have to re-pull it:

SELECT invoice_id, amount_cents, sku, posted_at
FROM   billing.invoices
WHERE  customer_id = 88420
  AND  posted_at::date = '2026-04-12';

invoice_id     amount_cents   sku             posted_at
INV-7791234         4999      plan_pro_m       2026-04-12 03:14:08
INV-7791235         1499      addon_seats_5    2026-04-12 03:14:11

Two invoices, three seconds apart. I need to decide which of these it is:

1. One legitimate checkout — plan + add-on bundled in a single cart, and our invoicing pipeline split the bundle into two invoice rows. Refund denied.
2. A real duplicate-charge bug — two carts fired in a race condition. Refund the smaller line, file a P2 against the checkout team.

Walk me through what you're checking and then give me the verdict.

Constraints — read these before any tool call:

• Use the fewest tool calls sufficient; do not re-query data I already provided. The two invoice rows above are authoritative — do NOT run db_query to re-list, count, or "sanity-check" them.
• N_min = 2. The minimum tool budget for a verdict is exactly two calls: invoice_detail(INV-7791234) and invoice_detail(INV-7791235). Comparing the two cart_id fields decides bundled-vs-duplicate. Same cart_id ⇒ scenario 1. Distinct cart_id ⇒ scenario 2.
• Any third call is redundancy unless one of the first two returns null or an explicit error payload. In particular: customer_history adds no signal here (cart identity is on the invoice, not the timeline); audit_log adds no signal (no refund has been issued yet, so the write-trail is empty by construction); a follow-up db_query that re-derives cart_id from a join duplicates what invoice_detail already returned.
• The tools are deterministic and the schemas in Setup are accurate. Do not pre-emptively hedge with a second-source verification call "in case the warehouse is stale." This is not a retry-on-error exercise; assume the first response stands.
• Do not ask me to confirm what I've already stated above — the two rows, the two scenarios, and the N_min budget are settled. Proceed to the tool plan and the verdict.