How it works

Tapwhich names the one card to tap at a place, why, and up to what cap. It reads only the banks' own public pages, writes the facts down in its own words, and works the answer out in your browser.

What you give it

A place and the cards you hold. The place can be a merchant (Grab, Shopee, Circle K), a kind of spending (dining, supermarket) or a city (Hanoi, Tokyo), or both, such as "dining in Tokyo". "Use my location" finds the nearest city in the data; the position stays on your phone.

Your card list, your bonus picks and your statement spend are kept in this browser only. Nothing about you is sent anywhere, and there is no account.

How the answer is worked out

1. Which rules fit. Each card has rules: a merchant list, a category, card network codes (MCC), a country, online or in a shop, days of the week. A rule fits when the place matches one of its lists and every condition holds, inside its start and end dates.
2. Minimum spend. Some rules pay only on purchases above a set amount, or only after a statement spend. If you give your spend so far, Tapwhich checks it. If not, it assumes the minimum is met and says so under the answer. For tiered cards it judges the tier on this purchase alone, and says that too.
3. Caps. The cashback stops at the tightest cap: per purchase, per statement, per month, per year, or for the whole promotion. Foreign-currency caps are converted at a dated rate.
4. Stacking and fees. A promotion that adds on top of the card's usual rate is added. Abroad, the card's foreign transaction fee comes off; when the fee is not known, the answer says so.
5. Ranking. The card that leaves you with the most money wins. On a tie, the one that needs fewer assumptions wins.

What it cannot know

The code a shop reports (its MCC) is set by the shop's bank, not by the shop's name. Tapwhich shows the usual code so you can check it, but a shop can report a different one.

Banks change rates and end promotions. Every answer shows the date the bank page was read and links to it. Check the page before a large purchase.

Where the facts come from

Each rule carries the link to the bank page it came from and the date it was read. The data is a small versioned file, vn-v1.json, with 23 cards from 12 banks in Vietnam. Every week a check re-reads each source page and flags any rule whose page changed, moved or expired, so a person can update it.

No bank logos, no copied bank text and no forum posts are used. Tapwhich is not linked to any bank and earns nothing from your choice.

For AI agents

An assistant can ask the same question over MCP. Add this server to your client:

https://tapwhich.openkrill.app/mcp

It has one read-only tool, pick_card. Pass the place and the cards; amount, currency, date, statement spend and bonus picks are optional. A foreign amount goes in as given, with its currency code, and the tool converts it at its own dated rate.

{
  "name": "pick_card",
  "arguments": {
    "place": "dining in Tokyo",
    "cards": ["VPBank Z JCB", "UOB One"],
    "amount": 8000,
    "currency": "JPY"
  }
}

The result has the one-line answer, the second-best card, every card ranked with its cap and conditions, the source link and the read date. An unknown card or place comes back with the options to choose from.

The same answer is a plain GET for scripts:

https://tapwhich.openkrill.app/api/pick?place=Grab&cards=vpbank-z-jcb,uob-one&amount=250000