The denomination problem, which is the real problem
Every visitor to northern Thailand meets the same small friction: the ATM gives you 1,000 baht notes, and the things you actually want to buy cost 40. A songthaew driver, a khao soi stall and a market vendor at nine in the morning will all struggle with a large note, and the resulting awkwardness is entirely avoidable.
Break big notes deliberately. Buy water or a coffee at a convenience store or a supermarket in the morning and pay with the largest note you have. Two minutes of planning gives you a day of frictionless small transactions.
Where cards genuinely work
The card-friendly Chiang Mai is real and easy to find: shopping centres, supermarkets, hotels, hospitals, the Nimmanhaemin cafés and restaurants, and larger tour operators. Contactless is normal in those places, and there is no reason to carry more cash than you need on a day spent among them.
Where you will still be asked for cash is anywhere with a family behind the counter, anywhere with a plastic stool, and anything that moves. That is not a poor-service pattern; it is the economics of card fees on a 50 baht sale.
ATMs and their fees
Thai banks add their own fee for a foreign-card withdrawal, set by each bank rather than fixed nationally, and charged per transaction. That means the cost of your cash depends far more on how many times you withdraw than on how much you take.
The fee is shown on screen before you confirm, and you can cancel at that point. Machines inside bank branches during opening hours are the ones to use if anything goes wrong, since there is somebody to ask.
QR codes, service charges and other small print
The QR codes propped on counters across the city belong to Thailand's domestic payment network, built around Thai bank accounts. The Bank of Thailand has opened cross-border links for users of certain foreign apps, starting with Chinese and Lao services, but there is no general scheme that lets every visitor pay this way.
In restaurants, if a service charge is added, it has to be clearly displayed alongside prices before you order. Thailand's consumer protection agency is explicit about that, and about your right to question a charge that was never shown. It comes up more in hotel restaurants than in the places where you will eat most of your meals.
One thing to check before you fly home
If you are carrying a significant amount of cash out of Thailand, note that there are published limits on how much Thai currency you may take across the border without declaring it, alongside a declaration threshold for foreign currency. It rarely affects a normal trip, but it is a real rule worth a glance if your last day involved a large withdrawal.