Which chips work with both Android and iPhone?
Any tag conforming to NFC Forum Type 2, without qualification. Compatibility follows the tag type rather than the chip brand or the manufacturer, which is why a listing naming a brand and omitting the type has told you nothing useful about whether phones will read it.
It is the type, not the brand
Phones implement the NFC Forum tag types. A chip is readable because it conforms to one of them, not because of who made it.
This is why brand names in listings are close to useless for the compatibility question. Two chips from the same family can present different tag types, and two chips from different manufacturers presenting the same type behave identically.
Android exposes the underlying technologies individually in its reference documentation, which shows how the layers stack. iOS is narrower and works from the same tag types.
The safe choice
Type 2. It is read by essentially every phone with NFC hardware, on both platforms, going back years.
The support is so uniform that a Type 2 tag that fails to read is almost always a physical problem rather than a compatibility one, which makes diagnosis considerably easier.
It is also the cheapest and the most widely stocked, so choosing it costs nothing.
The types that carry risk
Type 5, built on the vicinity card standard, is supported on many recent devices and not uniformly across the installed base. For a deployment aimed at the public with older phones in it, that is a real exposure.
Type 3 is tied to an ecosystem concentrated in Japan and is not a sensible default elsewhere.
Type 1 is largely historical and worth avoiding for anything new.
Type 4 is well supported and appears mostly in cards and emulation scenarios rather than in the stickers most deployments buy.
Checking before you commit
Order a sample, read it with a tool that reports the tag type, and tap it with the oldest iPhone and the oldest Android handset you can borrow.
The oldest, not the newest. Compatibility differences appear on older devices, and testing on a current flagship tells you nothing about the phone in a visitor's pocket.
Repeat it per delivery rather than once. Suppliers substitute, and a substitution that changes the tag type changes the compatibility of the whole batch without changing anything visible.
Frequently asked questions
Will a Type 2 tag work on every iPhone with NFC?
On every model able to read tags at all, yes. What varies by model is background reading, which is about how the tap is initiated rather than whether the tag is readable.
Is a MIFARE tag compatible with iPhone?
It depends entirely on which MIFARE product. Ultralight follows a Forum tag type and reads widely, while Classic uses a proprietary layer and should be avoided for consumer deployments.
Does a bigger memory chip risk compatibility?
Not by itself, as long as the tag type stays the same. Larger chips within the same type are read identically.
How do I read the tag type of a sample?
Any reading tool reports it alongside the chip and the free space. It takes seconds and it answers the most important purchasing question.