Does the iPhone read tags when locked?
Locked and asleep are not the same state, and that distinction decides the answer. A locked but awake phone can present a notification from a background read. A phone fully asleep in a pocket is not looking for anything at all, whatever its model.
Three states, not two
People say locked and mean one of three things.
Asleep, with the screen off and the phone doing as little as possible. Awake but locked, showing the lock screen. Or unlocked and in use.
Those behave differently, and a question phrased as locked or not cannot be answered because it merges the first two.
What actually happens
On a model with background reading, a phone that is awake enough to present a notification can produce one from a tag. Background reading is available from the iPhone XS and XR onwards.
A phone that is asleep is not in that situation, so nothing appears.
An application cannot read in any of the three states unless it is open and asking, which is the constraint that applies regardless of the lock.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
Payment is not the same thing
People point out that paying works without unlocking anything, and conclude that tag reading should too.
Payment is card emulation: the terminal produces the field and the phone answers as a card. Tag reading is the opposite role, with the phone producing the field.
They are different modes reached by different paths with different rules, so one working says nothing about the other.
This is the single most common false inference about iPhone NFC and it is worth stating plainly.
What to expect in public
Assume the person will wake their phone. Most people do it without thinking when they intend to tap something.
Design so that a deliberate tap on an awake phone works, and treat anything beyond that as a bonus rather than a requirement.
Print a short instruction. It converts a puzzled visitor holding a sleeping phone into a successful one, and it costs nothing.
And do not build a flow that depends on a sleeping phone reading a tag, because it works on some devices and not others, which is worse than a limit you designed around.
Frequently asked questions
Why does payment work when the phone is locked?
Payment is card emulation, where the terminal produces the field and the phone answers. Tag reading is the opposite role with its own rules.
Does the phone need to be unlocked?
It needs to be awake enough to present a notification. Unlocking is not the deciding factor, being asleep is.
Will a tag wake my phone?
No. A sleeping phone is not producing a field, so there is nothing for the tag to respond to.
Can I make it work from a pocket?
Not reliably, on either platform. Design for a deliberate tap on an awake phone instead.