My phone reads tags but cannot write them
Reading is often handled by the system while writing is always done by an application. So the limit is usually the software rather than the phone, and the second candidate is the tag itself refusing rather than the phone being unable.
Who does the writing
Reading a tag can happen without you choosing an application, because the system handles it and acts on the content.
Writing never happens by itself. It is always an explicit action inside an application that asked for the capability.
So a phone that reads without you doing anything and cannot write is behaving normally. The capability is there and nothing is using it.
The application may not write
Many applications only read. A tool built to show what a tag contains has no reason to include writing, and its absence is not a fault.
Some applications write only certain record types, so a tool that writes a web address may not write a contact or Wi-Fi credentials.
Check that you are using a tool that writes what you want to write, and that you are in its writing mode rather than its reading screen.
On an iPhone, writing requires an application in the foreground. There is no background writing, so a phone that reads a tag passively will not write one.
Or the tag is refusing
A locked tag reads perfectly and refuses every write, permanently.
A password protected tag does the same until the key is supplied.
A message larger than the free space is rejected rather than truncated, which looks like a phone that cannot write.
And a chip that is not an NFC Forum tag type will not be written by a phone, though it may not read either.
Read the tag first. It reports its writability directly, which separates a tag refusing from a phone not trying.
Writing needs a steadier hand
A write is a longer exchange than a read, so a gesture that reads reliably can be too brief to write.
Rest the phone on the tag rather than holding it, and keep it there until the application confirms.
Remove the case, which matters more for writing than for reading because the margin is thinner.
And read the tag back afterwards. A write that reports success and did not persist is a distinct failure, and only reading back catches it.
Frequently asked questions
Do all phones with NFC support writing?
Devices that read tags generally support writing them through an application. The limit is usually which application you are using.
Can an iPhone write tags?
Yes, through an application in the foreground. There is no background writing, so the app has to be open and asking.
Why does it read the tag and refuse to write it?
Usually the tag is locked, protected, or too small for the content. Read it first, since it reports its writability directly.
Does writing need a special app?
It needs an application that offers writing, since not every reader does. Reading and writing are separate capabilities in software.