[iphone] What is the maximum length of a Push Notification alert text?

What is the maximum length of the alert text of an iOS push notification?

The documentation states that the notification payload has to be under 256 bytes in total, but surely there must be a specific character limit for the alert text.

see my test here

I could send up to 33 Chinese characters and 13 bytes of custom values.

EDIT:

Updating the answer with latest information

The maximum size allowed for a notification payload depends on which provider API you employ.

When using the legacy binary interface, maximum payload size is 2KB (2048 bytes).

When using the HTTP/2 provider API, maximum payload size is 4KB (4096 bytes). For Voice over Internet Protocol (VoIP) notifications, the maximum size is 5KB (5120 bytes)

OLD ANSWER: According to the apple doc the payload for iOS 8 is 2 kilobytes (2048 bytes) and 256 bytes for iOS 7 and prior. (removed the link as it was an old doc and it's broken now)

So if you just send text you have 2028 (iOS 8+) or 236 (iOS 7-) characters available.

The Notification Payload

Each remote notification includes a payload. The payload contains information about how the system should alert the user as well as any custom data you provide. In iOS 8 and later, the maximum size allowed for a notification payload is 2 kilobytes; Apple Push Notification service refuses any notification that exceeds this limit. (Prior to iOS 8 and in OS X, the maximum payload size is 256 bytes.)

But I've tested and you can send 2 kilobytes to iOS 7 devices too, even in production configurations

For regular remote notifications, the maximum size is 4KB (4096 bytes) https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CreatingtheNotificationPayload.html

###iOS the size limit is 256 bytes, but since the introduction of iOS 8 has changed to 2kb!

https://forums.aws.amazon.com/ann.jspa?annID=2626

With iOS 8, Apple introduced new features that enable some rich new use cases for mobile push notifications — interactive push notifications, third party widgets, and larger (2 KB) payloads. Today, we are pleased to announce support for the new mobile push capabilities announced with iOS 8. We are publishing a new iOS 8 Sample App that demonstrates how these new features can be implemented with SNS, and have also implemented support for larger 2KB payloads.

According to updated Apple document (check my answer date):

"... When using the HTTP/2 provider API, maximum payload size is 4096 bytes. Using the legacy binary interface, maximum payload size is 2048 bytes. Apple Push Notification service (APNs) refuses any notification that exceeds the maximum size."

It should be 236 bytes. There is no restriction on the size of the alert text as far as I know, but only the total payload size. So considering if the payload is minimal and only contains the alert information, it should look like:

{"aps":{"alert":""}}

That takes up 20 characters (20 bytes), leaving 236 bytes to put inside the alert string. With ASCII that will be 236 characters, and could be lesser with UTF8 and UTF16.

Apple Updated Doc:

Each remote notification includes a payload. The payload contains information about how the system should alert the user as well as any custom data you provide. The maximum size allowed for a notification payload depends on which provider API you employ. When using the HTTP/2 provider API, maximum payload size is 4096 bytes. Using the legacy binary interface, maximum payload size is 2048 bytes. Apple Push Notification service (APNs) refuses any notification that exceeds the maximum size.

Apple push will reject a string for a variety of reasons. I tested a variety of scenarios for push delivery, and this was my working fix (in python):

#  Apple rejects push payloads > 256 bytes (truncate msg to < 120 bytes to be safe)
if len(push_str) > 120:
    push_str = push_str[0:120-3] + '...'

# Apple push rejects all quotes, remove them
import re
push_str = re.sub("[\"']", '', push_str)

# Apple push needs to newlines escaped
import MySQLdb
push_str = MySQLdb.escape_string(push_str)

# send it
import APNSWrapper
wrapper = APNSWrapper.APNSNotificationWrapper(certificate=...)
message = APNSWrapper.APNSNotification()
message.token(...)
message.badge(1)
message.alert(push_str)
message.sound("default")
wrapper.append(message)
wrapper.notify()

According to the WWDC 713_hd_whats_new_in_ios_notifications. The previous size limit of 256 bytes for a push payload has now been increased to 2 kilobytes for iOS 8.

Source: http://asciiwwdc.com/2014/sessions/713?q=notification#1414.0

The limit of the enhanced format notifications is documented here.

It explicitly states:

The payload must not exceed 256 bytes and must not be null-terminated.

ascandroli claims above that they were able to send messages with 1400 characters. My own testing with the new notification format showed that a message just 1 byte over the 256 byte limit was rejected. Given that the docs are very explicit on this point I suggest it is safer to use 256 regardless of what you may be able to achieve experimentally as there is no guarantee Apple won't change it to 256 in the future.

As for the alert text itself, if you can fit it in the 256 total payload size then it will be displayed by iOS. They truncate the message that shows up on the status bar, but if you open the notification center, the entire message is there. It even renders newline characters \n.

Here're some screenshots (banner, alert, & notification center)