Push notification payload generator

APNs wants a JSON object whose reserved aps dictionary holds the alert, sound, and badge, with your custom keys sitting beside it at the top level. Putting custom keys inside aps is the classic mistake: Apple reserves that dictionary, and your values are ignored rather than delivered. FCM's HTTP v1 API wraps everything in a message envelope with notification for the visible part and data for your key-value payload.

FCM adds a constraint that regularly surprises people: every value in data must be a string. A count of 3 or a flag of true that works fine in APNs custom data is rejected by FCM until it is sent as "3" or "true" and parsed back in the app.

Both platforms cap the payload at 4096 bytes. APNs rejects an oversized notification at the connection, and the limit applies to the encoded JSON, so it is measured in UTF-8 bytes rather than characters: an emoji in the body is one visible character and four bytes.

The budget goes faster than expected because everything shares it: the alert text, the custom keys and their names, and the JSON punctuation itself. Deep-link URLs and serialized objects in custom data are the usual overweight passengers. If a payload is close to the limit, move data out of the notification and have the app fetch it: a push is a doorbell, not a truck.