FCM vs APNs vs Web Push: what developers need to know

"Send a push notification" means three very different things depending on the platform. Here's what actually differs between Firebase Cloud Messaging (Android), Apple Push Notification service (iOS), and Web Push.

Authentication

• FCM: a Google service-account JSON; you mint short-lived OAuth tokens for the HTTP v1 API. (The legacy server-key API is deprecated.)
• APNs: a .p8 token-signing key + key ID + team ID, used to sign a JWT per connection over HTTP/2.
• Web Push: VAPID — you sign each request with your application's private key.

Token lifecycle

• FCM tokens rotate; you must handle onTokenRefresh and update your store.
• APNs device tokens change on reinstall/restore.
• Web Push subscriptions expire and must be re-created; the browser tells you via pushsubscriptionchange.

Payload limits

• APNs: 4 KB. FCM: 4 KB data. Web Push: 4 KB after encryption — keep payloads lean and fetch rich data client-side.

Error semantics (the important part)

• Unregistered / BadDeviceToken (APNs) and UNREGISTERED (FCM) mean the token is dead — remove it.
• Web Push 410 Gone is the equivalent.
• MismatchSenderId (FCM) means the token was issued for a different project — almost always a config mismatch.

A unified API's job is to normalize all of this: one send call, one set of canonical error codes, automatic dead-token cleanup, per-platform retries. That's exactly what OpenPushAPI's delivery engine does behind POST /v1/notifications.