Official documentation

Configuring open access for a custom keyboard

Apple, Apple Developer Documentation, 2026

Read on October 16, 2026

What it is: Apple's developer documentation on what a third-party iPhone keyboard can and cannot do, and what the Allow Full Access switch unlocks.
How much weight it carries: Official, written for developers. It describes the mechanism, not a user's day-to-day risk.

Facts taken from this source

Apple says custom keyboards run in a sandbox whose default configuration disallows network access and prevents writing to the containing app's shared group containers.
“This sandbox’s default configuration disallows access to the network and prevents writing to the containing app’s shared group containers (reading is permitted).” (Discussion)
Apple says users must explicitly turn on Allow Full Access for a keyboard in Settings before it can have open access.
“Users must explicitly allow your keyboard to have open access by turning the “Allow Full Access” switch on for your keyboard in Settings.” (Discussion)
Apple's developer documentation says a custom iPhone keyboard runs in a sandbox that by default disallows network access, and that users must switch on Allow Full Access to change this.
“Custom keyboards operate in a sandboxed environment running in an isolated process. This sandbox's default configuration disallows access to the network” (Discussion)