Following signs of new Side Button functionality in the iOS 26.2 beta 3 update, Apple developer documentation has confirmed that assigning a third-party voice assistant to the Side Button will be a feature available to iPhone users in Japan.
Developers are able to set up an App Intents framework that will allow voice-based conversational apps to be launched directly from the Side Button in Japan.
Right now, a long press on the Side Button activates Siri, and there is no option to change that. Japanese iPhone users will soon be able to select a new default, opting to activate Alexa, Gemini, or another voice assistant instead. App developers will need to expressly adopt this functionality for Side Button access to be available to consumers.
From Apple's developer documentation:
In Japan, people might place an action on the side button of iPhone that instantly launches your voice-based conversational app. People expect the voice-based conversational functionality to be instantly available when they launch your app with the side button, so make sure to let them immediately use it by starting an audio session - for example, with AVFoundation.
Apple makes it clear that the option to activate a third-party personal assistant using App Intents will only be available on the iPhone in Japan, not in other countries. To use the feature, the country or region of an Apple Account must be set to Japan and a user must be located in Japan.
iOS 26.2 also adds support for third-party App Stores and provides a Safari Search Engine choice screen during setup in Japan. With these features, Apple is complying with Mobile Software Competition Act Guidelines established by the Japan Fair Trade Commission.
When we initially shared details of the Side Button code, it looked like it might be available to iPhone users in the European Union under the Digital Markets Act. It's possible that might still happen eventually, but it's clear that the initial implementation will be limited to Japan.
