InboxSDK provides a set of APIs that allow browser extensions to "interact with and extend the Gmail and Inbox UI." InboxSDK provides the means for developers to build applications like the Streak extension and extend Gmail in other ways. A few examples of applications built using InboxSDK is DocSend, a tool designed to track, optimize, and control sent business documents and Screenleap, a cross-device screen sharing application.
A key and perhaps controversial feature of InboxSDK, is a high level API that makes it possible for developers to build applications using simple high level classes like ComposeView. ComposeView has to do with how applications interact with compose windows in Gmail and Inbox.
The high level API allows developers to build applications that extend Gmail without the need to hack Gmail's DOM. InboxSDK is essentially hacking Gmail's DOM for the developer, keeping up with Gmail's DOM element changes so that the developer doesn't have to. Developers no longer have to keep up with DOM changes and constantly patch applications.
There is some concern by developers that Google may take steps to shut down an application that is hacking Gmail's DOM as well as the constant maintenance involved with DOM hacking. According to InboxSDK, there are systems in place to catch DOM updates and prevent breaking changes from taking place. According to the official website, the current average number of SDK releases per week is 12 and the average number of new SDK features added per week is 3.
Developers interested in building application that extend the capabilities of Gmail and Inbox, visit https://www.inboxsdk.com/ for more information. Please note that at the time of this writing, Google Inbox support is currently in private beta. However, developers can email InboxSDK to request access.