Inbox by Gmail is a great product. But right now it works only in Chrome. When you open it in Safari: We can change the default User Agent to mock Google.

Usage

Please see the Important Update at first!

Just run such command in Terminal:

defaults write com.apple.Safari CustomUserAgent "\"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.57 Safari/537.36\""

Then you can open Inbox in a new tab in Safari and it should works.

If you just wanna use Inbox in Safari, please stop here and enjoy.

Find Other User Agent Strings

The other User Agent String can be found in:

/Applications/Safari.app/Contents/Resources/UserAgents.plist

How to Revert

To revert the change, you just need run this command:

defaults delete com.apple.Safari CustomUserAgent

Then restart Safari.

Important Update

Set global User Agent to Chrome can cause some critical issue, like abnormal behavior on YouTube.

So we need only set User Agent for inbox tab. You can use this AppleScript in Alfred, Automator/Services, Quicksilver, LaunchBar or whatever you like.