Google Assistant has started reading messages from non-Google apps like WhatsApp, Slack and Telegram.
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Google Assistant could always read SMS messages,sent through the Messages app or Hangouts. But many people use other services for day-to-day communication, and those apps weren't available in Assistant (some could send but not read). This has now changed.
To test a new feature, you need to sayto the assistant “read my messages.” You'll have to grant access to notifications, after which the Assistant will show your latest text messages, the app it came from, and the name of the sender. You can then type or dictate your answer. Once complete, the response will be sent immediately and the notification will disappear.
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This can be done with the last few.chats, with the Assistant showing a carousel of received messages and viewing them one by one. Images, video and audio are not available, the assistant will simply say "message contains an audio attachment" and will not play it.
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Most likely, the function is not yet available to everyone, but in the near future should appear for most users.