I need to create a link to open one particular Hangouts chat (not videocall). It is possible to open a chat in a separate window from the main Google+ page, but the URL of that window (something like https://talkgadget.google.com/u/0/talkgadget/_/frame?v=1395695842&hl=ru#tgtn_c71vh1) can't be opened again.
Is it possible to get a permanent URL?
As of April 2018, It actually is possible to create URL to link to a chat with a specific person. (But who knows when Google will break these given the ongoing incoherence of their messaging applications…)
First, you'll need to know that person's Gaia ID (an internal ID used by Google Hangouts and Google Plus)
https://plus.google.com/me. This will redirect to a URL of the form https://plus.google.com/1XXXXXXXXXX. The long number at the end of the URL is your Gaia ID. (If you don't have a Google Plus profile, it'll be an error page, but it'll still have the correct Gaia ID.)https://plus.google.com/u/0/1XXXXXXXXXXXX/... associated with their profile, you've got their Gaia ID.Anyway, once you've got someone's Gaia ID, it's pretty straightforward to create the correct URL to start a Hangout with them. Simply replace 1XXXXXXXXXXXX below with that user's Gaia ID:
https://hangouts.google.com/chat/person/1XXXXXXXXXXXX
https://hangouts.google.com/group/Lrjad6ru9e806l69AlpHaNuMer1c which you can create from the web interface itself. Likewise for group video calls, with links like /call/number.
– Dan Lenski
May 31 '18 at 20:40
As stated before by @Dan Leski
you'll need to know that person's Gaia ID (an internal ID used by Google).
I have added a detailed answer to an existing stackoverflow question in which I describe how to retrieve the Gaia ID using the People API.
TL;DR: Request metadata in the personFields, by using people.connections.list resources for your contacts, or the people/get resource for yourself.
Once you've got someone's Gaia ID, it's pretty straightforward to create the correct URL to start a Hangout with them. Simply replace 1XXXXXXXXXXXX below with that user's Gaia ID:
No, at this time it's not possible to get a permanent URL to a Google Hangouts "chat" but it's possible to get a permanent URL to a Google Hangout "chat history" and open the chat from the linked web page.
Google Hangouts "chats" could be accessed from several places (Gmail, Google+ Chrome Extension, Chrome App, Hangouts web app, mobile apps) but none of them offer a way to get a permanent URL for a Google Hangouts chat.
But Google Hangouts "chats" history have a permanent URL. From the corresponding page the chat could be opened.
To get the URL
The easiest way I know of right now to find Gaia ID is actually to go to get.google.com/albumarchive and the 21 numbers that reveal itself once you go to Album Archive will be your own and you can use it.
Once you get the id, it easy to create your hangout link.
https://hangouts.google.com/chat/person/[Gaia ID]
where [Gaia Id] = 1xxxxxxxxxxxxxxxxx (21 digits)
Just go to something like https://hangouts.google.com/call/cywnzqtjadjwhl55kfr5euvanyb.
Click on Join Hangout and then send the link to your partner.
Link as https://hangouts.google.com/hangouts/_/cywnzqtjadjwhl55kfr5euvanyb should work as well.
You may play with the address as the security feature here is the randomness of address.
In 2019, you can find someone's Gaia ID by going to a chat, starting up the developer tools, then look at the network log for a querypresence POST request; the URL may look like this:
https://chat-pa.clients6.google.com/chat/v1/presence/querypresence?key=blahblahblah&alt=protojson
The post data contains something like this (mangled):
[[[44,3,"chat_frontend_20190321.06_p1",1553297518],
["lcsw_hangouts_7D87E9D6","17D2A8EEB9EE1DD8"],null,"en"],
[["106512922239035206222"],["109501794533344715333"]],[2,3,10]]
The long numbers are Gaia ids; yours and your chat partner's.
