Is there a way to get all messages belonging to a thread?
We know from https://developers.google.com/hangouts/chat/reference/rest/v1/spaces.messages#Message that a Message belongs to a Thread, is there an API to fetch all messages for a given thread?
I believe that is not possible to get all the messages in a thread, by security design. A bot only receives an event when it is directly mentioned in a message and is only granted access to the text of that message.
The event object received by the bot includes a message object with details of the message, including a reference to the thread.
I wrote a simple Apps Script bot whose onMessage function returns the event object as JSON. The message part of the event provides very limited information about the thread:
"thread": {
"retentionSettings": {
"state": "PERMANENT"
},
"name": "spaces/sKkv0fAAAAE/threads/F6qY-rYiwr0"
},
This behaviour is the same for rooms and DMs.
Human users can scan rooms for threads and threads for messages but bots cannot.