### Mutual Rooms

{{% added-in v="1.19" %}}

{{% http-api spec="client-server" api="mutual_rooms" %}}

#### Server behaviour

The server may decide that the response to this endpoint is too large, and only return a
subset of the results. In this case, the server should populate the optional field `next_batch`
with an [opaque identifier](/appendices/#opaque-identifiers). The client may then supply
the identifier as the `from` query parameter in a subsequent request, along with the original
`user_id`, to fetch the next batch of responses. This will continue until the server no longer
inserts `next_batch`, meaning there are no further results.

Batch tokens generated by this endpoint SHOULD be valid for at least 10 minutes, after which,
tokens can expire. Expired tokens SHOULD be handled similar to invalid tokens, and return
a 400 response. When a client uses batch tokens, servers MAY omit rooms that the user joined
after the first token was generated.
