You should be reusing the content json of the original account data event, not directRoomMap(). directRoomMap is intended to be used as a way to quickly find which user a direct room is associated with. Your code here just convert things back and forth, and is inefficient and error-prone.

did not check whether userId is contained in content, can cause UB.

did not check whether rooms is a json array

Add test cases where:

• the m.direct content contains other users' rooms, then these rooms should be kept.
• the m.direct content contains the user id but another room, then the new room should be added while keeping existing room(s).

refer to the matrix spec for a better grammar for alias.

also, the regex should be static const and the isValidAlias function should be global and static

This line is not covered

Should this really be an Action in the ClientModel?

The problem with it being an action is that it cannot happen at the same time with other actions -- this is useful to avoid race conditions when the reducer needs to access the model and change it based on the current. But your reducer does not actually need to access the model except server url and access token (which we consider to be unlikely to change).

This means that it might have to wait for the potential heavy computing in another reducer before it can give you back the results.

Maybe a function that returns a BaseJob + a function that parses a Response will be a better idea.

BaseJob getRoomIdByAliasJob(std::string alias) const;
static std::string parseGetRoomIdByAliasResponse(...) const;