Firebird isql exclusive mode?
OK, so further testing seems to indicate that what happens is this:

If the database specified for isql's connection is just a local file path (possibly indirectly via an alias), isql will first attempt to connect by opening the file directly. This would be the kind of connection used for classic (and supeclassic?) mode. So it's not exclusive per se, but incompatible with superserver. If the file is already locked, as would be the case if the superserver has a connection to it, isql will try to connect via the superserver. In this case, the connection will obviously not block any other connections in any way.

So, to make sure other (superserver) connections are never blocked when using isql, isql's command line should specify a TCP/IP connection string, something like this:

isql -u usr -p pwd localhost/3050:MyDBAlias


If a connection string like that is used, other (superserver) connections work just fine.