I'm either using it incorrectly, or string interpolation appears to be broken. See below.

<tmp]$ echo {"foo":42} | jq '@(the input was %(.))'
error: Invalid character
@(the input was %(.))
^
error: syntax error, unexpected IDENT
@(the input was %(.))
^^^^^
error: Invalid character
@(the input was %(.))
^
3 compile errors