> To control a trace session you need its numerical ID.

What do you mean - "control a trace session" ?

> But how do I get it? isc_action_svc_trace_start mentions nothing about a session ID.

isc_action_svc_trace_list is your friend

> Should I parse the text stream? - it seems rather crude.

isc_action_svc_trace_list have very structured format and easy to parse.
Binary format for trace services planned for the next Firebird releases.

Regards,
Vlad