Okay, good ideas there: so the program has some standard dispatch function that can be called to talk to the debugger. Normally that does nothing. The debugger can recognize this function and stick a breakpoint in it to alter the behavior.

The function can do something like put a request into a mailbox buffer, and then check for a reply in the inbox buffer. When there is no debugger, there is never any reply.

When the debugger stops the show with the breakpoint, it processes the request and prepares a reply, flipping some "reply present" flag; when the program is resumed then, it gets the reply.

It would be good not have any of that damned Python monkey business involved in this.