> When is a program and its plug-ins considered a single combined program?

> It depends on how the main program invokes its plug-ins. If the main program uses fork and exec to invoke plug-ins, and they establish intimate communication by sharing complex data structures, or shipping complex data structures back and forth, that can make them one single combined program. A main program that uses simple fork and exec to invoke plug-ins and does not establish intimate communication between them results in the plug-ins being a separate program.

JSON data sent over the wire, particularly data with nested arrays and maps, and especially a bidirectional communication protocol, can reasonably count as "intimate communication" with "complex data structures" shipped "back and forth".

This is nonbinding guidance, but it is from the FSF, and it is legally untested afaik (not a lawyer). There's sufficient legal risk here that I'd be wary of using rich communication protocols with GPL programs, particularly if there isn't an explicit exception for that protocol.