On a complete aside ... I have been very pleased with RPC::Any as a basic framework for a recent project.   (It is not an exact fit to this requirement, but a very fine framework for RPC.   Just sayin’ FYI.)

Edit:   I have also learned to add the following features, as part of the descriptive information that goes back and forth (if it’s not already provided by the protocol):

1. A version-number.   If the format changes, change the number.   You never know when an old version of something is still out there wanting to talk to you.
2. A randomly generated string, produced by the client, which the host must return verbatim.   Excellent for matching up requests with responses.   (But don’t be tempted to use it as a reliable-identifier on the host-side.)