FWIW I didn't sent those stubs right away because I was thinking it needs at least some minor beginnings of systematic handling of those function providers and functions. An application can register function providers itself while builtin function provider can be queried GUID_SYSTEM_FUNCTIONPROVIDER. I am not sure where system provider functions reside, in msctf itself or there is another one (perhaps can be checked by looking what dlls are loaded in test?). I am not sure how much of details are needed for the empty stubs but I'd guess we'd at least want to check for provider GUID and only return ours for GUID_SYSTEM_FUNCTIONPROVIDER. And then introduce a table with guid / constructor for builin functions so that can be extended in a straightforward way. -- https://gitlab.winehq.org/wine/wine/-/merge_requests/10996#note_141338