I already included
gamehelpers; the compiler mentions that there's only a forward declaration of
CBaseHandle from the
IGameHelpers header; can't use the class directly as a (non-pointer) function argument / return type.
Pulled in a stub from
the SDK's basehandle include, and it looks like it's working as predicted.
Thanks for the help, duck!
I've set up a repository with just the relevant code here for future reference. This commit for the solution.
Edit: Looks like making MMS a requirement for the extension works too (had to pull in
FindEntityByClassname and found a redeclaration error).