ResourceLoader.h has two different callback methods. On Mac - virtual NSCachedURLResponse* willCacheResponse(ResourceHandle*, NSCachedURLResponse*); On Windows - virtual bool shouldCacheResponse(ResourceHandle*, CFCachedURLResponseRef); The method on Windows should match the one on Mac. It should be virtual CFCachedURLResponseRef willCacheResponse(ResourceHandle*, CFCachedURLResponseRef); I am adding the above method for Mac when using CFNetwork (see bug 51836). Windows code should use that.