Can you check file size locally without hydrating?

Yes. The first thing to change is GENERIC_READ: it requests access to file contents as well as metadata. For this query, request only FILE_READ_ATTRIBUTES.

HANDLE hff = CreateFileW( fn.c_str(), FILE_READ_ATTRIBUTES, FILE_SHARE_DELETE | FILE_SHARE_READ | FILE_SHARE_WRITE, nullptr, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, nullptr ); if (hff == INVALID_HANDLE_VALUE) { const DWORD error = GetLastError(); errno = ERRNO_FROM_WIN32(error); return -1; } BY_HANDLE_FILE_INFORMATION info{}; if (!GetFileInformationByHandle(hff, &info)) { const DWORD error = GetLastError(); CloseHandle(hff); errno = ERRNO_FROM_WIN32(error); return -1; } CloseHandle(hff); const ULONGLONG fileSize = (static_cast<ULONGLONG>(info.nFileSizeHigh) << 32) | info.nFileSizeLow;

For a locally present OneDrive Files On-Demand placeholder, this metadata-only access should avoid downloading its contents. The reported size is the logical file size, not the space currently occupied on disk.

If you only need size and timestamps, GetFileAttributesExW is another option that doesn’t require opening a handle.

One caveat: the file index is a local filesystem identifier, not a persistent OneDrive cloud ID. Last-access timestamps also follow Windows filesystem update rules, so they aren’t a reliable record of when someone last viewed the cloud file.

Reference: CreateFileW access rights.