Senior Windows Extension Developer (Contractor)

The Role

We are looking for a Senior Windows systems engineer to build and maintain a production-grade Windows sync client using Microsoft's Cloud Files API (CF API). You will own the low-level plumbing that makes seamless, on-demand file synchronization feel invisible to end users, from placeholder hydration and shell overlay icons to Explorer context menus and high-throughput I/O. This is a deeply technical IC role. You will work closest to the metal of the Windows platform, collaborating with product and backend teams to ship a sync client that rivals OneDrive and Dropbox in quality and reliability.

Key Responsabilities:

• Design and implement a Windows Cloud Files API (CF API) sync provider, including placeholder management, hydration/dehydration callbacks, and range-based partial hydration.
• Build robust Windows Shell integration: overlay icons, right-click context menus, and deep Windows Explorer integration via COM.
• Own Win32 and NTFS-level file system interactions, including extended attributes, reparse points, and oplocks.
• Implement high-performance I/O using IOCP and Windows thread pool APIs to handle concurrent file transfers reliably.
• Write and maintain COM servers and in-process shell extensions that meet Microsoft's strict reliability and compatibility requirements.
• Debug complex race conditions, kernel-mode interactions, and edge cases introduced by antivirus software, Group Policy, and diverse hardware configurations.
• Maintain backward compatibility across Windows 10 and Windows 11, including various enterprise and consumer SKUs.
• Instrument code with ETW tracing, WPR/WPA profiling, and crash dump analysis to diagnose production issues.
• Collaborate with QA to develop stress tests, fuzz tests, and automated regression suites for the sync engine.
• Contribute to architecture and design reviews, set engineering standards for the Windows client codebase

Required Skills:

• 5+ years of Windows systems programming in C++, with deep mastery of Win32 APIs, NTFS internals, and COM.
• Strong, hands-on experience with the Windows Cloud Files API (cfapi.h): placeholder files, CF_CALLBACK_TYPE hydration callbacks, pin/unpin semantics, and range-based partial hydration.
• Demonstrated expertise in Windows Shell integration: overlay icon handlers, context menu shell extensions, and Explorer namespace extensions via COM.
• Solid multithreading experience using IOCP, Windows thread pools, and synchronization primitives (events, mutexes, critical sections, slim reader/writer locks).
• Has shipped at least one production Windows sync client (cloud storage client, backup agent, or equivalent),  this is non-negotiable.

Nice-to-have Skills:

• Experience with Windows kernel-mode development (filter drivers, minifilters) or deep knowledge of the Windows I/O stack.
• Familiarity with Windows Security: ACLs, privilege management, integrity levels, and UAC.
• Prior work on enterprise software distributed via MSI/MSIX with silent install and Group Policy support.
• Experience contributing to or maintaining large-scale C++ codebases with CI/CD pipelines (GitHub Actions, Azure DevOps).
• Familiarity with WinRT APIs and bridging classic Win32 components to modern Windows Runtime environments.
• Background in network programming (HTTP/2, WebSockets, chunked transfer) in the context of cloud sync.

What we offer:

• Fully remote engagement across MX.
• Opportunity to work on high-impact client systems with real operational ownership.

Employment type:

• Contract