Application Compatibility Toolkit is a free diagnostic tool from Microsoft that finds and fixes problems when older software won't run on newer Windows versions

The Application Compatibility Toolkit (ACT) is a set of utilities designed to test whether programs written for older Windows systems will work on newer ones. It runs your software through compatibility checks, identifies what will break, and in some cases applies fixes automatically. If you're responsible for maintaining software across multiple Windows versions or preparing to upgrade a large number of machines, this tool saves you from discovering problems after the upgrade is already done.

Microsoft built ACT specifically for IT departments and software developers who need to know whether their applications will survive a Windows version jump. It's not a magic fix — some problems it finds have no solution — but it tells you exactly what those problems are before they affect users.

Key Takeaways

  • Application Compatibility Toolkit scans your software to find compatibility issues before you upgrade Windows, rather than discovering them afterward.
  • The toolkit includes a compatibility database that identifies known problems with thousands of applications and suggests fixes when they exist.
  • You can run ACT on individual machines or across an entire network to test multiple applications at once.
  • Some fixes are automatic (the toolkit applies them silently), while others require manual changes to how the software runs.
  • ACT is most useful when you're planning a large Windows upgrade or need to verify that custom or older business software will work on a new OS version.

How Application Compatibility Toolkit Works

The toolkit operates in two main ways. First, it scans your installed applications and compares them against Microsoft's compatibility database — a record of known issues with specific software versions on specific Windows versions. If your version of Adobe Reader, for example, has a documented problem on Windows 11, ACT will flag it.

Second, it can monitor your software while it runs and watch for compatibility problems in real time. This is called dynamic analysis. The toolkit records what the software tries to do — what system files it accesses, what registry keys it reads, what permissions it needs — and checks whether those actions will work on the target Windows version.

When ACT finds a problem, it offers three types of solutions. Some issues have automatic fixes that the toolkit can apply without you doing anything. Others have workarounds — ways to run the software in compatibility mode or with specific settings changed. A third category has no fix at all, and ACT simply reports that the software won't work and suggests you contact the vendor for an updated version.

Who Should Use Application Compatibility Toolkit

If you're an individual upgrading a single personal computer, you probably don't need ACT. Windows itself will warn you during setup if your installed software has known problems, and most mainstream applications (Microsoft Office, Chrome, Zoom) work across Windows versions without issues.

ACT becomes valuable in these situations: you manage a company network and need to test hundreds of machines before a Windows upgrade; you maintain custom business software written years ago; you support older specialized software (engineering tools, medical devices, point-of-sale systems) that may not have been updated in years; or you're a software developer preparing your application for a new Windows version.

The toolkit is also useful if you have a mix of commercial and custom applications and need to know which ones will cause problems before you commit to an upgrade. Testing with ACT takes hours. Discovering incompatibilities after upgrade takes weeks and costs money.

What Application Compatibility Toolkit Can and Cannot Fix

ACT can resolve many problems automatically. If software is looking for a file in the wrong place, or needs to run with administrator privileges, or expects an older version of a system library, the toolkit can often apply a shim — a small piece of code that intercepts the software's requests and redirects them to work on the new Windows version.

ACT cannot fix problems that require the software itself to be rewritten. If an application uses a technology that Microsoft removed entirely from Windows 11 (like 16-bit code, or certain graphics libraries), no shim will help. In those cases, you either need a newer version of the software, or you need to keep running it on the old Windows version.

The toolkit also cannot predict every possible problem. It tests against known issues and common patterns, but edge cases and unusual configurations sometimes slip through. ACT is a risk-reduction tool, not a may provide.

How to Get and Install Application Compatibility Toolkit

Microsoft distributes ACT as part of the Windows Assessment and Deployment Kit (ADK). You download the ADK installer from Microsoft's website, run it, and select only the Application Compatibility Toolkit component — you don't need the other tools unless you're doing large-scale Windows deployments.

The installation takes a few minutes and requires administrator access on your machine. Once installed, you launch the toolkit from your Start menu. The interface is straightforward: you point it at the applications you want to test, choose your target Windows version, and let it run. On a single machine with a few applications, a scan takes 10 to 30 minutes. On a network, you can set it to run overnight.

The toolkit is free and runs on Windows 7 and newer. You don't need special licensing or accounts to use it.

Reading and Acting on ACT Results

When the scan finishes, ACT generates a report listing every issue it found, organized by severity. A red flag means the software probably won't work. A yellow flag means it might have problems in specific situations. Green means no known issues were found.

For each issue, the report tells you what the problem is, what Windows versions it affects, and what you can do about it. If there's an automatic fix available, the report will say so. If there's a workaround, it explains the workaround. If there's no solution, it tells you that too.

The next step depends on what you find. If ACT reports that a critical business application won't work on Windows 11, you contact the vendor and ask for an updated version, or you plan to keep that machine on Windows 10. If it reports minor issues with automatic fixes, you apply those fixes before the upgrade. If it reports no issues, you can upgrade with confidence.

Alternatives and When to Use Them

Windows itself includes a basic compatibility checker built into the upgrade process. When you try to upgrade to a new Windows version, the installer scans your system and warns you about known problems. This is simpler than ACT but less detailed — it tells you what will break, but not always how to fix it.

If you're upgrading just one or two machines, the built-in checker is usually enough. If you're planning a large upgrade and need detailed reports, testing across a network, or the ability to apply fixes before the upgrade starts, ACT is worth the time to set up.

Some third-party software vendors offer their own compatibility testing tools. These are usually more detailed for that specific software but don't help with the rest of your applications. ACT is the standard tool for testing across your entire software inventory.

Frequently Asked Questions

Can I use Application Compatibility Toolkit to test software that isn't installed yet?

Yes. ACT can scan executable files and installation packages directly without installing them. This is useful if you're evaluating new software before you buy it, or if you want to test an application on a machine where you don't have permission to install it.

Does Application Compatibility Toolkit work for testing Mac or Linux software?

No. ACT only tests Windows software for compatibility with Windows versions. It cannot test software written for other operating systems.

What if ACT says my software has no compatibility issues but it still fails after I upgrade?

This can happen. ACT tests against known issues and common patterns, but unusual configurations or edge cases sometimes cause problems that the toolkit didn't predict. If this occurs, contact the software vendor to report the issue, and ask whether they have a compatibility fix or updated version available.

Do I need to uninstall Application Compatibility Toolkit after I finish testing?

No, but you don't need to keep it installed either. It doesn't run in the background or use system resources when you're not actively using it. You can leave it installed if you think you'll test again, or uninstall it through Control Panel to free up disk space.

Can Application Compatibility Toolkit test software running on remote computers?

Yes. ACT includes tools for testing across a network. You can deploy it to multiple machines and collect results centrally, which is useful if you're testing compatibility across an entire company before a Windows upgrade.