The setting buried in Office options
Most people meet COM add-ins by accident. Outlook feels slow, someone online says to check your COM add-ins, and you end up in a dialog under File, Options, Add-ins that you have never seen before, full of items you did not knowingly install. So what is a COM add-in? In short, it is an older kind of Office add-in built on a Microsoft technology called the Component Object Model, installed directly on a Windows machine, with deep access to that machine. It is the powerful, legacy predecessor of today's web add-ins. This guide explains what COM add-ins are, how they differ from VSTO and modern add-ins, how to find and manage them, and why they matter in 2026 even as they fade.
Key Takeaways
COM add-ins are desktop add-ins
They are built on the Component Object Model and install on a Windows machine, running inside classic desktop Office with deep access.
They are Windows-only and legacy
COM add-ins do not run on Mac, on Office for the web, or in the new Outlook for Windows. They predate the web add-in model.
VSTO is a managed cousin
VSTO add-ins are a .NET-based way to build COM add-ins, so they are essentially a friendlier wrapper over the same underlying model.
You manage them in Office options
Under File, Options, Add-ins, the COM Add-ins dialog lets you see, enable, and disable them, which is a common performance fix.
The new Outlook does not run them
Because the new Outlook for Windows is web-based, COM and VSTO add-ins do not load in it at all.
New work uses web add-ins
For anything built today, the modern web add-in is the supported, cross-platform replacement.
What is a COM add-in?
A COM add-in is an Office add-in built on Microsoft's Component Object Model, a long-standing Windows technology for letting software components talk to each other. It installs on a Windows machine, loads inside the classic desktop Office applications, and runs with deep access to the machine. COM add-ins are the older, Windows-only predecessor of today's web-based Office add-ins.
The Component Object Model, or COM, is a way Windows software has shared functionality for decades. A COM add-in uses that mechanism to hook into Office. When you install one, it registers itself with Windows, and the next time you open the relevant Office app, the add-in loads in-process, meaning it runs as part of the application itself.
That in-process, registered nature is what gives COM add-ins their power and their problems. Because the add-in runs as part of Office on the machine, it can reach the file system, drive other applications, and do heavy work with full access. That made COM add-ins the go-to for serious business tools for years. It also means a poorly written one can slow down or destabilise Office, which is why they have a reputation in IT departments.
Typically COM add-ins are written in lower-level languages like C++ or in .NET languages like C#. You do not need to know the languages to use one, but it helps to know they are real installed programs on your computer, not the lightweight web panels that modern add-ins use.
How is a COM add-in different from a VSTO add-in?
This trips people up, because the two overlap. VSTO, which stands for Visual Studio Tools for Office, is a framework Microsoft created to make building COM add-ins easier using managed .NET code rather than raw COM programming. So a VSTO add-in is, underneath, still a COM add-in. VSTO is the comfortable modern way to build one, with Visual Studio tooling and the .NET framework behind it.
In practice, when people say COM add-in they often mean one written the older, lower-level way, and when they say VSTO add-in they mean one built with the .NET framework. Both install on Windows, both load into classic desktop Office, and both share the same fundamental limitation: they are Windows-only and do not run in the new Outlook or on the web.
For a user, the distinction rarely matters. For anyone deciding what to build or maintain, it matters a lot, because both are on the same deprecation path. We go deep on this in our overview of VSTO add-ins in 2026.
Where do you see COM add-ins, and how do you manage them?
In a desktop Office app, go to File, Options, Add-ins. At the bottom, set Manage to COM Add-ins and select Go to open a dialog listing the installed COM add-ins, where you can enable or disable each one. Disabling a problematic COM add-in is a common fix for a slow or unstable Outlook.
The COM Add-ins dialog is where most people encounter them. Open a desktop Office application, go to File, then Options, then the Add-ins tab. At the bottom there is a Manage dropdown; choose COM Add-ins and click Go. The dialog that opens lists every COM add-in installed for that app, with a checkbox to turn each on or off.
This is genuinely useful for troubleshooting. Outlook in particular can be slowed or destabilised by a misbehaving COM add-in, and disabling them one at a time to find the culprit is a standard fix. Office itself will sometimes disable an add-in that loads too slowly or crashes, to protect your experience, which is why an add-in you rely on can quietly stop appearing.
If you see COM add-ins you do not recognise, they were usually installed by other software: a PDF tool, a meeting app, an antivirus, or a CRM client often add their own. You can disable ones you do not need, though it is worth knowing what an add-in belongs to before removing it.
How are COM add-ins different from modern web add-ins?
This is the contrast that defines where things are heading. A COM add-in is installed desktop software, Windows-only, with deep machine access. A modern web add-in is a small web app that runs in a sandbox inside Office, works across Windows, Mac, and the web, and cannot freely touch your machine. They are almost opposites in design.
The web model trades raw power for reach and safety. A web add-in cannot do everything a COM add-in can on the local machine, but it runs everywhere, it is far easier for IT to deploy and update, and it cannot destabilise Office the way a bad COM add-in can. If you want the fuller picture of the modern model, our guide to what Office add-ins are walks through it.
The single most important practical difference in 2026 is the new Outlook. Because the new Outlook for Windows is built on web technology, it does not load COM or VSTO add-ins at all. Many organisations are moving to it, which is why a COM Outlook add-in that works today may simply be absent for those users tomorrow.
Why are COM add-ins being phased out?
Everything that made COM add-ins powerful ties them to a world Microsoft is moving away from. They run on the Windows desktop, in-process, with deep access, and Office is heading toward a cross-platform, web-based, sandboxed future that works the same on a MacBook, a browser, and a phone. COM add-ins cannot follow it there.
The new Outlook is the clearest signal. It does not run COM or VSTO add-ins, and it became the default experience for enterprise Microsoft 365 in April 2026, with classic Outlook positioned as a temporary fallback rather than a permanent home. Excel, Word, and PowerPoint COM add-ins still run in their classic desktop apps for now, but the strategic direction across the suite is identical.
None of this means your COM add-ins stop working today. It means the platform underneath them is being retired, and anything business-critical should have a plan. For mail add-ins especially, that plan is increasingly urgent.
If you have discovered that a tool your business depends on is a COM or VSTO add-in, and you are now wondering how exposed you are to the new Outlook switch, that assessment is the cheapest first step. We map what the add-in does, what survives a move to the web, and what needs rethinking. See how we handle it on our page.Office Add-in development services
Should you still build a COM add-in in 2026?
For almost all new work, no. The reasons to build a COM add-in have narrowed to a single situation: you need deep local-machine access that only desktop code can provide, and the add-in will only ever run on a managed Windows fleet, never on Mac, the web, or the new Outlook. That is a small and shrinking set of cases.
For everything else, a web add-in is the right choice, because it runs everywhere, survives the new Outlook, is easier to deploy, and is where Microsoft is investing. Building a new COM add-in today usually means signing up for a migration later, which is a strange decision to make on purpose.
If you already have a COM or VSTO add-in, the question is not whether to build a new one but when to migrate the old one. The honest answer for mail add-ins is sooner rather than later, and our playbook on migrating a VSTO Outlook add-in to the web lays out exactly how that is done.
COM add-in vs VSTO add-in vs web add-in
| Factor | COM add-in | VSTO add-in | Web add-in |
|---|---|---|---|
| Technology | Component Object Model | .NET over COM | Web (Office.js) |
| Installed on the machine | Yes | Yes | No (sandboxed) |
| Platforms | Windows only | Windows only | Windows, Mac, web, mobile |
| Runs in new Outlook | No | No | Yes |
| Machine access | Deep | Deep | Limited by design |
| Status | Legacy | Deprecating | Supported direction |
Frequently asked questions
What is a COM add-in in simple terms?
It is an older type of Office add-in built on Microsoft's Component Object Model. It installs on a Windows machine and loads inside the classic desktop Office apps with deep access to the computer. It is the Windows-only predecessor of today's web-based Office add-ins.
What are COM add-ins in Outlook?
They are installed add-ins that hook into the desktop Outlook app, often added by other software like PDF tools, meeting apps, or CRMs. You can view and disable them under File, Options, Add-ins, then Manage COM Add-ins. Disabling a faulty one is a common fix for a slow Outlook.
How do I disable a COM add-in?
In a desktop Office app, go to File, then Options, then Add-ins. Set the Manage dropdown to COM Add-ins and click Go. In the dialog, untick the add-in you want to disable and confirm. This is a standard step when troubleshooting performance problems in Outlook.
Is a VSTO add-in the same as a COM add-in?
Essentially yes. VSTO is a .NET framework for building COM add-ins more easily, so a VSTO add-in is a COM add-in underneath. Both install on Windows, run in classic desktop Office, and share the same limitation of being Windows-only and on a deprecation path.
Do COM add-ins work in the new Outlook?
No. The new Outlook for Windows is web-based and does not load COM or VSTO add-ins at all. Since the new Outlook became the enterprise default in April 2026, a COM Outlook add-in that works today may be absent for users who have switched.
Should I build a new COM add-in?
Almost never. The only remaining case is deep local-machine access on a Windows-only fleet. For everything else, a web add-in is the right choice because it runs across platforms, survives the new Outlook, and is the model Microsoft supports going forward.
Powerful, but on the way out
A COM add-in is the older, Windows-only kind of Office add-in: installed on the machine, deeply capable, and built on a technology that predates the web. VSTO add-ins are the friendlier .NET way to build the same thing. For years they were how serious Office tools got made, but the platform has moved on, and the new Outlook not running them at all is the clearest sign of where things are going. If you have a COM or VSTO add-in your business relies on, the smart move in 2026 is to understand how exposed it is and plan a migration on your own timeline rather than during a forced switch. Tell us what your add-in does and we will give you a straight assessment. Reach out through our contact page whenever you are ready.