Microsoft 365 add-ins are web-based extensions that an Office app loads using a manifest. Outlook, Word, and Excel can host Office Add-ins, but an add-in will not necessarily work the same way in all three: support depends on its manifest, the client and platform, and the Office.js APIs and features it uses. Teams is related through Microsoft 365 app packaging, but an Office Add-in does not automatically run as a Teams app.
How an Office Add-in works
An Office host reads an add-in’s manifest to learn how it integrates with the app and what commands or other entry points it provides. The host then loads the add-in’s web interface and logic, commonly built with HTML, CSS, and JavaScript. The add-in can use Office.js APIs to interact with supported Office content or features. Its web application and data may be hosted outside the Office client and accessed over HTTPS. Microsoft’s Office Add-ins overview explains this model.
This shared web foundation makes it possible to build for more than one Office client. It does not guarantee identical features everywhere: each host and client must support the integration point and APIs the add-in uses.
What the manifest determines
The manifest describes an add-in and its integration with Microsoft 365 apps. Microsoft documents two relevant formats:
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| Manifest | Format | What it is for |
|---|---|---|
| Add-in-only manifest | XML | Production Office Add-ins for hosts including Word, Excel, and Outlook. |
| Unified manifest for Microsoft 365 | JSON | Extends the Teams app manifest model and can package an Office Add-in together with other Microsoft 365 app types. |
The choice affects client and feature support, so check the relevant documentation for the manifest and target clients rather than assuming the formats are interchangeable. See Microsoft’s add-in manifest documentation and unified manifest overview.
How support differs by app
Word and Excel
Word and Excel can host Office Add-ins. Whether a particular add-in can install and activate depends on its declared host requirements, the Office.js requirement sets it uses, and the client version and platform. A task pane or other extension point should not be assumed to be available in every Word or Excel client. Check Microsoft’s current host and API requirement documentation for the specific features and clients involved.
Outlook
Outlook web add-ins use a manifest and web-based interface and logic. Microsoft documents support across Outlook on the web, Windows, Mac, and mobile, subject to client and feature compatibility. This is a different technology from older COM and VSTO add-ins: Microsoft says COM and VSTO add-ins are not supported in new Outlook for Windows. For the distinctions and client details, see Microsoft’s Outlook add-ins documentation.
Teams
Teams is part of the broader Microsoft 365 app-extension ecosystem, but it is not simply another Office host for every Office.js add-in. A unified app package can bring an Office Add-in and a Teams app together; that packaging relationship does not mean an Office task pane or Outlook-specific experience automatically appears or behaves as a Teams app. Check the Teams app’s own host, manifest, and feature support. Microsoft describes the packaging model in its unified manifest overview.
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Will the same add-in work in Outlook, Word, Excel, and Teams?
Not necessarily. An Office Add-in may target multiple Office hosts if it is built and declared for them, but support still depends on each host, client, platform, and feature. Teams requires a Teams app experience; inclusion in a unified package is not proof that the Office Add-in itself runs inside Teams.
When assessing an add-in, compare these factors:
- Host: Outlook, Word, Excel, or a separately implemented Teams app.
- Manifest: add-in-only XML or unified Microsoft 365 JSON, including the support limits for the chosen format.
- Client and platform: web, Windows, Mac, iPad, or mobile; distinguish Outlook clients in particular.
- APIs and extension points: whether the client supports the Office.js APIs and entry points the add-in needs.
- Distribution: sideloading, Marketplace publication, or organizational deployment.
- Tenant conditions: licensing, Exchange Online mailboxes, identity configuration, and admin-portal availability where centralized deployment is used.
Compatibility can change as clients and features evolve. Confirm the current support information for the exact add-in, feature, and client combination in Microsoft’s host and API requirements, manifest documentation, and Outlook documentation.
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How organizations distribute Office Add-ins
Developers can sideload add-ins for testing, publish them through the Marketplace, or distribute eligible add-ins within an organization. Centralized deployment lets an administrator assign supported Office Add-ins to users or groups, but it is not available under every tenant or client condition. Microsoft lists prerequisites that include eligible Microsoft 365 licensing, Exchange Online mailboxes, and a directory in or federated to Microsoft Entra ID.
Microsoft recommends the integrated apps portal as the most feature-rich central administration route for most customers; availability and government-cloud exceptions apply. Deployment may take up to 24 hours to appear across clients, according to Microsoft. Administrators should check the current centralized deployment requirements and publishing and distribution guidance for their tenant and target clients.
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