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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Microsoft 365 add-ins are web-based extensions that an Office app loads from a manifest and can connect to supported Office features through JavaScript APIs. Outlook, Word, and Excel can host Office Add-ins, but the same add-in does not automatically run in all three—or in Teams. Compatibility depends on its manifest, APIs, target host, client, and deployment method. Teams can be included alongside an Office Add-in in a Microsoft 365 app package, but that is not the same as running an Office Add-in inside Teams.
How an Office Add-in works
An Office application reads an add-in manifest that describes how the extension integrates with the host, including supported entry points and commands. The host then loads the add-in’s web experience, commonly built with HTML, CSS, and JavaScript. Through Office.js, the add-in can interact with document or mailbox features that the host and client support. Its web application and data can be hosted outside Office and accessed over HTTPS. Microsoft’s Office Add-ins overview describes this model.
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This shared web foundation makes it possible to build for more than one Office application or client. It does not guarantee identical behavior everywhere: the host must support the APIs and extension points the add-in uses.
What the manifest determines
Microsoft documents two manifest formats with different packaging implications:
| 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 | A Microsoft 365 app package that can combine an Office Add-in with other app types, such as a Teams app. |
The manifest documentation and unified manifest overview explain the formats and their support. A unified package does not make every included experience available in every host; client and feature support still matters.
How support differs by app
Word and Excel
Word and Excel can host Office Add-ins. Whether a particular add-in installs and works depends on its declared host requirements, the Office.js APIs and extension points it uses, and the client platform and version. A task pane or other interface should not be assumed to be available in every Word or Excel client. Check Microsoft’s Office Add-in availability and manifest guidance for the specific host and features.
Outlook
Outlook web add-ins use the web-based manifest and interface model, with support across Outlook clients subject to feature and client limitations. “Outlook add-in” can also refer to older COM or VSTO extensions, which are a different technology. Microsoft says COM and VSTO add-ins are not supported in new Outlook for Windows; do not assume that a legacy desktop extension will carry over as a web add-in.
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Teams
Teams is part of Microsoft’s wider app-extension ecosystem, but it is not simply another Office.js host. A Microsoft 365 app package may include both an Office Add-in and a Teams app. The Teams experience has its own host and support requirements; packaging it with an Office Add-in does not cause the Office task pane or Outlook-specific experience to appear in Teams. Consult the unified manifest guidance for package support and the documentation for the specific Teams capability you intend to use.
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How to tell whether the same add-in will work in all four apps
Usually, do not assume one add-in works unchanged in Outlook, Word, Excel, and Teams. Check the exact add-in and the clients your organization uses against these factors:
- Host: Is it built for Outlook, Word, Excel, or a separately packaged Teams app?
- Manifest: Does it use the XML add-in-only manifest or the JSON unified manifest, and does the target client support the needed features?
- Client and platform: Is the target Outlook, Word, or Excel client on the web, Windows, Mac, iPad, or mobile? Support can vary among clients, particularly among Outlook editions.
- APIs and extension points: Does each client support the Office.js requirement set and interface entry point the add-in relies on?
- Distribution and tenant: Is the add-in sideloaded, obtained through the Marketplace, or centrally deployed? Do the tenant, identity, licensing, and mailbox conditions for that route apply?
Microsoft’s manifest documentation and availability guidance are the relevant starting points for checking host and feature support. Verify the current compatibility information for each client rather than relying on the phrase “supports Microsoft 365.”
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How add-ins are deployed
Microsoft documents sideloading for development and testing, Marketplace publication, and organization-level distribution. Centralized deployment lets an administrator assign supported Office Add-ins to users or groups, but it is not available under every tenant and host configuration. Microsoft’s centralized deployment guidance lists prerequisites including eligible Microsoft 365 licensing, Exchange Online mailboxes, and a directory in or federated to Microsoft Entra ID.
Microsoft says centrally deployed add-ins can take up to 24 hours to appear across clients. Its deployment documentation describes the integrated apps portal as the recommended, most feature-rich administration route for most customers, while also documenting availability and government-cloud exceptions. Administrators should confirm the current tenant requirements and client compatibility before choosing a rollout path.
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