Install: sideload Agent Outlook

Agent Outlook is not in the Microsoft marketplace. You install it by uploading its XML manifest to your own mailbox — a sideload. Takes about two minutes, and needs no admin rights on a normal mailbox.

1. Open the sideload dialog

Go to https://aka.ms/olksideload.

This link is the whole trick, and it is worth reading twice. It loads Outlook on the web and then pops the Add-Ins for Outlook dialog — the one surface that still accepts an XML manifest. That dialog is no longer reachable from Outlook's own menus. In particular, Apps → Manage your apps is the Marketplace surface and cannot sideload an XML manifest at all. If you go looking there you will not find it, and nothing is wrong with your mailbox.

2. Upload the manifest

In the dialog:

1. My add-ins
2. Scroll to the bottom, to Custom Addins
3. Add a custom add-in → Add from File
4. Choose manifest.xml

Add from URL was removed from this dialog — upload the file.

If you are running your own instance, generate the manifest first with node scripts/build-manifest.js. It reads PUBLIC_BASE_URL and renders public/manifest.xml from the template, so the manifest always points at the instance you are actually running. The file is deliberately not checked in: it is per-environment.

3. Find the button

The add-in does not appear on the ribbon after install. This is the single most common "it didn't work" — it did work, the button is somewhere else.

Open a message in read mode (the manifest activates on ItemIs Message / Read, so drafts and calendar items show nothing at all), then look in either of these:

• More apps on the ribbon or nav bar
• The Apps button on the message's own action bar

Right-click Agent Outlook there and choose Pin to promote it onto the ribbon and the message action bar permanently.

A fresh sideload usually needs a hard refresh (Ctrl+Shift+R) before the add-in list updates. If the add-in is not in the list at all, refresh before concluding anything.

Removing it

Same dialog → Custom Addins → ... → Remove.

Reinstalling under the same manifest <Id> replaces the old entry rather than adding a second one — but remove the old one first when you are switching an instance between environments, or Outlook can keep serving the previous URL.

Other Outlook clients

New Outlook desktop (Windows) and Outlook for Mac pick up the same sideloaded manifest automatically. They host the pane in a WebView2/WKWebView rather than a cross-origin iframe, so framing and cookie behaviour differ from the web client — which is exactly why the add-in avoids cookies entirely.

Classic Outlook on Windows can take up to 24 hours to show a manually sideloaded add-in, because of caching. That is Outlook, not the add-in.

If "Add a custom add-in" is missing

The mailbox lacks the Exchange My Custom Apps role, or the tenant has custom add-ins disabled.

The fallback that always works, for a tenant admin: Microsoft 365 admin center → Settings → Integrated apps → Upload custom apps, scoped to a single user. Slower to iterate with, but it needs no per-mailbox sideload right. See Deploy to a whole tenant for the full walkthrough.

Next

Quickstart — connect your account and track a first email.