What's the difference between an Office add-in and an old VSTO plugin?

VSTO/COM plugins are Windows-only and install per machine. Modern Office add-ins are web apps built on Office.js that run in Outlook, Word, Excel, and PowerPoint across Windows, Mac, and the web — the supported, future-proof model.

Do add-ins work on desktop, web, and Mac?

Yes. A single Office.js add-in runs across Office on Windows, Mac, iPad, and the web, with capability checks where a host differs.

Can you integrate Microsoft Graph for mail, calendar, and files?

Yes — Microsoft Graph integration is central to my work, covering mail, calendar, files, users, and Teams with delegated or application permissions.

Do you set up SSO with Entra ID?

Yes. I implement single sign-on with MSAL and Entra ID (Azure AD) using least-privilege scopes suitable for enterprise environments.

Can you publish to AppSource or deploy internally?

Both. I prepare the manifest, pass Microsoft's validation, and either publish to AppSource or deploy privately via centralized deployment in your admin center.

Microsoft 365

Microsoft 365 Developer

Custom software inside Microsoft 365 — Office add-ins across Outlook, Word, Excel, and PowerPoint, Microsoft Graph integrations, and SSO with Entra ID — built with Office.js and delivered to AppSource or your tenant.

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What I build

What's included

Office add-in development services

Task panes and ribbon commands across Outlook, Word, Excel, and PowerPoint using Office.js — one codebase, all hosts.

Microsoft Graph integration

Reading and automating mail, calendar, files, users, and Teams through the Microsoft Graph API with delegated or application permissions.

SSO & Entra ID

Single sign-on with MSAL and Entra ID (Azure AD), least-privilege scopes, and admin-consent flows ready for enterprise rollout.

AppSource & deployment

Manifest authoring, Microsoft's validation, and publishing to AppSource or centralized deployment through the 365 admin center.

Problems this solves

Sound familiar?

Business logic that lives outside Office, so staff constantly switch apps and re-key data.
An add-in that has to work identically on Outlook/Excel desktop, web, and Mac.
Enterprise SSO and admin-consent requirements that block a naive add-in from shipping.
AppSource submissions bouncing on validation because the manifest or permissions are wrong.

Example use cases

What this looks like in practice

CRM context in Outlook

An Outlook add-in that shows the sender's deal and ticket history on the open email, pulled live via Microsoft Graph.

Live figures in Excel

An Excel add-in with custom functions streaming live data from an internal API straight into cells.

Org-wide rollout

Deployed an add-in to an entire tenant through the admin center with Entra ID SSO and no per-user install.

Tech stack

The tools I use for this

Add-ins

Office.js
React
TypeScript

Platform

Microsoft Graph
MSAL
Entra ID SSO

Hosting

Azure Static Web Apps
Azure Functions

Delivery

Unified manifest
AppSource
Centralized deployment

Explore microsoft 365

What microsoft 365 developer covers

Microsoft 365 is broad. Here are the specific add-in services under it — go deeper on the host you need:

office plugin development

Custom Office add-ins for Word, Excel, Outlook, and PowerPoint built with Office.js — the modern, cross-platform replacement for old COM/VSTO plugins, running on Windows, Mac, iPad, and the web.

outlook plugin development

Custom Outlook add-ins that live in the reading pane and compose window — surfacing data, automating replies, and acting on messages — built with Office.js and Microsoft Graph across desktop, web, and Mac.

excel add in developer

Custom Excel add-ins that add your own worksheet functions, pull live data into cells, and automate reporting — built with Office.js custom functions, ranges, and worksheet events across desktop, web, and Mac.

document automation software

Custom document automation that generates, reviews, and routes documents from your templates and data — Word add-ins with Office.js content controls, server-side DOCX generation, and approval and e-signature flows.

Process

How it works

Scope the workflow

Clarify Office apps, Graph data, permissions, and deployment target.

Build the add-in

Office.js + React task panes and a secure Graph backend.

Secure & validate

SSO, scopes, and manifest hardening against Microsoft's rules.

Deploy & support

AppSource or tenant deployment, with docs and handoff.

Related services

FAQ

Questions, answered

What's the difference between an Office add-in and an old VSTO plugin?: VSTO/COM plugins are Windows-only and install per machine. Modern Office add-ins are web apps built on Office.js that run in Outlook, Word, Excel, and PowerPoint across Windows, Mac, and the web — the supported, future-proof model.
Do add-ins work on desktop, web, and Mac?: Yes. A single Office.js add-in runs across Office on Windows, Mac, iPad, and the web, with capability checks where a host differs.
Can you integrate Microsoft Graph for mail, calendar, and files?: Yes — Microsoft Graph integration is central to my work, covering mail, calendar, files, users, and Teams with delegated or application permissions.
Do you set up SSO with Entra ID?: Yes. I implement single sign-on with MSAL and Entra ID (Azure AD) using least-privilege scopes suitable for enterprise environments.
Can you publish to AppSource or deploy internally?: Both. I prepare the manifest, pass Microsoft's validation, and either publish to AppSource or deploy privately via centralized deployment in your admin center.

Let's build it together

Email me with what you need — I'll reply with a clear plan and next steps. Prefer chat? WhatsApp works too.