What Happens When You Open Mica
The first time you open Mica, two things happen at once: a small program starts on your machine, and a browser tab opens in front of you. It is a fair question to wonder whether the second means the first is a web app in disguise. It is not. The program is the part that does the work; the tab is the part you look at. Everything that matters — reading your workbook, planning the steps, executing the task, writing the changes back into Excel — happens in that program, on your machine. The tab only carries the conversation.
If you have used a modern cloud app, a browser tab is familiar — which is exactly why it can feel misleading. A cloud app is the browser: your file goes to the vendor's server, the work runs there, and the tab is the whole product. Mica is the reverse shape. The file never makes that trip, the work does not run on a server, and the tab is a window onto a program that lives on your computer. If you are new to Mica, what Mica is is the two-minute overview of the product behind this design.
The Window and the Engine
Think of it as a car: the dashboard is what you look at, the engine is what moves you. The dashboard does not carry your cargo and does not replace the engine — it shows you what is happening and lets you steer. Mica is the same shape. The browser tab is the dashboard; the local program is the engine, and it does the spreadsheet work in your Excel file on your machine. The cargo — your workbook — never leaves the car.
Because the engine is local, Mica behaves like a desktop application in every way that matters: tasks execute in your workbook on your machine, the file never uploads, and the loop runs without your data crossing a server. The window is a matter of interface, not architecture. how Mica works walks through the task flow — plan, approve, execute — that the engine carries out.
Why the Window Is a Browser
An interface that stays current on its own. Because the window is served as a web page, the interface you see is always the latest one we have shipped — nothing to download, no version number to chase. The moment an interface improvement is ready, it is what you see the next time you open the tab. The engine on your machine stays where it is; the window stays current by itself.
One window, every machine. The interface looks the same wherever you open it — on your laptop, on a shared office PC, on a colleague's screen, the layout and the steps are identical, because the window is the same everywhere and only the machine doing the work changes. It is also the reason the same window will be ready wherever the engine ships next — the interface is already platform-neutral by design.
A light window and a heavy engine, where the weight belongs. The window carries no spreadsheet logic and no copy of your data; every byte of your workbook stays in the engine on your machine. That is why the interface can stay slim while the automation is powerful — each part does what it is best at.
And the reason we chose this shape is simple: we would rather spend our effort where it changes your day. We are a small, focused team, and we treat that as an advantage we choose deliberately — effort goes into the experience of doing the task (the plan, the steps, the permission review) instead of into building and maintaining a separate native interface for every platform. A window that updates itself means the improvements we ship reach you the next time you open it, and the engineering time that would have gone into interface upkeep goes into making the Excel work faster and clearer instead. That is a trade we would make again. How to use Mica shows the whole thing from install to your first task.
What Stays on Your Machine
The question underneath all of this is the one worth answering plainly: where does your data go? Your workbook stays on your machine. The engine reads it, runs the task, and writes the result back in the same place the file has always lived. The window never receives a copy of the workbook — it only displays the conversation and the steps.
The only thing that travels is the conversation itself, and that is true of every AI tool: describing a task to a language model means sending that description over the connection. What does not travel is the file — no workbook upload, no stored copy, no background sync. Mica security & privacy documents this exactly, including how you can verify it yourself.
So when you see the browser tab open, read it the right way: the window is the part that could not possibly hold your data, and the engine that does hold it is the part running on your desk.
How This Compares
The same question — where the work runs, and where the window lives — is what separates the whole category. The cloud add-ins run both in their clouds: your file is uploaded to their servers, the work happens there, and the interface lives inside their platform — an add-in in a ribbon, a page in their app. A local MCP setup keeps your data on your machine but hands you the assembly: you configure a server, manage its connection, and keep the pieces working together yourself.
Mica sits between the two in the way that matters: your data stays local like the MCP setup, and the assembly disappears like the add-in. The engine ships as one install, the window stays current by itself, and there is nothing to configure between them. The brand-by-brand comparison shows the whole field; the short version is that most tools make you choose between local-and-fiddly and easy-but-cloud. Mica is the version where local does not mean fiddly.
Frequently Asked Questions
Why does Mica open in my browser?
The browser tab is the interface — the window you talk to Mica through. The work itself runs in the local program on your machine, which reads and writes your Excel file where it lives. The tab is how you see it, not where it runs.
If the interface is a web page, is my Excel data uploaded?
No. Your workbook stays on your machine — the local engine reads, processes, and writes the file on your device, and the browser window never receives a copy of it. Only the task description travels, as with any AI conversation.
Does that make Mica a web app?
No — it makes the window a web page, which is a different thing. A web app stores and processes your data on a server; Mica stores and processes nothing about your workbook anywhere but your machine. The tab is just the window.
Why not just a normal desktop window?
A native window would work too, but it would bring back version chasing, per-platform interface work, and updates you have to install. The window we chose updates itself, looks the same on any machine, and lets us put engineering time into the Excel work instead of the interface upkeep.
Do I need to keep the browser open?
The window is where the conversation and the approval happen, so keep it open while you work with Mica. The Excel work is done by the local engine on your machine — the tab is the cockpit, not the engine.