If you use Claude for more than one thing at a time -- a work account and a personal one, or a client's workspace next to your own -- you've probably wished you could have more than one Claude instance open at once. By default the desktop app runs a single instance, but one simple flag lets you open multiple Claude instances simultaneously, each fully isolated from the others.

The trick is the --user-data-dir flag. It tells Claude to store its data in a specific folder, so pointing each instance at its own folder gives you separate, sandboxed copies running side by side.

What the --user-data-dir flag does

Every instance you launch with --user-data-dir gets its own data folder. That folder holds everything local to that instance: the account you're signed into, your chat history, and your connected tools. Point two launchers at two different folders and you get two independent Claude apps that never touch each other's data.

Open a second instance on macOS

Follow these three steps:

1. Completely quit the Claude app.
2. Open your Terminal app.
3. Paste and run the command below. Use one folder name per account (change the name at the end to spin up another).

Same command, different folder = a separate, isolated instance.

Open a second instance on Windows

On Windows you launch claude.exe with the same flag:

1. Press Win + R to open the Run dialog.
2. Paste the command below and hit OK.
3. Log in with your second account.

Want a permanent launcher? Right-click the desktop, choose New › Shortcut, and paste the same command. Now that instance is one double-click away.

Each instance is its own profile

Here's the part worth understanding before you set this up: each instance points at its own data folder, so it keeps its own everything. That means separate chat histories -- across both Cowork and Claude Code -- along with a separate signed-in account and its own connected tools (MCP servers). Nothing carries over between instances.

They behave very much like separate profiles in a web browser: switch profiles and you get a clean, self-contained environment every time. The folders are created for you automatically the first time you launch each instance, so you don't need to make them by hand.

Two folders, two profiles. Neither can see the other's data.

One more thing: your original Claude setup is untouched. When you open the app normally (no flag), it keeps using the default folder it always has. The flag only adds new, separate instances alongside it.