Set up the local helper

Downloading happens on your own machine. Pick the guide that suits you.

For people comfortable with Terminal commands.

Do this once

A one-time installation. You will not need these steps again.

1Get the helper

Double-click the file in your Downloads folder to unzip it. You should end up with a folder called yt-local-helper inside Downloads — the next steps move it to its final home.

2Move the helper out of Downloads
mkdir -p ~/Tools && mv ~/Downloads/yt-local-helper ~/Tools/
This gives it a proper home in a Tools folder inside your user folder. It will no longer be in Downloads, so it stays put even when you clean that folder out.
3Install Homebrew (skip if you already have it)
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
Homebrew is what installs everything else. It asks for your Mac password. When it finishes it may print a “Next steps” section — run the lines it shows, then close Terminal and open it again.
4Install Node.js and the download tools (skip if you already have them)
brew install node yt-dlp ffmpeg
Node.js provides the npm command used in the next step. If you later see “command not found: npm”, this step did not finish — open a new Terminal window and run it again.
5Prepare the helper
cd ~/Tools/yt-local-helper && npm install
This downloads the few small pieces the helper needs. You never have to run it again, unless you download a newer version of the helper.

Do this every time

Whenever you want to download something, open Terminal and run this one command.

cd ~/Tools/yt-local-helper && npm start

Leave this Terminal window open while you use the app. When you are done, press Control-C in that window, or just close it.

Connection

The port number in the address is editable — click it to type a different one.

http://127.0.0.1:

Port 4545 already in use? Change it here:

Troubleshooting

The terminal running the helper must stay open.
To save somewhere other than Downloads, start it with a command that opens a Finder window to pick the folder — no typing of paths needed:
cd ~/Tools/yt-local-helper && OUTPUT_DIR="$(osascript -e 'POSIX path of (choose folder)')" npm start
A small Finder window pops up — choose the folder where downloads should be saved (you can also click New Folder in that window to create one) and click Choose. The helper starts with that folder, and the address input below on this page updates to match. This choice lasts for this launch only; next time you run plain npm start, downloads go to Downloads again.
Need both at once — a different port and a different folder? Use one command that does both (it uses the port set in the Connection box above):
cd ~/Tools/yt-local-helper && PORT=4545 OUTPUT_DIR="$(osascript -e 'POSIX path of (choose folder)')" npm start
Just like before, a Finder window pops up to pick the folder, and the helper starts on port 4545 saving there. Both settings last for this launch only.