According to the Susy 2.1.2 documentation, installing Susy is as simple as running npm install susy --save-dev then altering your Gulpfile in accordance with the afore-mentioned doc. You might suspect this would install 2.1.2.
However, this operation installs Susy 3. Susy 3 introduced breaking changes compared to Susy 2, which is preventing my client's SASS code from compiling. There is no documentation for how to install Susy 2 in the Susy 3 docs.
How can I install Susy 2 using npm for use with Gulp?
to force susy to stay in 2.x you can run
npm install --save susy@2.x
this will install the latest version of 2.x but not 3.x+