adapter.rb

# This line makes me sad
Faraday::Adapter.register_middleware File.expand_path(File.dirname(__FILE__)), 
  :certificate_pinning => [:CertificatePinning, 'certificate_pinning/certificate_pinning'] 

Faraday.new(:url => 'https://fancy-startup.io/api') do |f|
      
  f.use :http_cache, store: @cache
  f.request  :url_encoded
  f.response :logger
      
  f.adapter  :certificate_pinning # I want to use my custom Adapter subclass here
end

How about this?

f.adapter My::Namespace::CertificatePinning

Tried it first, but I don't have namespaces.

require_relative 'certificate_pinning/certificate_pinning'

Faraday.new(:url => 'https://shotsky.io') do |f|
  f.use :http_cache, store: @cache
  f.request  :url_encoded
  f.response :logger
  f.adapter  CertificatePinning
end

"CertificatePinning is not registered on Faraday::Adapter"