diff --git a/Library/Homebrew/api.rb b/Library/Homebrew/api.rb
index 38d2748db5..53b0bf1dd7 100644
--- a/Library/Homebrew/api.rb
+++ b/Library/Homebrew/api.rb
@@ -4,6 +4,7 @@
 require "api/analytics"
 require "api/bottle"
 require "api/cask"
+require "api/cask-source"
 require "api/formula"
 require "api/versions"
 require "extend/cachable"
@@ -21,15 +22,19 @@ module Homebrew
 
     API_DOMAIN = "https://formulae.brew.sh/api"
 
-    sig { params(endpoint: String).returns(T.any(String, Hash)) }
-    def fetch(endpoint)
+    sig { params(endpoint: String, json: T::Boolean).returns(T.any(String, Hash)) }
+    def fetch(endpoint, json: true)
       return cache[endpoint] if cache.present? && cache.key?(endpoint)
 
       api_url = "#{API_DOMAIN}/#{endpoint}"
       output = Utils::Curl.curl_output("--fail", "--max-time", "5", api_url)
       raise ArgumentError, "No file found at #{Tty.underline}#{api_url}#{Tty.reset}" unless output.success?
 
-      cache[endpoint] = JSON.parse(output.stdout)
+      cache[endpoint] = if json
+        JSON.parse(output.stdout)
+      else
+        output.stdout
+      end
     rescue JSON::ParserError
       raise ArgumentError, "Invalid JSON file: #{Tty.underline}#{api_url}#{Tty.reset}"
     end
diff --git a/Library/Homebrew/api/cask-source.rb b/Library/Homebrew/api/cask-source.rb
new file mode 100644
index 0000000000..86e380e252
--- /dev/null
+++ b/Library/Homebrew/api/cask-source.rb
@@ -0,0 +1,28 @@
+# typed: false
+# frozen_string_literal: true
+
+module Homebrew
+  module API
+    # Helper functions for using the cask source API.
+    #
+    # @api private
+    module CaskSource
+      class << self
+        extend T::Sig
+
+        sig { params(token: String).returns(Hash) }
+        def fetch(token)
+          Homebrew::API.fetch "cask-source/#{token}.rb", json: false
+        end
+
+        sig { params(token: String).returns(T::Boolean) }
+        def available?(token)
+          fetch token
+          true
+        rescue ArgumentError
+          false
+        end
+      end
+    end
+  end
+end
diff --git a/Library/Homebrew/api/versions.rb b/Library/Homebrew/api/versions.rb
index 0bfc51c505..3c18824348 100644
--- a/Library/Homebrew/api/versions.rb
+++ b/Library/Homebrew/api/versions.rb
@@ -44,7 +44,12 @@ module Homebrew
         def latest_cask_version(token)
           return unless casks.key? token
 
-          Version.new(casks[token]["version"])
+          version = if casks[token]["versions"].key? MacOS.version.to_sym.to_s
+            casks[token]["versions"][MacOS.version.to_sym.to_s]
+          else
+            casks[token]["version"]
+          end
+          Version.new(version)
         end
       end
     end
diff --git a/Library/Homebrew/cask/cask.rb b/Library/Homebrew/cask/cask.rb
index d7f811d263..2575b04530 100644
--- a/Library/Homebrew/cask/cask.rb
+++ b/Library/Homebrew/cask/cask.rb
@@ -6,6 +6,7 @@ require "cask/config"
 require "cask/dsl"
 require "cask/metadata"
 require "searchable"
+require "api"
 
 module Cask
   # An instance of a cask.
@@ -19,7 +20,7 @@ module Cask
     extend Searchable
     include Metadata
 
-    attr_reader :token, :sourcefile_path, :config, :default_config
+    attr_reader :token, :sourcefile_path, :source, :config, :default_config
 
     def self.each(&block)
       return to_enum unless block
@@ -37,9 +38,10 @@ module Cask
       @tap
     end
 
-    def initialize(token, sourcefile_path: nil, tap: nil, config: nil, &block)
+    def initialize(token, sourcefile_path: nil, source: nil, tap: nil, config: nil, &block)
       @token = token
       @sourcefile_path = sourcefile_path
+      @source = source
       @tap = tap
       @block = block
 
@@ -147,14 +149,21 @@ module Cask
         return []
       end
 
+      latest_version = if ENV["HOMEBREW_JSON_CORE"].present? &&
+                          (latest_cask_version = Homebrew::API::Versions.latest_cask_version(token))
+        DSL::Version.new latest_cask_version.to_s
+      else
+        version
+      end
+
       installed = versions
       current   = installed.last
 
       # not outdated unless there is a different version on tap
-      return [] if current == version
+      return [] if current == latest_version
 
       # collect all installed versions that are different than tap version and return them
-      installed.reject { |v| v == version }
+      installed.reject { |v| v == latest_version }
     end
 
     def outdated_info(greedy, verbose, json, greedy_latest, greedy_auto_updates)
diff --git a/Library/Homebrew/cask/cask_loader.rb b/Library/Homebrew/cask/cask_loader.rb
index fa7eed16ae..0b231fcbc9 100644
--- a/Library/Homebrew/cask/cask_loader.rb
+++ b/Library/Homebrew/cask/cask_loader.rb
@@ -40,7 +40,7 @@ module Cask
       private
 
       def cask(header_token, **options, &block)
-        Cask.new(header_token, **options, config: @config, &block)
+        Cask.new(header_token, source: content, **options, config: @config, &block)
       end
     end
 
diff --git a/Library/Homebrew/cask/installer.rb b/Library/Homebrew/cask/installer.rb
index ec1a004058..9377f89958 100644
--- a/Library/Homebrew/cask/installer.rb
+++ b/Library/Homebrew/cask/installer.rb
@@ -399,10 +399,10 @@ module Cask
     def save_caskfile
       old_savedir = @cask.metadata_timestamped_path
 
-      return unless @cask.sourcefile_path
+      return if @cask.source.blank?
 
       savedir = @cask.metadata_subdir("Casks", timestamp: :now, create: true)
-      FileUtils.copy @cask.sourcefile_path, savedir
+      (savedir/"#{@cask.token}.rb").write @cask.source
       old_savedir&.rmtree
     end
 
diff --git a/Library/Homebrew/cli/named_args.rb b/Library/Homebrew/cli/named_args.rb
index 79c7fd822d..7ef5b5a695 100644
--- a/Library/Homebrew/cli/named_args.rb
+++ b/Library/Homebrew/cli/named_args.rb
@@ -45,18 +45,18 @@ module Homebrew
       # the formula and prints a warning unless `only` is specified.
       sig {
         params(
-          only:                     T.nilable(Symbol),
-          ignore_unavailable:       T.nilable(T::Boolean),
-          method:                   T.nilable(Symbol),
-          uniq:                     T::Boolean,
-          prefer_loading_from_json: T::Boolean,
+          only:                    T.nilable(Symbol),
+          ignore_unavailable:      T.nilable(T::Boolean),
+          method:                  T.nilable(Symbol),
+          uniq:                    T::Boolean,
+          prefer_loading_from_api: T::Boolean,
         ).returns(T::Array[T.any(Formula, Keg, Cask::Cask)])
       }
       def to_formulae_and_casks(only: parent&.only_formula_or_cask, ignore_unavailable: nil, method: nil, uniq: true,
-                                prefer_loading_from_json: false)
+                                prefer_loading_from_api: false)
         @to_formulae_and_casks ||= {}
         @to_formulae_and_casks[only] ||= downcased_unique_named.flat_map do |name|
-          load_formula_or_cask(name, only: only, method: method, prefer_loading_from_json: prefer_loading_from_json)
+          load_formula_or_cask(name, only: only, method: method, prefer_loading_from_api: prefer_loading_from_api)
         rescue FormulaUnreadableError, FormulaClassUnavailableError,
                TapFormulaUnreadableError, TapFormulaClassUnavailableError,
                Cask::CaskUnreadableError
@@ -90,11 +90,11 @@ module Homebrew
         end.uniq.freeze
       end
 
-      def load_formula_or_cask(name, only: nil, method: nil, prefer_loading_from_json: false)
+      def load_formula_or_cask(name, only: nil, method: nil, prefer_loading_from_api: false)
         unreadable_error = nil
 
         if only != :cask
-          if prefer_loading_from_json && ENV["HOMEBREW_JSON_CORE"].present? &&
+          if prefer_loading_from_api && ENV["HOMEBREW_JSON_CORE"].present? &&
              Homebrew::API::Bottle.available?(name)
             Homebrew::API::Bottle.fetch_bottles(name)
           end
@@ -133,9 +133,14 @@ module Homebrew
         end
 
         if only != :formula
+          if prefer_loading_from_api && ENV["HOMEBREW_JSON_CORE"].present? &&
+             Homebrew::API::CaskSource.available?(name)
+            contents = Homebrew::API::CaskSource.fetch(name)
+          end
+
           begin
             config = Cask::Config.from_args(@parent) if @cask_options
-            cask = Cask::CaskLoader.load(name, config: config)
+            cask = Cask::CaskLoader.load(contents || name, config: config)
 
             if unreadable_error.present?
               onoe <<~EOS
diff --git a/Library/Homebrew/cmd/install.rb b/Library/Homebrew/cmd/install.rb
index e04ced8c99..7c102bb598 100644
--- a/Library/Homebrew/cmd/install.rb
+++ b/Library/Homebrew/cmd/install.rb
@@ -155,7 +155,7 @@ module Homebrew
     end
 
     begin
-      formulae, casks = args.named.to_formulae_and_casks(prefer_loading_from_json: true)
+      formulae, casks = args.named.to_formulae_and_casks(prefer_loading_from_api: true)
                             .partition { |formula_or_cask| formula_or_cask.is_a?(Formula) }
     rescue FormulaOrCaskUnavailableError, Cask::CaskUnavailableError => e
       retry if Tap.install_default_cask_tap_if_necessary(force: args.cask?)
diff --git a/Library/Homebrew/cmd/reinstall.rb b/Library/Homebrew/cmd/reinstall.rb
index c95693917e..791b299ed3 100644
--- a/Library/Homebrew/cmd/reinstall.rb
+++ b/Library/Homebrew/cmd/reinstall.rb
@@ -85,6 +85,9 @@ module Homebrew
   def reinstall
     args = reinstall_args.parse
 
+    # We need to use the bottle API instead of just using the formula file
+    # from an installed keg because it will not contain bottle information.
+    # As a consequence, `brew reinstall` will also upgrade outdated formulae
     if ENV["HOMEBREW_JSON_CORE"].present?
       args.named.each do |name|
         formula = Formulary.factory(name)
diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index cc4ae0d1c2..4c7918ab39 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -648,7 +648,8 @@ EOS
     # HOMEBREW_UPDATE_PREINSTALL wasn't modified in subshell.
     # shellcheck disable=SC2031
     if [[ -n "${HOMEBREW_JSON_CORE}" ]] && [[ -n "${HOMEBREW_UPDATE_PREINSTALL}" ]] &&
-       [[ "${DIR}" = "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" ]]
+       [[ "${DIR}" = "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" ||
+          "${DIR}" = "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-cask" ]]
     then
       continue
     fi
diff --git a/Library/Homebrew/cmd/upgrade.rb b/Library/Homebrew/cmd/upgrade.rb
index cbc10fb6b1..c673975f63 100644
--- a/Library/Homebrew/cmd/upgrade.rb
+++ b/Library/Homebrew/cmd/upgrade.rb
@@ -225,6 +225,15 @@ module Homebrew
   def upgrade_outdated_casks(casks, args:)
     return false if args.formula?
 
+    if ENV["HOMEBREW_JSON_CORE"].present?
+      casks = casks.map do |cask|
+        next cask if cask.tap.present? && cask.tap != "homebrew/cask"
+        next cask unless Homebrew::API::CaskSource.available?(cask.token)
+
+        Cask::CaskLoader.load Homebrew::API::CaskSource.fetch(cask.token)
+      end
+    end
+
     Cask::Cmd::Upgrade.upgrade_casks(
       *casks,
       force:          args.force?,
diff --git a/Library/Homebrew/extend/os/mac/tap.rb b/Library/Homebrew/extend/os/mac/tap.rb
index ca2701238a..d2e5b89212 100644
--- a/Library/Homebrew/extend/os/mac/tap.rb
+++ b/Library/Homebrew/extend/os/mac/tap.rb
@@ -4,7 +4,7 @@
 class Tap
   def self.install_default_cask_tap_if_necessary(force: false)
     return false if default_cask_tap.installed?
-
+    return false if ENV["HOMEBREW_JSON_CORE"].present?
     return false if !force && Tap.untapped_official_taps.include?(default_cask_tap.name)
 
     default_cask_tap.install
diff --git a/Library/Homebrew/test/api/versions_spec.rb b/Library/Homebrew/test/api/versions_spec.rb
index 28b66d65f6..c27ec833dd 100644
--- a/Library/Homebrew/test/api/versions_spec.rb
+++ b/Library/Homebrew/test/api/versions_spec.rb
@@ -4,22 +4,21 @@
 require "api"
 
 describe Homebrew::API::Versions do
-  let(:versions_formulae_json) {
-    <<~EOS
-      {
-        "foo":{"version":"1.2.3","revision":0},
-        "bar":{"version":"1.2","revision":4}
-      }
-    EOS
-  }
-  let(:versions_casks_json) { '{"foo":{"version":"1.2.3"}}' }
-
   def mock_curl_output(stdout: "", success: true)
     curl_output = OpenStruct.new(stdout: stdout, success?: success)
     allow(Utils::Curl).to receive(:curl_output).and_return curl_output
   end
 
   describe "::latest_formula_version" do
+    let(:versions_formulae_json) {
+      <<~EOS
+        {
+          "foo":{"version":"1.2.3","revision":0},
+          "bar":{"version":"1.2","revision":4}
+        }
+      EOS
+    }
+
     it "returns the expected `PkgVersion` when the revision is 0" do
       mock_curl_output stdout: versions_formulae_json
       pkg_version = described_class.latest_formula_version("foo")
@@ -39,16 +38,34 @@ describe Homebrew::API::Versions do
     end
   end
 
-  describe "::latest_cask_version" do
+  describe "::latest_cask_version", :needs_macos do
+    let(:versions_casks_json) {
+      <<~EOS
+        {
+          "foo":{"version":"1.2.3","versions":{}},
+          "bar":{"version":"1.2.3","versions":{"#{MacOS.version.to_sym}":"1.2.0"}},
+          "baz":{"version":"1.2.3","versions":{"test_os":"1.2.0"}}
+        }
+      EOS
+    }
+
     it "returns the expected `Version`" do
       mock_curl_output stdout: versions_casks_json
       version = described_class.latest_cask_version("foo")
       expect(version.to_s).to eq "1.2.3"
     end
 
-    it "returns `nil` when the cask is not in the JSON file" do
+    it "returns the expected `Version` for an OS with a non-default version" do
       mock_curl_output stdout: versions_casks_json
       version = described_class.latest_cask_version("bar")
+      expect(version.to_s).to eq "1.2.0"
+      version = described_class.latest_cask_version("baz")
+      expect(version.to_s).to eq "1.2.3"
+    end
+
+    it "returns `nil` when the cask is not in the JSON file" do
+      mock_curl_output stdout: versions_casks_json
+      version = described_class.latest_cask_version("doesnotexist")
       expect(version).to be_nil
     end
   end
diff --git a/Library/Homebrew/test/api_spec.rb b/Library/Homebrew/test/api_spec.rb
index 68a01074fc..6d70ef6626 100644
--- a/Library/Homebrew/test/api_spec.rb
+++ b/Library/Homebrew/test/api_spec.rb
@@ -14,6 +14,12 @@ describe Homebrew::API do
   end
 
   describe "::fetch" do
+    it "fetches a text file" do
+      mock_curl_output stdout: text
+      fetched_text = described_class.fetch("foo.txt", json: false)
+      expect(fetched_text).to eq text
+    end
+
     it "fetches a JSON file" do
       mock_curl_output stdout: json
       fetched_json = described_class.fetch("foo.json")