diff --git a/Library/Homebrew/api.rb b/Library/Homebrew/api.rb
index ef06312bff..4b154f4432 100644
--- a/Library/Homebrew/api.rb
+++ b/Library/Homebrew/api.rb
@@ -116,11 +116,13 @@ module Homebrew
       end
     end
 
-    sig { params(name: String, git_head: T.nilable(String), sha256: T.nilable(String)).returns(String) }
-    def self.fetch_homebrew_cask_source(name, git_head: nil, sha256: nil)
+    sig {
+      params(name: String, path: T.any(Pathname, String), git_head: String,
+             sha256: T.nilable(String)).returns(String)
+    }
+    def self.fetch_homebrew_cask_source(name, path:, git_head:, sha256: nil)
       # TODO: unify with formula logic (https://github.com/Homebrew/brew/issues/14746)
-      git_head = "master" if git_head.blank?
-      raw_endpoint = "#{git_head}/Casks/#{name}.rb"
+      raw_endpoint = "#{git_head}/#{path}"
       return cache[raw_endpoint] if cache.present? && cache.key?(raw_endpoint)
 
       # This API sometimes returns random 404s so needs a fallback at formulae.brew.sh.
diff --git a/Library/Homebrew/api/cask.rb b/Library/Homebrew/api/cask.rb
index ccec8f083e..32be78af09 100644
--- a/Library/Homebrew/api/cask.rb
+++ b/Library/Homebrew/api/cask.rb
@@ -20,9 +20,12 @@ module Homebrew
           Homebrew::API.fetch "cask/#{token}.json"
         end
 
-        sig { params(token: String, git_head: T.nilable(String), sha256: T.nilable(String)).returns(String) }
-        def fetch_source(token, git_head: nil, sha256: nil)
-          Homebrew::API.fetch_homebrew_cask_source token, git_head: git_head, sha256: sha256
+        sig {
+          params(token: String, path: T.any(String, Pathname), git_head: String,
+                 sha256: T.nilable(String)).returns(String)
+        }
+        def fetch_source(token, path:, git_head:, sha256: nil)
+          Homebrew::API.fetch_homebrew_cask_source token, path: path, git_head: git_head, sha256: sha256
         end
 
         sig { returns(T::Boolean) }
diff --git a/Library/Homebrew/cask/cask.rb b/Library/Homebrew/cask/cask.rb
index e2597c98b5..c2924025a3 100644
--- a/Library/Homebrew/cask/cask.rb
+++ b/Library/Homebrew/cask/cask.rb
@@ -241,6 +241,15 @@ module Cask
       end
     end
 
+    def ruby_source_path
+      return @ruby_source_path if defined?(@ruby_source_path)
+
+      return unless sourcefile_path
+      return unless tap
+
+      @ruby_source_path = sourcefile_path.relative_path_from(tap.path)
+    end
+
     def ruby_source_checksum
       @ruby_source_checksum ||= {
         "sha256" => Digest::SHA256.file(sourcefile_path).hexdigest,
@@ -259,7 +268,9 @@ module Cask
       raise ArgumentError, "Expected cask to be loaded from the API" unless loaded_from_api?
 
       @languages = json_cask[:languages]
-      @tap_git_head = json_cask[:tap_git_head]
+      @tap_git_head = json_cask.fetch(:tap_git_head, "HEAD")
+
+      @ruby_source_path = json_cask[:ruby_source_path]
       @ruby_source_checksum = json_cask[:ruby_source_checksum].freeze
     end
 
@@ -308,6 +319,7 @@ module Cask
         "auto_updates"         => auto_updates,
         "tap_git_head"         => tap_git_head,
         "languages"            => languages,
+        "ruby_source_path"     => ruby_source_path,
         "ruby_source_checksum" => ruby_source_checksum,
       }
     end
diff --git a/Library/Homebrew/cask/installer.rb b/Library/Homebrew/cask/installer.rb
index e5d7ea2559..1321e8b32e 100644
--- a/Library/Homebrew/cask/installer.rb
+++ b/Library/Homebrew/cask/installer.rb
@@ -564,8 +564,12 @@ on_request: true)
     end
 
     def load_cask_from_source_api!
-      options = { git_head: @cask.tap_git_head, sha256: @cask.ruby_source_checksum["sha256"] }
-      cask_source = Homebrew::API::Cask.fetch_source(@cask.token, **options)
+      cask_source = Homebrew::API::Cask.fetch_source(
+        @cask.token,
+        path:     @cask.ruby_source_path || "Casks/#{@cask.token}.rb",
+        git_head: @cask.tap_git_head,
+        sha256:   @cask.ruby_source_checksum["sha256"],
+      )
       @cask = CaskLoader::FromContentLoader.new(cask_source, tap: @cask.tap).load(config: @cask.config)
     end
   end
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 6b73e7127c..0e4feeb1c6 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -2199,6 +2199,7 @@ class Formula
         "sha256" => resource("ruby-source").checksum.hexdigest,
       }
     elsif !self.class.loaded_from_api && path.exist?
+      hsh["ruby_source_path"] = path.relative_path_from(tap.path).to_s
       hsh["ruby_source_checksum"] = {
         "sha256" => Digest::SHA256.file(path).hexdigest,
       }
diff --git a/Library/Homebrew/formulary.rb b/Library/Homebrew/formulary.rb
index 6724e6db7e..349b5b5041 100644
--- a/Library/Homebrew/formulary.rb
+++ b/Library/Homebrew/formulary.rb
@@ -255,10 +255,12 @@ module Formulary
       end
 
       resource "ruby-source" do
-        url "https://raw.githubusercontent.com/Homebrew/homebrew-core/#{json_formula["tap_git_head"]}/Formula/#{name}.rb"
-        if (ruby_source_sha256 = json_formula.dig("ruby_source_checksum", "sha256")).present?
-          sha256 ruby_source_sha256
-        end
+        tap_git_head = json_formula.fetch("tap_git_head", "HEAD")
+        ruby_source_path = json_formula.fetch("ruby_source_path", "Formula/#{name}.rb")
+        ruby_source_sha256 = json_formula.dig("ruby_source_checksum", "sha256")
+
+        url "https://raw.githubusercontent.com/Homebrew/homebrew-core/#{tap_git_head}/#{ruby_source_path}"
+        sha256 ruby_source_sha256 if ruby_source_sha256
       end
 
       def install
diff --git a/Library/Homebrew/test/api/cask_spec.rb b/Library/Homebrew/test/api/cask_spec.rb
index c5a555d0e6..3d476483a4 100644
--- a/Library/Homebrew/test/api/cask_spec.rb
+++ b/Library/Homebrew/test/api/cask_spec.rb
@@ -51,9 +51,9 @@ describe Homebrew::API::Cask do
     it "fetches the source of a cask (defaulting to master when no `git_head` is passed)" do
       curl_output = instance_double(SystemCommand::Result, stdout: "foo", success?: true)
       expect(Utils::Curl).to receive(:curl_output)
-        .with("--fail", "https://raw.githubusercontent.com/Homebrew/homebrew-cask/master/Casks/foo.rb")
+        .with("--fail", "https://raw.githubusercontent.com/Homebrew/homebrew-cask/HEAD/Casks/foo.rb")
         .and_return(curl_output)
-      described_class.fetch_source("foo", git_head: nil)
+      described_class.fetch_source("foo", path: "Casks/foo.rb", git_head: "HEAD")
     end
   end
 end
diff --git a/Library/Homebrew/test/api_spec.rb b/Library/Homebrew/test/api_spec.rb
index d1c02111f0..2ff4ff5c32 100644
--- a/Library/Homebrew/test/api_spec.rb
+++ b/Library/Homebrew/test/api_spec.rb
@@ -72,14 +72,14 @@ describe Homebrew::API do
   describe "::fetch_file_source" do
     it "fetches a file" do
       mock_curl_output stdout: json
-      fetched_json = described_class.fetch_homebrew_cask_source("foo", git_head: "master")
+      fetched_json = described_class.fetch_homebrew_cask_source("foo", path: "Casks/foo.rb", git_head: "HEAD")
       expect(fetched_json).to eq json
     end
 
     it "raises an error if the file does not exist" do
       mock_curl_output success: false
       expect do
-        described_class.fetch_homebrew_cask_source("bar", git_head: "master")
+        described_class.fetch_homebrew_cask_source("bar", path: "Casks/bar.rb", git_head: "HEAD")
       end.to raise_error(ArgumentError, /No valid file found/)
     end
   end
diff --git a/Library/Homebrew/test/support/fixtures/cask/everything-systemsettings-caveats.json b/Library/Homebrew/test/support/fixtures/cask/everything-systemsettings-caveats.json
index 4accb46a89..e7291d2cc3 100644
--- a/Library/Homebrew/test/support/fixtures/cask/everything-systemsettings-caveats.json
+++ b/Library/Homebrew/test/support/fixtures/cask/everything-systemsettings-caveats.json
@@ -93,6 +93,7 @@
     "en",
     "eo"
   ],
+  "ruby_source_path": "Formula/everything.rb",
   "ruby_source_checksum": {
     "sha256": "b2707d1952f02c3fa566b7ad2a707a847a959d36f51d3dee642dbe5deec12f27"
   }
diff --git a/Library/Homebrew/test/support/fixtures/cask/everything.json b/Library/Homebrew/test/support/fixtures/cask/everything.json
index 9ac6b3d522..f69b490717 100644
--- a/Library/Homebrew/test/support/fixtures/cask/everything.json
+++ b/Library/Homebrew/test/support/fixtures/cask/everything.json
@@ -93,6 +93,7 @@
     "en",
     "eo"
   ],
+  "ruby_source_path": "Casks/everything.rb",
   "ruby_source_checksum": {
     "sha256": "b2707d1952f02c3fa566b7ad2a707a847a959d36f51d3dee642dbe5deec12f27"
   }