diff --git a/Library/Homebrew/api.rb b/Library/Homebrew/api.rb
index 8417784fa0..1221d42d78 100644
--- a/Library/Homebrew/api.rb
+++ b/Library/Homebrew/api.rb
@@ -4,6 +4,7 @@
 require "api/analytics"
 require "api/cask"
 require "api/formula"
+require "api/internal"
 require "base64"
 
 module Homebrew
@@ -151,6 +152,30 @@ module Homebrew
       json.except("variations")
     end
 
+    sig { params(download_queue: T.nilable(DownloadQueue), stale_seconds: Integer).void }
+    def self.fetch_api_files!(download_queue: nil, stale_seconds: Homebrew::EnvConfig.api_auto_update_secs.to_i)
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.fetch_formula_api!(download_queue:, stale_seconds:)
+        Homebrew::API::Internal.fetch_cask_api!(download_queue:, stale_seconds:)
+      else
+        Homebrew::API::Formula.fetch_api!(download_queue:, stale_seconds:)
+        Homebrew::API::Formula.fetch_tap_migrations!(download_queue:, stale_seconds:)
+        Homebrew::API::Cask.fetch_api!(download_queue:, stale_seconds:)
+        Homebrew::API::Cask.fetch_tap_migrations!(download_queue:, stale_seconds:)
+      end
+    end
+
+    sig { void }
+    def self.write_names_and_aliases
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.write_formula_names_and_aliases
+        Homebrew::API::Internal.write_cask_names
+      else
+        Homebrew::API::Formula.write_names_and_aliases
+        Homebrew::API::Cask.write_names
+      end
+    end
+
     sig { params(names: T::Array[String], type: String, regenerate: T::Boolean).returns(T::Boolean) }
     def self.write_names_file!(names, type, regenerate:)
       names_path = HOMEBREW_CACHE_API/"#{type}_names.txt"
@@ -216,6 +241,69 @@ module Homebrew
 
       Tap.fetch(org, repo)
     end
+
+    sig { returns(T::Array[String]) }
+    def self.formula_names
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.formula_arrays.keys
+      else
+        Homebrew::API::Formula.all_formulae.keys
+      end
+    end
+
+    sig { returns(T::Hash[String, String]) }
+    def self.formula_aliases
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.formula_aliases
+      else
+        Homebrew::API::Formula.all_aliases
+      end
+    end
+
+    sig { returns(T::Hash[String, String]) }
+    def self.formula_renames
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.formula_renames
+      else
+        Homebrew::API::Formula.all_renames
+      end
+    end
+
+    sig { returns(T::Hash[String, String]) }
+    def self.formula_tap_migrations
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.formula_tap_migrations
+      else
+        Homebrew::API::Formula.tap_migrations
+      end
+    end
+
+    sig { returns(T::Array[String]) }
+    def self.cask_tokens
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.cask_hashes.keys
+      else
+        Homebrew::API::Cask.all_casks.keys
+      end
+    end
+
+    sig { returns(T::Hash[String, String]) }
+    def self.cask_renames
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.cask_renames
+      else
+        Homebrew::API::Cask.all_renames
+      end
+    end
+
+    sig { returns(T::Hash[String, String]) }
+    def self.cask_tap_migrations
+      if Homebrew::EnvConfig.use_internal_api?
+        Homebrew::API::Internal.cask_tap_migrations
+      else
+        Homebrew::API::Cask.tap_migrations
+      end
+    end
   end
 
   sig { params(block: T.proc.returns(T.untyped)).returns(T.untyped) }
diff --git a/Library/Homebrew/brew.rb b/Library/Homebrew/brew.rb
index aced177b90..f38ef7ec7e 100644
--- a/Library/Homebrew/brew.rb
+++ b/Library/Homebrew/brew.rb
@@ -95,10 +95,7 @@ begin
         require "api/cask"
         download_queue = Homebrew::DownloadQueue.new
         stale_seconds = 86400 # 1 day
-        Homebrew::API::Formula.fetch_api!(download_queue:, stale_seconds:)
-        Homebrew::API::Formula.fetch_tap_migrations!(download_queue:, stale_seconds:)
-        Homebrew::API::Cask.fetch_api!(download_queue:, stale_seconds:)
-        Homebrew::API::Cask.fetch_tap_migrations!(download_queue:, stale_seconds:)
+        Homebrew::API.fetch_api_files!(download_queue:, stale_seconds:)
         begin
           download_queue.fetch
         ensure
diff --git a/Library/Homebrew/cask/cask_loader.rb b/Library/Homebrew/cask/cask_loader.rb
index 18666f40c4..905ec292a6 100644
--- a/Library/Homebrew/cask/cask_loader.rb
+++ b/Library/Homebrew/cask/cask_loader.rb
@@ -306,8 +306,8 @@ module Cask
         return if Homebrew::EnvConfig.no_install_from_api?
         return unless ref.is_a?(String)
         return unless (token = ref[HOMEBREW_DEFAULT_TAP_CASK_REGEX, :token])
-        if !Homebrew::API::Cask.all_casks.key?(token) &&
-           !Homebrew::API::Cask.all_renames.key?(token)
+        if Homebrew::API.cask_tokens.exclude?(token) &&
+           !Homebrew::API.cask_renames.key?(token)
           return
         end
 
diff --git a/Library/Homebrew/cli/named_args.rb b/Library/Homebrew/cli/named_args.rb
index 1047b10265..4500547035 100644
--- a/Library/Homebrew/cli/named_args.rb
+++ b/Library/Homebrew/cli/named_args.rb
@@ -207,13 +207,13 @@ module Homebrew
               if formula_path.exist? ||
                  (!Homebrew::EnvConfig.no_install_from_api? &&
                  !CoreTap.instance.installed? &&
-                 Homebrew::API::Formula.all_formulae.key?(path.basename.to_s))
+                 Homebrew::API.formula_names.include?(path.basename.to_s))
                 paths << formula_path
               end
               if cask_path.exist? ||
                  (!Homebrew::EnvConfig.no_install_from_api? &&
                  !CoreCaskTap.instance.installed? &&
-                 Homebrew::API::Cask.all_casks.key?(path.basename.to_s))
+                 Homebrew::API.cask_tokens.include?(path.basename.to_s))
                 paths << cask_path
               end
 
diff --git a/Library/Homebrew/cmd/update-report.rb b/Library/Homebrew/cmd/update-report.rb
index 38c84d2e02..a7d9ce6b60 100644
--- a/Library/Homebrew/cmd/update-report.rb
+++ b/Library/Homebrew/cmd/update-report.rb
@@ -134,10 +134,7 @@ module Homebrew
         end
 
         # Check if we can parse the JSON and do any Ruby-side follow-up.
-        unless Homebrew::EnvConfig.no_install_from_api?
-          Homebrew::API::Formula.write_names_and_aliases
-          Homebrew::API::Cask.write_names
-        end
+        Homebrew::API.write_names_and_aliases unless Homebrew::EnvConfig.no_install_from_api?
 
         Homebrew.failed = true if ENV["HOMEBREW_UPDATE_FAILED"]
         return if Homebrew::EnvConfig.disable_load_formula?
diff --git a/Library/Homebrew/dev-cmd/edit.rb b/Library/Homebrew/dev-cmd/edit.rb
index bee0caf795..d3e8072657 100644
--- a/Library/Homebrew/dev-cmd/edit.rb
+++ b/Library/Homebrew/dev-cmd/edit.rb
@@ -110,7 +110,7 @@ module Homebrew
 
           raise TapUnavailableError, "#{tap_match[:user]}/#{tap_match[:repo]}"
         elsif cask || core_cask_path?(path)
-          if !CoreCaskTap.instance.installed? && Homebrew::API::Cask.all_casks.key?(name)
+          if !CoreCaskTap.instance.installed? && Homebrew::API.cask_tokens.include?(name)
             command = "brew tap --force #{CoreCaskTap.instance.name}"
             action = "tap #{CoreCaskTap.instance.name}"
           else
@@ -119,7 +119,7 @@ module Homebrew
           end
         elsif core_formula_path?(path) &&
               !CoreTap.instance.installed? &&
-              Homebrew::API::Formula.all_formulae.key?(name)
+              Homebrew::API.formula_names.include?(name)
           command = "brew tap --force #{CoreTap.instance.name}"
           action = "tap #{CoreTap.instance.name}"
         else
diff --git a/Library/Homebrew/env_config.rb b/Library/Homebrew/env_config.rb
index 47a11688a5..74679b02fd 100644
--- a/Library/Homebrew/env_config.rb
+++ b/Library/Homebrew/env_config.rb
@@ -662,5 +662,10 @@ module Homebrew
 
       [concurrency, 1].max
     end
+
+    sig { returns(T::Boolean) }
+    def use_internal_api?
+      ENV["HOMEBREW_USE_INTERNAL_API"].present?
+    end
   end
 end
diff --git a/Library/Homebrew/formulary.rb b/Library/Homebrew/formulary.rb
index b436f80030..e325ecbdca 100644
--- a/Library/Homebrew/formulary.rb
+++ b/Library/Homebrew/formulary.rb
@@ -875,9 +875,9 @@ module Formulary
       return if Homebrew::EnvConfig.no_install_from_api?
       return unless ref.is_a?(String)
       return unless (name = ref[HOMEBREW_DEFAULT_TAP_FORMULA_REGEX, :name])
-      if !Homebrew::API::Formula.all_formulae.key?(name) &&
-         !Homebrew::API::Formula.all_aliases.key?(name) &&
-         !Homebrew::API::Formula.all_renames.key?(name)
+      if Homebrew::API.formula_names.exclude?(name) &&
+         !Homebrew::API.formula_aliases.key?(name) &&
+         !Homebrew::API.formula_renames.key?(name)
         return
       end
 
diff --git a/Library/Homebrew/tap.rb b/Library/Homebrew/tap.rb
index 4d5d2df15e..989a1fff1d 100644
--- a/Library/Homebrew/tap.rb
+++ b/Library/Homebrew/tap.rb
@@ -1345,13 +1345,18 @@ class CoreTap < AbstractCoreTap
     end, T.nilable(Pathname))
   end
 
-  sig { override.params(name: String).returns(Pathname) }
-  def new_formula_path(name)
-    formula_subdir = if name.start_with?("lib")
+  sig { params(name: String).returns(String) }
+  def new_formula_subdirectory(name)
+    if name.start_with?("lib")
       "lib"
     else
       name[0].to_s
     end
+  end
+
+  sig { override.params(name: String).returns(Pathname) }
+  def new_formula_path(name)
+    formula_subdir = new_formula_subdirectory(name)
 
     return super unless (formula_dir/formula_subdir).directory?
 
@@ -1373,7 +1378,7 @@ class CoreTap < AbstractCoreTap
         ensure_installed!
         super
       else
-        Homebrew::API::Formula.all_renames
+        Homebrew::API.formula_renames
       end,
       T.nilable(T::Hash[String, String]),
     )
@@ -1386,7 +1391,7 @@ class CoreTap < AbstractCoreTap
         ensure_installed!
         super
       else
-        Homebrew::API::Formula.tap_migrations
+        Homebrew::API.formula_tap_migrations
       end,
       T.nilable(T::Hash[String, T.untyped]),
     )
@@ -1443,7 +1448,7 @@ class CoreTap < AbstractCoreTap
       if Homebrew::EnvConfig.no_install_from_api?
         super
       else
-        Homebrew::API::Formula.all_aliases
+        Homebrew::API.formula_aliases
       end,
       T.nilable(T::Hash[String, String]),
     )
@@ -1460,7 +1465,7 @@ class CoreTap < AbstractCoreTap
   def formula_names
     return super if Homebrew::EnvConfig.no_install_from_api?
 
-    Homebrew::API::Formula.all_formulae.keys
+    Homebrew::API.formula_names
   end
 
   sig { override.returns(T::Hash[String, Pathname]) }
@@ -1469,13 +1474,12 @@ class CoreTap < AbstractCoreTap
 
     @formula_files_by_name ||= T.let(
       begin
-        tap_path = path.to_s
-        Homebrew::API::Formula.all_formulae.each_with_object({}) do |item, hash|
-          name, formula_hash = item
+        formula_directory_path = formula_dir.to_s
+        Homebrew::API.formula_names.each_with_object({}) do |name, hash|
           # If there's more than one item with the same path: use the longer one to prioritise more specific results.
           existing_path = hash[name]
           # Pathname equivalent is slow in a tight loop
-          new_path = File.join(tap_path, formula_hash.fetch("ruby_source_path"))
+          new_path = File.join(formula_directory_path, new_formula_subdirectory(name), "#{name.downcase}.rb")
           hash[name] = Pathname(new_path) if existing_path.nil? || existing_path.to_s.length < new_path.length
         end
       end,
@@ -1500,14 +1504,18 @@ class CoreCaskTap < AbstractCoreTap
     true
   end
 
-  sig { override.params(token: String).returns(Pathname) }
-  def new_cask_path(token)
-    cask_subdir = if token.start_with?("font-")
+  sig { params(token: String).returns(String) }
+  def new_cask_subdirectory(token)
+    if token.start_with?("font-")
       "font/font-#{token.delete_prefix("font-")[0]}"
     else
       token[0].to_s
     end
-    cask_dir/cask_subdir/"#{token.downcase}.rb"
+  end
+
+  sig { override.params(token: String).returns(Pathname) }
+  def new_cask_path(token)
+    cask_dir/new_cask_subdirectory(token)/"#{token.downcase}.rb"
   end
 
   sig { override.returns(T::Array[Pathname]) }
@@ -1521,7 +1529,7 @@ class CoreCaskTap < AbstractCoreTap
   def cask_tokens
     return super if Homebrew::EnvConfig.no_install_from_api?
 
-    Homebrew::API::Cask.all_casks.keys
+    Homebrew::API.cask_tokens
   end
 
   sig { override.returns(T::Hash[String, Pathname]) }
@@ -1530,13 +1538,12 @@ class CoreCaskTap < AbstractCoreTap
 
     @cask_files_by_name ||= T.let(
       begin
-        tap_path = path.to_s
-        Homebrew::API::Cask.all_casks.each_with_object({}) do |item, hash|
-          name, cask_hash = item
+        cask_directory_path = cask_dir.to_s
+        Homebrew::API.cask_tokens.each_with_object({}) do |name, hash|
           # If there's more than one item with the same path: use the longer one to prioritise more specific results.
           existing_path = hash[name]
           # Pathname equivalent is slow in a tight loop
-          new_path = File.join(tap_path, cask_hash.fetch("ruby_source_path"))
+          new_path = File.join(cask_directory_path, new_cask_subdirectory(name), "#{name.downcase}.rb")
           hash[name] = Pathname(new_path) if existing_path.nil? || existing_path.to_s.length < new_path.length
         end
       end,
@@ -1550,7 +1557,7 @@ class CoreCaskTap < AbstractCoreTap
       if Homebrew::EnvConfig.no_install_from_api?
         super
       else
-        Homebrew::API::Cask.all_renames
+        Homebrew::API.cask_renames
       end,
       T.nilable(T::Hash[String, String]),
     )
@@ -1562,7 +1569,7 @@ class CoreCaskTap < AbstractCoreTap
       if Homebrew::EnvConfig.no_install_from_api?
         super
       else
-        Homebrew::API::Cask.tap_migrations
+        Homebrew::API.cask_tap_migrations
       end,
       T.nilable(T::Hash[String, T.untyped]),
     )
diff --git a/Library/Homebrew/uninstall.rb b/Library/Homebrew/uninstall.rb
index ce19765f81..6d88d2fc3b 100644
--- a/Library/Homebrew/uninstall.rb
+++ b/Library/Homebrew/uninstall.rb
@@ -68,7 +68,7 @@ module Homebrew
 
               unversioned_name = f.name.gsub(/@.+$/, "")
               maybe_paths = Dir.glob("#{f.etc}/#{unversioned_name}*")
-              excluded_names = Homebrew::API::Formula.all_formulae.keys
+              excluded_names = Homebrew::API.formula_names
               maybe_paths = maybe_paths.reject do |path|
                 # Remove extension only if a file
                 # (e.g. directory with name "openssl@1.1" will be trimmed to "openssl@1")
diff --git a/Library/Homebrew/utils/analytics.rb b/Library/Homebrew/utils/analytics.rb
index 305af3f6df..a22d3f8bae 100644
--- a/Library/Homebrew/utils/analytics.rb
+++ b/Library/Homebrew/utils/analytics.rb
@@ -329,7 +329,7 @@ module Utils
 
         require "api"
 
-        return unless Homebrew::API::Formula.all_formulae.key? formula.name
+        return unless Homebrew::API.formula_names.include? formula.name
 
         json = Homebrew::API::Formula.formula_json formula.name
         return if json.blank? || json["analytics"].blank?
@@ -347,7 +347,7 @@ module Utils
 
         require "api"
 
-        return unless Homebrew::API::Cask.all_casks.key? cask.token
+        return unless Homebrew::API.cask_tokens.include? cask.token
 
         json = Homebrew::API::Cask.cask_json cask.token
         return if json.blank? || json["analytics"].blank?