diff --git a/lib/bitsmithy/auth/config.rb b/lib/bitsmithy/auth/config.rb
index 4ba940c..28edd7a 100644
--- a/lib/bitsmithy/auth/config.rb
+++ b/lib/bitsmithy/auth/config.rb
@@ -2,6 +2,7 @@
 
 require_relative "errors"
 require_relative "stores/memory_store"
+require_relative "stores/rails_cache_store"
 
 module Bitsmithy
   module Auth
@@ -10,6 +11,8 @@ class Config
       JWT_ALGORITHM = "HS256"
       DEFAULT_SESSION_DURATION = 86_400 # 24h per ADR-0001
       DEFAULT_RATE_LIMIT = { per_phone: 5, window: 3_600 }.freeze
+      CACHE_MISSING_WARNING = "[bitsmithy-auth] Rails.cache is nil — configure config.cache_store " \
+                              "for cross-worker rate limiting. Falling back to MemoryStore."
 
       attr_accessor :signing_key, :otp_adapter, :session_duration, :rate_limit,
                     :twilio_account_sid, :twilio_auth_token, :twilio_verify_service_sid,
@@ -26,7 +29,16 @@ def initialize
       end
 
       def rate_limit_store
-        @rate_limit_store ||= Stores::MemoryStore.new
+        @rate_limit_store ||= if defined?(Rails) && Rails.respond_to?(:cache)
+                                if Rails.cache
+                                  Stores::RailsCacheStore.new(Rails.cache)
+                                else
+                                  warn CACHE_MISSING_WARNING
+                                  Stores::MemoryStore.new
+                                end
+                              else
+                                Stores::MemoryStore.new
+                              end
       end
 
       def validate!
diff --git a/lib/bitsmithy/auth/stores/rails_cache_store.rb b/lib/bitsmithy/auth/stores/rails_cache_store.rb
new file mode 100644
index 0000000..9136c4c
--- /dev/null
+++ b/lib/bitsmithy/auth/stores/rails_cache_store.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Bitsmithy
+  module Auth
+    module Stores
+      # Wraps an ActiveSupport::Cache::Store (typically Rails.cache) so the
+      # rate limiter works across worker processes when a distributed cache
+      # (Redis, Memcached) is configured.
+      #
+      # Falls back gracefully: if Rails.cache is nil the caller should use
+      # MemoryStore instead — see Config#rate_limit_store.
+      class RailsCacheStore
+        def initialize(cache)
+          @cache = cache
+        end
+
+        # Increment the counter for +key+ and return the new value.
+        # Expires the entry after +window_seconds+.
+        def increment(key, window_seconds)
+          @cache.increment(key, 1, expires_in: window_seconds)
+        end
+      end
+    end
+  end
+end
diff --git a/test/dummy/config/application.rb b/test/dummy/config/application.rb
index e66d51d..0602335 100644
--- a/test/dummy/config/application.rb
+++ b/test/dummy/config/application.rb
@@ -13,6 +13,8 @@ class Application < Rails::Application
     config.hosts.clear
     config.action_controller.allow_forgery_protection = false
 
+    config.cache_store = :memory_store
+
     # Point Rails.root at the dummy app so config/routes.rb is found
     config.root = File.expand_path("..", __dir__)
   end
diff --git a/test/engine/test_rails_cache_store.rb b/test/engine/test_rails_cache_store.rb
new file mode 100644
index 0000000..150a887
--- /dev/null
+++ b/test/engine/test_rails_cache_store.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require_relative "../engine_helper"
+
+class RailsCacheStoreTest < ActiveSupport::TestCase
+  setup do
+    @store = Bitsmithy::Auth::Stores::RailsCacheStore.new(Rails.cache)
+  end
+
+  test "increment returns 1 on first call" do
+    assert_equal 1, @store.increment("counter:a", 60)
+  end
+
+  test "increment returns cumulative count on subsequent calls" do
+    @store.increment("counter:b", 60)
+    @store.increment("counter:b", 60)
+
+    assert_equal 3, @store.increment("counter:b", 60)
+  end
+
+  test "different keys are independent" do
+    @store.increment("counter:c", 60)
+    @store.increment("counter:c", 60)
+
+    assert_equal 1, @store.increment("counter:d", 60)
+  end
+end
diff --git a/test/engine_helper.rb b/test/engine_helper.rb
index bb14562..df2c230 100644
--- a/test/engine_helper.rb
+++ b/test/engine_helper.rb
@@ -3,3 +3,9 @@
 require_relative "dummy/config/environment"
 require "rails/test_help"
 require "mocha/minitest"
+
+module ActiveSupport
+  class TestCase
+    setup { Rails.cache.clear }
+  end
+end