diff --git a/README.md b/README.md
index 05601f2..0141c56 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,10 @@ account [here](https://www.checkout.com/get-test-account).
 
 **PLEASE NEVER SHARE OR PUBLISH YOUR CHECKOUT CREDENTIALS.**
 
+### Subdomain value
+
+Requests must be made through your merchant-specific subdomain (MSSD): the first 8 characters of your client ID (excluding `cli_`). For example, if your client ID is `cli_vkuhvk4vjn2edkps7dfsq6emqm`, your subdomain is `vkuhvk4v`. When `with_environment_subdomain` is set the SDK sends requests to `https://vkuhvk4v.api.checkout.com`. See [Base URLs](https://api-reference.checkout.com/#section/Base-URLs) and [API endpoints](https://www.checkout.com/docs/developer-resources/api/api-endpoints) for further details, and for where to find your unique client ID.
+
 ### Default
 
 Default keys client instantiation can be done as follows:
@@ -79,6 +83,7 @@ api = CheckoutSdk.builder
                  .with_secret_key('secret_key')
                  .with_public_key('public_key') # optional, only required for operations related with tokens
                  .with_environment(CheckoutSdk::Environment.sandbox)
+                 .with_environment_subdomain('subdomain') # required, the first 8 characters of your client ID
                  .build
 ```
 
@@ -93,6 +98,7 @@ api = CheckoutSdk.builder
                  .with_client_credentials("client_id", "client_secret")
                  .with_scopes([CheckoutSdk::OAuthScopes::VAULT, CheckoutSdk::OAuthScopes::GATEWAY]) # array of scopes
                  .with_environment(CheckoutSdk::Environment.sandbox)
+                 .with_environment_subdomain('subdomain') # required, the first 8 characters of your client ID
                  .build
 ```
 
@@ -107,6 +113,7 @@ api = CheckoutSdk.builder
                  .with_secret_key('secret_key')
                  .with_public_key('public_key') # optional, only required for operations related with tokens
                  .with_environment(CheckoutSdk::Environment.sandbox)
+                 .with_environment_subdomain('subdomain') # optional for the Previous platform
                  .build
 ```
 
@@ -267,6 +274,23 @@ If you absolutely need to skip the pre-commit hooks:
 OVERCOMMIT_DISABLE=1 git commit -m "your message"
 ```
 
+## Legacy domain (emergency use only)
+
+> :warning: **Only use if merchant specific sub domains are causing issues.** Connecting through your merchant-specific subdomain (see [Subdomain value](#subdomain-value)) is the supported way of using the Checkout.com API, and non-subdomain usage will be deprecated.
+
+If, in exceptional circumstances, you cannot use your merchant-specific subdomain, you can explicitly opt out by calling `with_legacy_domain` instead of `with_environment_subdomain`:
+
+```ruby
+api = CheckoutSdk.builder
+                 .static_keys
+                 .with_secret_key('secret_key')
+                 .with_environment(CheckoutSdk::Environment.sandbox)
+                 .with_legacy_domain # deprecated, emergency fallback only
+                 .build
+```
+
+This routes requests to `api.checkout.com` (or `api.sandbox.checkout.com`) and `access.checkout.com` (or `access.sandbox.checkout.com`). The method prints a deprecation warning. Exactly one of `with_environment_subdomain` or `with_legacy_domain` must be set: the SDK raises a `CheckoutSdk::CheckoutArgumentException` if both, or neither, are. The Previous (ABC) platform predates merchant-specific subdomains and is exempt from this requirement.
+
 ## Code of Conduct
 
 Please refer to [Code of Conduct](CODE_OF_CONDUCT.md)
diff --git a/lib/checkout_sdk/abstract_checkout_sdk_builder.rb b/lib/checkout_sdk/abstract_checkout_sdk_builder.rb
index 12e26c6..dd230f8 100644
--- a/lib/checkout_sdk/abstract_checkout_sdk_builder.rb
+++ b/lib/checkout_sdk/abstract_checkout_sdk_builder.rb
@@ -8,9 +8,9 @@ class AbstractCheckoutSdkBuilder
     #   @return [Faraday::Connection]
     # @!attribute multipart_http_client
     #   @return [Faraday::Connection]
-    # @!attribute environment_subdomain
-    #   @return [EnvironmentSubdomain, nil]
-    attr_accessor :environment, :http_client, :multipart_http_client, :logger, :environment_subdomain
+    # @!attribute subdomain
+    #   @return [String, nil]
+    attr_accessor :environment, :http_client, :multipart_http_client, :logger, :subdomain
 
     # @param [Environment] environment
     def with_environment(environment)
@@ -37,10 +37,39 @@ def with_logger(logger)
 
     # @param [String, nil] subdomain
     def with_environment_subdomain(subdomain)
-      @environment_subdomain = EnvironmentSubdomain.new(@environment, subdomain)
+      @subdomain = subdomain
       self
     end
 
+    # Opts out of the merchant-specific subdomain, sending every request to the shared hosts
+    # instead (api.checkout.com and access.checkout.com, or their sandbox equivalents).
+    #
+    # @deprecated This is an emergency fallback for the rare case where the merchant-specific
+    #   subdomain cannot be used, and will be removed in a future release. Call
+    #   {#with_environment_subdomain} instead.
+    #   See https://api-reference.checkout.com/#section/Base-URLs
+    def with_legacy_domain
+      warn '[DEPRECATION] with_legacy_domain is deprecated and will be removed in a future ' \
+           'release. It is intended only as an emergency fallback when the merchant-specific ' \
+           'subdomain cannot be used. Call with_environment_subdomain instead. ' \
+           'See https://api-reference.checkout.com/#section/Base-URLs'
+      @use_legacy_domain = true
+      self
+    end
+
+    # @return [EnvironmentSubdomain, nil]
+    def environment_subdomain
+      return nil if subdomain.nil?
+
+      EnvironmentSubdomain.new(environment, subdomain)
+    end
+
+    # Whether this builder requires the merchant-specific subdomain to be configured. The
+    # Previous (ABC) platform predates merchant-specific subdomains, so it overrides this.
+    def requires_environment_subdomain?
+      true
+    end
+
     def build
       with_environment(Environment.sandbox) if environment.nil?
       if http_client.nil?
@@ -58,6 +87,24 @@ def build
         end
       end
       @logger = SimpleLogger.new.logger if @logger.nil?
+      validate_environment_settings
+    end
+
+    private
+
+    def validate_environment_settings
+      if !subdomain.nil? && @use_legacy_domain
+        raise CheckoutArgumentException,
+              'with_environment_subdomain and with_legacy_domain cannot both be set - provide ' \
+              'only your merchant-specific subdomain'
+      end
+      return unless subdomain.nil? && !@use_legacy_domain && requires_environment_subdomain?
+
+      raise CheckoutArgumentException,
+            'environment subdomain is required - provide your merchant-specific subdomain (the ' \
+            'first 8 characters of your client ID, see ' \
+            'https://api-reference.checkout.com/#section/Base-URLs), or call with_legacy_domain ' \
+            'to opt out only if merchant specific sub domains are causing issues'
     end
   end
 end
diff --git a/lib/checkout_sdk/environment_subdomain.rb b/lib/checkout_sdk/environment_subdomain.rb
index f5dab5f..965a625 100644
--- a/lib/checkout_sdk/environment_subdomain.rb
+++ b/lib/checkout_sdk/environment_subdomain.rb
@@ -23,36 +23,35 @@ def initialize(environment, subdomain)
 
     private
 
-    # Applies subdomain transformation to any given URI.
-    # If the subdomain is valid (alphanumeric pattern), prepends it to the host.
-    # Otherwise, returns the original URI unchanged.
+    # Applies subdomain transformation to any given URI, prepending the subdomain to the host.
     #
     # @param original_url [String] The original URL to transform.
     # @param subdomain [String] The subdomain to prepend to the host.
-    # @return [String] The transformed URL with subdomain, or original URL if subdomain is invalid.
+    # @return [String] The transformed URL with subdomain.
+    # @raise [CheckoutArgumentException] If the subdomain is not a valid merchant-specific
+    #   subdomain.
     def create_url_with_subdomain(original_url, subdomain)
-      new_environment = original_url
-
-      if subdomain =~ /^(?:pl-)?[a-z0-9]+$/
-        url_parts = URI.parse(original_url)
-        new_host = "#{subdomain}.#{url_parts.host}"
-
-        port = url_parts.scheme == 'https' && url_parts.port == 443 ? nil : url_parts.port
+      unless subdomain =~ /^(?:pl-)?[a-z0-9]+$/
+        raise CheckoutArgumentException,
+              'invalid environment subdomain - provide your merchant-specific subdomain, the ' \
+              'first 8 characters of your client ID (see ' \
+              'https://api-reference.checkout.com/#section/Base-URLs)'
+      end
 
-        new_url_parts = URI::Generic.build(
-          scheme: url_parts.scheme,
-          userinfo: url_parts.userinfo,
-          host: new_host,
-          port: port,
-          path: url_parts.path,
-          query: url_parts.query,
-          fragment: url_parts.fragment
-        )
+      url_parts = URI.parse(original_url)
+      new_host = "#{subdomain}.#{url_parts.host}"
 
-        new_environment = new_url_parts.to_s
-      end
+      port = url_parts.scheme == 'https' && url_parts.port == 443 ? nil : url_parts.port
 
-      new_environment
+      URI::Generic.build(
+        scheme: url_parts.scheme,
+        userinfo: url_parts.userinfo,
+        host: new_host,
+        port: port,
+        path: url_parts.path,
+        query: url_parts.query,
+        fragment: url_parts.fragment
+      ).to_s
     end
   end
 end
diff --git a/lib/checkout_sdk/previous/checkout_previous_static_keys_sdk_builder.rb b/lib/checkout_sdk/previous/checkout_previous_static_keys_sdk_builder.rb
index 1985602..74371d8 100644
--- a/lib/checkout_sdk/previous/checkout_previous_static_keys_sdk_builder.rb
+++ b/lib/checkout_sdk/previous/checkout_previous_static_keys_sdk_builder.rb
@@ -7,6 +7,12 @@ class CheckoutPreviousStaticKeysSdkBuilder < StaticKeysBuilder
       PUBLIC_KEY_PATTERN = '^pk_(test_)?(\\w{8})-(\\w{4})-(\\w{4})-(\\w{4})-(\\w{12})$'
       private_constant :SECRET_KEY_PATTERN, :PUBLIC_KEY_PATTERN
 
+      # The Previous (ABC) platform predates merchant-specific subdomains, so it is exempt from
+      # the mandatory with_environment_subdomain/with_legacy_domain configuration.
+      def requires_environment_subdomain?
+        false
+      end
+
       def build
         @secret_key_pattern = SECRET_KEY_PATTERN
         @public_key_pattern = PUBLIC_KEY_PATTERN
diff --git a/spec/checkout_sdk/accounts/accounts_integration_spec.rb b/spec/checkout_sdk/accounts/accounts_integration_spec.rb
index 3129fc6..9712167 100644
--- a/spec/checkout_sdk/accounts/accounts_integration_spec.rb
+++ b/spec/checkout_sdk/accounts/accounts_integration_spec.rb
@@ -367,9 +367,14 @@ def payout_schedules_checkout_api
              .oauth
              .with_client_credentials(
                ENV.fetch('CHECKOUT_DEFAULT_OAUTH_PAYOUT_SCHEDULE_CLIENT_ID', nil),
-               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_PAYOUT_SCHEDULE_CLIENT_SECRET', nil))
+               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_PAYOUT_SCHEDULE_CLIENT_SECRET', nil)
+             )
              .with_scopes([CheckoutSdk::OAuthScopes::MARKETPLACE])
              .with_environment(CheckoutSdk::Environment.sandbox)
+             # The sandbox OAuth clients are not provisioned for the merchant-specific subdomain,
+             # so the token request would come back invalid_client. Opting out explicitly until
+             # they are.
+             .with_legacy_domain
              .build
 end
 
@@ -378,9 +383,12 @@ def accounts_checkout_api
              .oauth
              .with_client_credentials(
                ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_ID', nil),
-               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_SECRET', nil))
+               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_SECRET', nil)
+             )
              .with_scopes([CheckoutSdk::OAuthScopes::ACCOUNTS, CheckoutSdk::OAuthScopes::FILES])
              .with_environment(CheckoutSdk::Environment.sandbox)
+             # See payout_schedules_checkout_api above for why the legacy domain is used here.
+             .with_legacy_domain
              .build
 end
 
@@ -389,8 +397,11 @@ def files_checkout_api
              .oauth
              .with_client_credentials(
                ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_ID', nil),
-               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_SECRET', nil))
+               ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_SECRET', nil)
+             )
              .with_scopes([CheckoutSdk::OAuthScopes::FILES])
              .with_environment(CheckoutSdk::Environment.sandbox)
+             # See payout_schedules_checkout_api above for why the legacy domain is used here.
+             .with_legacy_domain
              .build
 end
diff --git a/spec/checkout_sdk/configuration/configuration_spec.rb b/spec/checkout_sdk/configuration/configuration_spec.rb
index 314375c..5fffd10 100644
--- a/spec/checkout_sdk/configuration/configuration_spec.rb
+++ b/spec/checkout_sdk/configuration/configuration_spec.rb
@@ -72,38 +72,12 @@ class FakeLogger
     end
   end
 
-  [
-    ['', 'https://api.sandbox.checkout.com/'],
-    [' ', 'https://api.sandbox.checkout.com/'],
-    ['  ', 'https://api.sandbox.checkout.com/'],
-    [' - ', 'https://api.sandbox.checkout.com/'],
-    ['a b', 'https://api.sandbox.checkout.com/'],
-    ['ab bc1', 'https://api.sandbox.checkout.com/'],
-    ['foo-', 'https://api.sandbox.checkout.com/'],
-    ['-foo', 'https://api.sandbox.checkout.com/'],
-    ['FOO', 'https://api.sandbox.checkout.com/'],
-    ['Foo-Bar', 'https://api.sandbox.checkout.com/'],
-    ['test-123', 'https://api.sandbox.checkout.com/'],
-    ['foo-bar', 'https://api.sandbox.checkout.com/'],
-    ['pl-', 'https://api.sandbox.checkout.com/']
-  ].each do |subdomain, expected_url|
-    it "should create configuration with bad subdomain #{subdomain}" do
-      environment_subdomain = CheckoutSdk::EnvironmentSubdomain.new(CheckoutSdk::Environment.sandbox, subdomain)
-
-      configuration = CheckoutSdk::CheckoutConfiguration.new(
-        @credentials,
-        CheckoutSdk::Environment.sandbox,
-        @http_client,
-        @multipart_http_client,
-        @logger,
-        environment_subdomain
-      )
-
-      expect(configuration.credentials).to eq(@credentials)
-      expect(configuration.environment.base_uri).to eq(CheckoutSdk::Environment.sandbox.base_uri)
-      expect(configuration.http_client).to eq(@http_client)
-      expect(configuration.environment_subdomain.base_uri).to eq(expected_url)
-      expect(configuration.environment_subdomain.authorization_uri).to eq('https://access.sandbox.checkout.com/connect/token')
+  ['', ' ', '  ', ' - ', 'a b', 'ab bc1', 'foo-', '-foo', 'FOO', 'Foo-Bar', 'test-123', 'foo-bar',
+   'pl-'].each do |subdomain|
+    it "should fail with bad subdomain #{subdomain}" do
+      expect do
+        CheckoutSdk::EnvironmentSubdomain.new(CheckoutSdk::Environment.sandbox, subdomain)
+      end.to raise_error(CheckoutSdk::CheckoutArgumentException, /invalid environment subdomain/)
     end
   end
 
diff --git a/spec/checkout_sdk/issuing/issuing_helper.rb b/spec/checkout_sdk/issuing/issuing_helper.rb
index 0d02956..8e52926 100644
--- a/spec/checkout_sdk/issuing/issuing_helper.rb
+++ b/spec/checkout_sdk/issuing/issuing_helper.rb
@@ -3,17 +3,23 @@
 module IssuingHelper
   def get_issuing_api
     if @issuing_api.nil?
-      @issuing_api = CheckoutSdk.builder
-                                .oauth
-                                .with_client_credentials(ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_ID', nil),
-                                                         ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_SECRET', nil))
-                                .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
-                                              CheckoutSdk::OAuthScopes::ISSUING_CLIENT,
-                                              CheckoutSdk::OAuthScopes::ISSUING_CARD_MGMT,
-                                              CheckoutSdk::OAuthScopes::ISSUING_CONTROLS_READ,
-                                              CheckoutSdk::OAuthScopes::ISSUING_CONTROLS_WRITE])
-                                .with_environment(CheckoutSdk::Environment.sandbox)
-                                .build
+      builder = CheckoutSdk.builder
+                           .oauth
+                           .with_client_credentials(
+                             ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_ID', nil),
+                             ENV.fetch('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_SECRET', nil)
+                           )
+                           .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
+                                         CheckoutSdk::OAuthScopes::ISSUING_CLIENT,
+                                         CheckoutSdk::OAuthScopes::ISSUING_CARD_MGMT,
+                                         CheckoutSdk::OAuthScopes::ISSUING_CONTROLS_READ,
+                                         CheckoutSdk::OAuthScopes::ISSUING_CONTROLS_WRITE])
+                           .with_environment(CheckoutSdk::Environment.sandbox)
+                           # The sandbox OAuth clients are not provisioned for the
+                           # merchant-specific subdomain, so the token request would come back
+                           # invalid_client. Opting out explicitly until they are.
+                           .with_legacy_domain
+      @issuing_api = builder.build
     end
     @issuing_api
   end
diff --git a/spec/checkout_sdk_spec.rb b/spec/checkout_sdk_spec.rb
index 961e69f..04a94e5 100644
--- a/spec/checkout_sdk_spec.rb
+++ b/spec/checkout_sdk_spec.rb
@@ -15,6 +15,7 @@
                                  .with_secret_key(ENV['CHECKOUT_DEFAULT_SECRET_KEY'])
                                  .with_public_key(ENV['CHECKOUT_DEFAULT_PUBLIC_KEY'])
                                  .with_environment(CheckoutSdk::Environment.production)
+                                 .with_environment_subdomain('12345domain')
                                  .build
         expect(default_sdk.class).to eq(CheckoutSdk::CheckoutApi)
       end
@@ -35,6 +36,7 @@
                                  .static_keys
                                  .with_secret_key(ENV['CHECKOUT_DEFAULT_SECRET_KEY'])
                                  .with_environment(CheckoutSdk::Environment.production)
+                                 .with_environment_subdomain('12345domain')
                                  .build
         expect(default_sdk.class).to eq(CheckoutSdk::CheckoutApi)
       end
@@ -48,6 +50,63 @@
                                  .build
         expect(default_sdk.class).to eq(CheckoutSdk::CheckoutApi)
       end
+
+      it 'builds default sdk with the legacy domain opt-out' do
+        default_sdk = CheckoutSdk.builder
+                                 .static_keys
+                                 .with_secret_key(ENV.fetch('CHECKOUT_DEFAULT_SECRET_KEY', nil))
+                                 .with_environment(CheckoutSdk::Environment.production)
+                                 .with_legacy_domain
+                                 .build
+        expect(default_sdk.class).to eq(CheckoutSdk::CheckoutApi)
+      end
+    end
+
+    context 'when the domain configuration is missing or contradictory' do
+      it 'raises without a subdomain and without the legacy domain opt-out' do
+        expect do
+          CheckoutSdk.builder
+                     .static_keys
+                     .with_secret_key(ENV.fetch('CHECKOUT_DEFAULT_SECRET_KEY', nil))
+                     .with_environment(CheckoutSdk::Environment.production)
+                     .build
+        end.to raise_error(CheckoutSdk::CheckoutArgumentException,
+                           /environment subdomain is required/)
+      end
+
+      it 'raises when both the subdomain and the legacy domain are set' do
+        expect do
+          CheckoutSdk.builder
+                     .static_keys
+                     .with_secret_key(ENV.fetch('CHECKOUT_DEFAULT_SECRET_KEY', nil))
+                     .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('12345domain')
+                     .with_legacy_domain
+                     .build
+        end.to raise_error(CheckoutSdk::CheckoutArgumentException, /cannot both be set/)
+      end
+
+      it 'raises with an invalid subdomain' do
+        expect do
+          CheckoutSdk.builder
+                     .static_keys
+                     .with_secret_key(ENV.fetch('CHECKOUT_DEFAULT_SECRET_KEY', nil))
+                     .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('not a subdomain')
+                     .build
+        end.to raise_error(CheckoutSdk::CheckoutArgumentException,
+                           /invalid environment subdomain/)
+      end
+
+      it 'does not require a subdomain on the Previous platform' do
+        previous_sdk = CheckoutSdk.builder
+                                  .previous
+                                  .static_keys
+                                  .with_secret_key(ENV.fetch('CHECKOUT_PREVIOUS_SECRET_KEY', nil))
+                                  .with_environment(CheckoutSdk::Environment.sandbox)
+                                  .build
+        expect(previous_sdk.class).to eq(CheckoutSdk::Previous::CheckoutApi)
+      end
     end
 
     context 'when building default sdk with incorrect parameters' do
@@ -57,6 +116,7 @@
                      .static_keys
                      .with_secret_key('my wrong key')
                      .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException)
       end
@@ -68,6 +128,7 @@
                      .with_secret_key(ENV['CHECKOUT_DEFAULT_SECRET_KEY'])
                      .with_public_key('my wrong key')
                      .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException)
       end
@@ -85,6 +146,7 @@
                                .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
                                              CheckoutSdk::OAuthScopes::GATEWAY])
                                .with_environment(CheckoutSdk::Environment.sandbox)
+                               .with_environment_subdomain('12345domain')
                                .build
 
         expect(oauth_sdk.class).to eq(CheckoutSdk::CheckoutApi)
@@ -118,6 +180,7 @@
                                .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
                                              CheckoutSdk::OAuthScopes::GATEWAY])
                                .with_environment(CheckoutSdk::Environment.sandbox)
+                               .with_environment_subdomain('12345domain')
                                .with_http_client(http_client)
                                .build
 
@@ -136,6 +199,7 @@
                      .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
                                    CheckoutSdk::OAuthScopes::GATEWAY])
                      .with_environment(CheckoutSdk::Environment.sandbox)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException,
                            'Invalid OAuth "client_id" or "client_secret"')
@@ -151,6 +215,7 @@
                      .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
                                    CheckoutSdk::OAuthScopes::GATEWAY])
                      .with_environment(CheckoutSdk::Environment.sandbox)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException,
                            'Invalid OAuth "client_id" or "client_secret"')
@@ -165,6 +230,7 @@
                      .with_scopes([CheckoutSdk::OAuthScopes::VAULT,
                                    CheckoutSdk::OAuthScopes::GATEWAY])
                      .with_authorization_uri('https://invalid.checkout.com/')
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException)
       end
@@ -215,6 +281,7 @@
                      .static_keys
                      .with_secret_key('my wrong key')
                      .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException)
       end
@@ -227,6 +294,7 @@
                      .with_secret_key(ENV['CHECKOUT_PREVIOUS_SECRET_KEY'])
                      .with_public_key('my wrong key')
                      .with_environment(CheckoutSdk::Environment.production)
+                     .with_environment_subdomain('12345domain')
                      .build
         end.to raise_error(CheckoutSdk::CheckoutArgumentException)
       end
diff --git a/spec/support/sandbox_test_fixture.rb b/spec/support/sandbox_test_fixture.rb
index 9680c98..b0491b8 100644
--- a/spec/support/sandbox_test_fixture.rb
+++ b/spec/support/sandbox_test_fixture.rb
@@ -8,9 +8,13 @@ module SandboxTestFixture
     def default_sdk
       CheckoutSdk.builder
                  .static_keys
-                 .with_secret_key(ENV['CHECKOUT_DEFAULT_SECRET_KEY'])
-                 .with_public_key(ENV['CHECKOUT_DEFAULT_PUBLIC_KEY'])
+                 .with_secret_key(ENV.fetch('CHECKOUT_DEFAULT_SECRET_KEY', nil))
+                 .with_public_key(ENV.fetch('CHECKOUT_DEFAULT_PUBLIC_KEY', nil))
                  .with_environment(CheckoutSdk::Environment.sandbox)
+                 # The sandbox OAuth clients are not provisioned for the merchant-specific
+                 # subdomain, so the token request would come back invalid_client. Opting out
+                 # explicitly until they are.
+                 .with_legacy_domain
                  .build
     end
 
@@ -29,10 +33,14 @@ def oauth_sdk
         @oauth_sdk = CheckoutSdk.builder
                                 .oauth
                                 .with_client_credentials(
-                                  ENV['CHECKOUT_DEFAULT_OAUTH_CLIENT_ID'],
-                                  ENV['CHECKOUT_DEFAULT_OAUTH_CLIENT_SECRET'])
+                                  ENV.fetch('CHECKOUT_DEFAULT_OAUTH_CLIENT_ID', nil),
+                                  ENV.fetch('CHECKOUT_DEFAULT_OAUTH_CLIENT_SECRET', nil))
                                 .with_scopes(get_oauth_scopes)
                                 .with_environment(CheckoutSdk::Environment.sandbox)
+                                # The sandbox OAuth clients are not provisioned for the
+                                # merchant-specific subdomain, so the token request would come
+                                # back invalid_client. Opting out explicitly until they are.
+                                .with_legacy_domain
                                 .build
       end
       @oauth_sdk