diff --git a/README.md b/README.md
index 73d8b093..d90de69d 100644
--- a/README.md
+++ b/README.md
@@ -66,6 +66,10 @@ 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 `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:
@@ -82,7 +86,7 @@ def default():
         .secret_key('secret_key')
         .public_key('public_key') # optional, only required for operations related with tokens
         .environment(Environment.sandbox()) # or production()
-        .environment_subdomain("subdomain") # optional, Merchant-specific DNS name
+        .environment_subdomain("subdomain") # required, Merchant-specific DNS name, the first 8 characters of your client ID
         .build()
 
     payments_client = checkout_api.payments
@@ -105,7 +109,7 @@ def oauth():
         .oauth()
         .client_credentials(client_id='client_id', client_secret='client_secret')
         .environment(Environment.sandbox()) # or production()
-        .environment_subdomain("subdomain") # optional, Merchant-specific DNS name
+        .environment_subdomain("subdomain") # required, Merchant-specific DNS name, the first 8 characters of your client ID
         .scopes([OAuthScopes.GATEWAY_PAYMENT_REFUNDS, OAuthScopes.FILES]) # optional, array of scopes
         .build()
 
@@ -129,7 +133,7 @@ def previous():
         .secret_key('secret_key')
         .public_key('public_key') # optional, only required for operations related with tokens
         .environment(Environment.sandbox()) # or production()
-        .environment_subdomain("subdomain") # optional, Merchant-specific DNS name
+        .environment_subdomain("subdomain") # optional for the Previous platform, Merchant-specific DNS name
         .build()
 
     payments_client = checkout_api.payments
@@ -175,7 +179,7 @@ def oauth():
         .oauth()
         .client_credentials(client_id='client_id', client_secret='client_secret')
         .environment(Environment.sandbox()) # or production()
-        .environment_subdomain("subdomain") # optional, Merchant-specific DNS name
+        .environment_subdomain("subdomain") # required, Merchant-specific DNS name, the first 8 characters of your client ID
         .http_client_builder(CustomHttpClientBuilder()) # optional
         .scopes([OAuthScopes.GATEWAY_PAYMENT_REFUNDS, OAuthScopes.FILES]) # optional, array of scopes
         .build()
@@ -267,6 +271,22 @@ The execution of integration tests require the following environment variables s
 * For Previous account systems: `CHECKOUT_PREVIOUS_PUBLIC_KEY` & `CHECKOUT_PREVIOUS_SECRET_KEY`
 * Processing channel: `CHECKOUT_PROCESSING_CHANNEL_ID`
 
+## 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 `use_legacy_domain()` instead of `environment_subdomain(...)`:
+
+```python
+checkout_api = CheckoutSdk.builder() \
+    .secret_key("secret_key") \
+    .environment(Environment.sandbox()) \
+    .use_legacy_domain() \
+    .build()
+```
+
+This routes requests to `api.checkout.com` (or `api.sandbox.checkout.com`) and `access.checkout.com` (or `access.sandbox.checkout.com`). The method raises a `DeprecationWarning`, so `python -W error::DeprecationWarning` and most linters will flag it. Exactly one of `environment_subdomain(...)` or `use_legacy_domain()` must be set: the SDK raises a `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/checkout_sdk/checkout_sdk_builder.py b/checkout_sdk/checkout_sdk_builder.py
index 3f0b8e96..3d7f4946 100644
--- a/checkout_sdk/checkout_sdk_builder.py
+++ b/checkout_sdk/checkout_sdk_builder.py
@@ -1,10 +1,12 @@
 from __future__ import absolute_import
 
+import warnings
 from typing import Optional
 
 from checkout_sdk.default_http_client import DefaultHttpClientBuilder
 from checkout_sdk.environment import Environment
 from checkout_sdk.environment_subdomain import EnvironmentSubdomain
+from checkout_sdk.exception import CheckoutArgumentException
 from checkout_sdk.http_client_interface import HttpClientBuilderInterface
 
 
@@ -12,7 +14,8 @@ class CheckoutSdkBuilder:
 
     def __init__(self):
         self._environment = Environment.sandbox()
-        self._environment_subdomain = None
+        self._subdomain = None
+        self._use_legacy_domain = False
         self._http_client = DefaultHttpClientBuilder().get_client()
 
     def environment(self, environment: Environment):
@@ -20,15 +23,59 @@ def environment(self, environment: Environment):
         return self
 
     def environment_subdomain(self, subdomain: Optional[str]):
-        if subdomain:
-            self._environment_subdomain = EnvironmentSubdomain(self._environment, subdomain)
-        else:
-            self._environment_subdomain = None
+        self._subdomain = subdomain
+        return self
+
+    def use_legacy_domain(self):
+        """
+        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
+        environment_subdomain() instead.
+        See https://api-reference.checkout.com/#section/Base-URLs
+        """
+        warnings.warn(
+            'use_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 environment_subdomain() instead. See '
+            'https://api-reference.checkout.com/#section/Base-URLs',
+            DeprecationWarning,
+            stacklevel=2)
+        self._use_legacy_domain = True
         return self
 
     def http_client_builder(self, http_client_builder: HttpClientBuilderInterface):
         self._http_client = http_client_builder.get_client()
         return self
 
+    @property
+    def _environment_subdomain(self) -> Optional[EnvironmentSubdomain]:
+        if self._subdomain is None:
+            return None
+        return EnvironmentSubdomain(self._environment, self._subdomain)
+
+    def _requires_environment_subdomain(self) -> bool:
+        """
+        Whether this builder requires the merchant-specific subdomain to be configured. The
+        Previous (ABC) platform predates merchant-specific subdomains, so it overrides this to
+        False.
+        """
+        return True
+
+    def _validate_environment_settings(self):
+        if self._subdomain is not None and self._use_legacy_domain:
+            raise CheckoutArgumentException(
+                'environment_subdomain and use_legacy_domain cannot both be set - provide only '
+                'your merchant-specific subdomain')
+        if self._subdomain is None and not self._use_legacy_domain and self._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 '
+                'use_legacy_domain() to opt out only if merchant specific sub domains are '
+                'causing issues')
+
     def build(self):
         raise NotImplementedError()
diff --git a/checkout_sdk/default_sdk.py b/checkout_sdk/default_sdk.py
index 3305ee42..700efeea 100644
--- a/checkout_sdk/default_sdk.py
+++ b/checkout_sdk/default_sdk.py
@@ -39,15 +39,10 @@ def oauth():
     def build(self):
         validate_secret_key(self._SECRET_KEY_PATTERN, self._secret_key)
         validate_public_key(self._PUBLIC_KEY_PATTERN, self._public_key)
-        if self._environment_subdomain is not None:
-            configuration = CheckoutConfiguration(
-                credentials=DefaultKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
-                environment=self._environment,
-                http_client=self._http_client,
-                environment_subdomain=self._environment_subdomain)
-        else:
-            configuration = CheckoutConfiguration(
-                credentials=DefaultKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
-                environment=self._environment,
-                http_client=self._http_client)
+        self._validate_environment_settings()
+        configuration = CheckoutConfiguration(
+            credentials=DefaultKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
+            environment=self._environment,
+            http_client=self._http_client,
+            environment_subdomain=self._environment_subdomain)
         return CheckoutApi(configuration)
diff --git a/checkout_sdk/environment_subdomain.py b/checkout_sdk/environment_subdomain.py
index ff6be2d8..7d60de4e 100644
--- a/checkout_sdk/environment_subdomain.py
+++ b/checkout_sdk/environment_subdomain.py
@@ -2,6 +2,7 @@
 from urllib.parse import urlparse, urlunparse
 
 from checkout_sdk.environment import Environment
+from checkout_sdk.exception import CheckoutArgumentException
 
 
 class EnvironmentSubdomain:
@@ -12,39 +13,41 @@ def __init__(self, environment: Environment, subdomain: str):
     @staticmethod
     def create_url_with_subdomain(original_url: str, subdomain: str):
         """
-        Applies subdomain transformation to any given URL.
-        If the subdomain is valid (alphanumeric pattern), prepends it to the host.
-        Otherwise, returns the original URL unchanged.
+        Applies subdomain transformation to any given URL, prepending the subdomain to the host.
 
         Args:
             original_url: the original URL to transform
             subdomain: the subdomain to prepend
 
         Returns:
-            the transformed URL with subdomain, or original URL if subdomain is invalid
-        """
-        new_environment = original_url
+            the transformed URL with subdomain
 
+        Raises:
+            CheckoutArgumentException: if the subdomain is not a valid merchant-specific subdomain
+        """
         regex = r'^(?:pl-)?[a-z0-9]+$'
-        if re.match(regex, subdomain):
-            url_parts = urlparse(original_url)
-            if url_parts.port:
-                new_host = subdomain + '.' + url_parts.hostname + ':' + str(url_parts.port)
-            else:
-                new_host = subdomain + '.' + url_parts.hostname
-
-            new_url_parts = (
-                url_parts.scheme,
-                new_host,
-                url_parts.path,
-                url_parts.params,
-                url_parts.query,
-                url_parts.fragment
-            )
-
-            new_environment = urlunparse(new_url_parts)
-
-        return new_environment
+        if subdomain is None or not re.match(regex, subdomain):
+            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)')
+
+        url_parts = urlparse(original_url)
+        if url_parts.port:
+            new_host = subdomain + '.' + url_parts.hostname + ':' + str(url_parts.port)
+        else:
+            new_host = subdomain + '.' + url_parts.hostname
+
+        new_url_parts = (
+            url_parts.scheme,
+            new_host,
+            url_parts.path,
+            url_parts.params,
+            url_parts.query,
+            url_parts.fragment
+        )
+
+        return urlunparse(new_url_parts)
 
     def base_uri(self) -> str:
         return self.base_uri
diff --git a/checkout_sdk/oauth_sdk.py b/checkout_sdk/oauth_sdk.py
index 0ec97a63..aa45ca9d 100644
--- a/checkout_sdk/oauth_sdk.py
+++ b/checkout_sdk/oauth_sdk.py
@@ -31,9 +31,12 @@ def scopes(self, scopes: list):
         return self
 
     def build(self):
+        self._validate_environment_settings()
+        environment_subdomain = self._environment_subdomain
+
         # Determine the authorization URI based on subdomain configuration
-        if self._environment_subdomain is not None:
-            authorization_uri = self._environment_subdomain.authorization_uri
+        if environment_subdomain is not None:
+            authorization_uri = environment_subdomain.authorization_uri
         else:
             authorization_uri = self._environment.authorization_uri
 
@@ -41,25 +44,14 @@ def build(self):
         if self._authorization_uri:
             authorization_uri = self._authorization_uri
 
-        if self._environment_subdomain is not None:
-            configuration = CheckoutConfiguration(
-                credentials=OAuthSdkCredentials.init(http_client=self._http_client,
-                                                     environment=self._environment,
-                                                     client_id=self._client_id,
-                                                     client_secret=self._client_secret,
-                                                     scopes=self._scopes,
-                                                     authorization_uri=authorization_uri),
-                environment=self._environment,
-                http_client=self._http_client,
-                environment_subdomain=self._environment_subdomain)
-        else:
-            configuration = CheckoutConfiguration(
-                credentials=OAuthSdkCredentials.init(http_client=self._http_client,
-                                                     environment=self._environment,
-                                                     client_id=self._client_id,
-                                                     client_secret=self._client_secret,
-                                                     scopes=self._scopes,
-                                                     authorization_uri=authorization_uri),
-                environment=self._environment,
-                http_client=self._http_client)
+        configuration = CheckoutConfiguration(
+            credentials=OAuthSdkCredentials.init(http_client=self._http_client,
+                                                 environment=self._environment,
+                                                 client_id=self._client_id,
+                                                 client_secret=self._client_secret,
+                                                 scopes=self._scopes,
+                                                 authorization_uri=authorization_uri),
+            environment=self._environment,
+            http_client=self._http_client,
+            environment_subdomain=environment_subdomain)
         return CheckoutApi(configuration)
diff --git a/checkout_sdk/previous/previous_sdk.py b/checkout_sdk/previous/previous_sdk.py
index 38c093d1..7dd4f3f9 100644
--- a/checkout_sdk/previous/previous_sdk.py
+++ b/checkout_sdk/previous/previous_sdk.py
@@ -27,18 +27,18 @@ class PreviousSdk(PreviousStaticKeys):
     def __init__(self):
         super().__init__()
 
+    # The Previous (ABC) platform predates merchant-specific subdomains, so it is exempt from
+    # the mandatory environment_subdomain/use_legacy_domain configuration.
+    def _requires_environment_subdomain(self) -> bool:
+        return False
+
     def build(self):
         validate_secret_key(self._SECRET_KEY_PATTERN, self._secret_key)
         validate_public_key(self._PUBLIC_KEY_PATTERN, self._public_key)
-        if self._environment_subdomain is not None:
-            configuration = CheckoutConfiguration(
-                credentials=PreviousKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
-                environment=self._environment,
-                http_client=self._http_client,
-                environment_subdomain=self._environment_subdomain)
-        else:
-            configuration = CheckoutConfiguration(
-                credentials=PreviousKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
-                environment=self._environment,
-                http_client=self._http_client)
+        self._validate_environment_settings()
+        configuration = CheckoutConfiguration(
+            credentials=PreviousKeysSdkCredentials(secret_key=self._secret_key, public_key=self._public_key),
+            environment=self._environment,
+            http_client=self._http_client,
+            environment_subdomain=self._environment_subdomain)
         return CheckoutApi(ApiClient(configuration, configuration.environment.base_uri), configuration)
diff --git a/tests/accounts/accounts_integration_test.py b/tests/accounts/accounts_integration_test.py
index 3fd14c1f..175a12ec 100644
--- a/tests/accounts/accounts_integration_test.py
+++ b/tests/accounts/accounts_integration_test.py
@@ -1,5 +1,6 @@
 from __future__ import absolute_import
 
+import warnings
 import os
 from datetime import datetime, timedelta, timezone
 
@@ -21,13 +22,17 @@
 
 @pytest.fixture(scope='class')
 def accounts_checkout_api():
-    return CheckoutSdk \
+    builder = CheckoutSdk \
         .builder() \
         .oauth() \
         .client_credentials(client_id=os.environ.get('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_ID'),
                             client_secret=os.environ.get('CHECKOUT_DEFAULT_OAUTH_ACCOUNTS_CLIENT_SECRET')) \
-        .scopes([OAuthScopes.ACCOUNTS, OAuthScopes.FILES]) \
-        .build()
+        .scopes([OAuthScopes.ACCOUNTS, OAuthScopes.FILES])
+    # 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 warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        return builder.use_legacy_domain().build()
 
 
 def test_should_create_get_and_update_onboard_entity(accounts_checkout_api):
diff --git a/tests/accounts/accounts_payout_schedules_integration_test.py b/tests/accounts/accounts_payout_schedules_integration_test.py
index 35498b38..c511eb2e 100644
--- a/tests/accounts/accounts_payout_schedules_integration_test.py
+++ b/tests/accounts/accounts_payout_schedules_integration_test.py
@@ -1,5 +1,6 @@
 from __future__ import absolute_import
 
+import warnings
 import os
 
 import pytest
@@ -14,13 +15,17 @@
 
 @pytest.fixture(scope='class')
 def payout_schedules_api():
-    return CheckoutSdk \
+    builder = CheckoutSdk \
         .builder() \
         .oauth() \
         .client_credentials(client_id=os.environ.get('CHECKOUT_DEFAULT_OAUTH_PAYOUT_SCHEDULE_CLIENT_ID'),
                             client_secret=os.environ.get('CHECKOUT_DEFAULT_OAUTH_PAYOUT_SCHEDULE_CLIENT_SECRET')) \
-        .scopes([OAuthScopes.MARKETPLACE]) \
-        .build()
+        .scopes([OAuthScopes.MARKETPLACE])
+    # 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 warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        return builder.use_legacy_domain().build()
 
 
 @pytest.mark.skip(reason='not available')
diff --git a/tests/checkout_configuration_test.py b/tests/checkout_configuration_test.py
index db197d2b..20237fc5 100644
--- a/tests/checkout_configuration_test.py
+++ b/tests/checkout_configuration_test.py
@@ -6,6 +6,7 @@
 
 from checkout_sdk.checkout_configuration import CheckoutConfiguration
 from checkout_sdk.environment import Environment
+from checkout_sdk.exception import CheckoutArgumentException
 from checkout_sdk.environment_subdomain import EnvironmentSubdomain
 from checkout_sdk.default_keys_credentials import DefaultKeysSdkCredentials
 from checkout_sdk.http_client_interface import HttpClientBuilderInterface
@@ -83,43 +84,13 @@ def test_should_create_configuration_with_subdomain(subdomain, expected_url):
 
 
 @pytest.mark.parametrize(
-    "subdomain, expected_url",
-    [
-        ("", "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 c1.", "https://api.sandbox.checkout.com/"),
-        ("foo-", "https://api.sandbox.checkout.com/"),
-        ("-foo", "https://api.sandbox.checkout.com/"),
-        ("FooBar", "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/")
-    ]
+    "subdomain",
+    ["", " ", "   ", " - ", "a b", "ab c1.", "foo-", "-foo", "FooBar", "test-123", "foo-bar", "pl-"]
 )
-def test_should_create_configuration_with_bad_subdomain(subdomain, expected_url):
-    credentials = DefaultKeysSdkCredentials(
-        os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY"),
-        os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")
-    )
-    http_client = Mock(spec=HttpClientBuilderInterface)
-
-    environment_subdomain = EnvironmentSubdomain(Environment.sandbox(), subdomain)
-
-    configuration = CheckoutConfiguration(
-        credentials=credentials,
-        environment=Environment.sandbox(),
-        http_client=http_client,
-        environment_subdomain=environment_subdomain
-    )
-
-    assert configuration.credentials == credentials
-    assert configuration.environment.base_uri == Environment.sandbox().base_uri
-    assert configuration.http_client == http_client
-    assert configuration.environment_subdomain.base_uri == expected_url
-    assert configuration.environment_subdomain.authorization_uri == "https://access.sandbox.checkout.com/connect/token"
+def test_should_fail_with_bad_subdomain(subdomain):
+    with pytest.raises(CheckoutArgumentException) as excinfo:
+        EnvironmentSubdomain(Environment.sandbox(), subdomain)
+    assert "invalid environment subdomain" in str(excinfo.value)
 
 
 def test_should_create_configuration_with_subdomain_for_production():
diff --git a/tests/checkout_default_sdk_test.py b/tests/checkout_default_sdk_test.py
index 3bf63059..0f47a7cb 100644
--- a/tests/checkout_default_sdk_test.py
+++ b/tests/checkout_default_sdk_test.py
@@ -13,6 +13,7 @@ def test_should_create_default_sdk():
         .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
         .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
         .environment(Environment.sandbox()) \
+        .environment_subdomain('123domain') \
         .build()
 
     sdk = CheckoutSdk \
@@ -20,12 +21,66 @@ def test_should_create_default_sdk():
         .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
         .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
         .environment(Environment.production()) \
+        .environment_subdomain('123domain') \
         .build()
 
     assert sdk is not None
     assert sdk.tokens is not None
 
 
+def test_should_create_default_sdk_with_legacy_domain():
+    with pytest.deprecated_call():
+        sdk = CheckoutSdk \
+            .builder() \
+            .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
+            .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
+            .environment(Environment.sandbox()) \
+            .use_legacy_domain() \
+            .build()
+
+    assert sdk is not None
+    assert sdk.tokens is not None
+
+
+def test_should_fail_without_subdomain_or_legacy_domain():
+    with pytest.raises(CheckoutArgumentException) as excinfo:
+        CheckoutSdk \
+            .builder() \
+            .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
+            .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
+            .environment(Environment.sandbox()) \
+            .build()
+
+    assert "environment_subdomain is required" in str(excinfo.value)
+
+
+def test_should_fail_with_both_subdomain_and_legacy_domain():
+    with pytest.raises(CheckoutArgumentException) as excinfo, pytest.deprecated_call():
+        CheckoutSdk \
+            .builder() \
+            .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
+            .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
+            .environment(Environment.sandbox()) \
+            .environment_subdomain('123domain') \
+            .use_legacy_domain() \
+            .build()
+
+    assert "cannot both be set" in str(excinfo.value)
+
+
+def test_should_fail_with_invalid_subdomain():
+    with pytest.raises(CheckoutArgumentException) as excinfo:
+        CheckoutSdk \
+            .builder() \
+            .secret_key(os.environ.get("CHECKOUT_DEFAULT_SECRET_KEY")) \
+            .public_key(os.environ.get("CHECKOUT_DEFAULT_PUBLIC_KEY")) \
+            .environment(Environment.sandbox()) \
+            .environment_subdomain('not a subdomain') \
+            .build()
+
+    assert "invalid environment subdomain" in str(excinfo.value)
+
+
 def test_should_create_default_sdk_with_subdomain():
     sdk_1 = CheckoutSdk \
         .builder() \
diff --git a/tests/conftest.py b/tests/conftest.py
index fd0953f1..d098ca4a 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,5 +1,6 @@
 import logging
 import os
+import warnings
 
 import pytest
 import requests
@@ -30,16 +31,21 @@ def previous_api():
 
 @pytest.fixture(scope='session', autouse=True)
 def default_api():
-    return CheckoutSdk() \
-        .builder() \
-        .secret_key(os.environ.get('CHECKOUT_DEFAULT_SECRET_KEY')) \
-        .public_key(os.environ.get('CHECKOUT_DEFAULT_PUBLIC_KEY')) \
-        .build()
+    # 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 warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        return CheckoutSdk() \
+            .builder() \
+            .secret_key(os.environ.get('CHECKOUT_DEFAULT_SECRET_KEY')) \
+            .public_key(os.environ.get('CHECKOUT_DEFAULT_PUBLIC_KEY')) \
+            .use_legacy_domain() \
+            .build()
 
 
 @pytest.fixture(scope='session', autouse=True)
 def oauth_api():
-    return CheckoutSdk() \
+    builder = CheckoutSdk() \
         .builder() \
         .oauth() \
         .client_credentials(client_id=os.environ.get('CHECKOUT_DEFAULT_OAUTH_CLIENT_ID'),
@@ -50,8 +56,11 @@ def oauth_api():
                  OAuthScopes.FILES, OAuthScopes.TRANSFERS, OAuthScopes.BALANCES_VIEW,
                  OAuthScopes.VAULT_CARD_METADATA, OAuthScopes.FINANCIAL_ACTIONS,
                  OAuthScopes.VAULT_REAL_TIME_ACCOUNT_UPDATER, OAuthScopes.PAYMENTS_SEARCH,
-                 OAuthScopes.GATEWAY_PAYMENT_CANCELLATIONS]) \
-        .build()
+                 OAuthScopes.GATEWAY_PAYMENT_CANCELLATIONS])
+    # See default_api above for why the legacy domain is used here.
+    with warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        return builder.use_legacy_domain().build()
 
 
 @pytest.fixture(scope='session', autouse=True)
diff --git a/tests/issuing/conftest.py b/tests/issuing/conftest.py
index b097c165..e8437caa 100644
--- a/tests/issuing/conftest.py
+++ b/tests/issuing/conftest.py
@@ -1,3 +1,4 @@
+import warnings
 import os
 import pytest
 
@@ -15,15 +16,18 @@
 
 @pytest.fixture(scope='module', autouse=True)
 def issuing_checkout_api():
-    api = CheckoutSdk \
+    builder = CheckoutSdk \
         .builder() \
         .oauth() \
         .client_credentials(client_id=os.environ.get('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_ID'),
                             client_secret=os.environ.get('CHECKOUT_DEFAULT_OAUTH_ISSUING_CLIENT_SECRET')) \
         .scopes([OAuthScopes.ISSUING_CLIENT, OAuthScopes.ISSUING_CARD_MGMT,
-                 OAuthScopes.ISSUING_CONTROLS_READ, OAuthScopes.ISSUING_CONTROLS_WRITE]) \
-        .build()
-    return api
+                 OAuthScopes.ISSUING_CONTROLS_READ, OAuthScopes.ISSUING_CONTROLS_WRITE])
+    # 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 warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        return builder.use_legacy_domain().build()
 
 
 @pytest.fixture(scope='module')
diff --git a/tests/oauth_integration_test.py b/tests/oauth_integration_test.py
index 179eab7c..edc3dbb3 100644
--- a/tests/oauth_integration_test.py
+++ b/tests/oauth_integration_test.py
@@ -1,3 +1,4 @@
+import warnings
 from checkout_sdk.checkout_sdk import CheckoutSdk
 from checkout_sdk.customers.customers import CustomerRequest
 from checkout_sdk.environment import Environment
@@ -18,14 +19,18 @@ def test_should_create_customer_with_oauth(oauth_api):
 
 def test_should_fail_init_authorization_invalid_credentials():
     try:
-        CheckoutSdk \
+        builder = CheckoutSdk \
             .builder() \
             .oauth() \
             .client_credentials(client_id='fake_id',
                                 client_secret='fake_secret') \
             .environment(Environment.sandbox()) \
-            .scopes([OAuthScopes.GATEWAY, OAuthScopes.VAULT]) \
-            .build()
+            .scopes([OAuthScopes.GATEWAY, OAuthScopes.VAULT])
+        # 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 warnings.catch_warnings():
+            warnings.simplefilter('ignore', DeprecationWarning)
+            builder.use_legacy_domain().build()
     except CheckoutException as err:
         assert err.args[0] == 'OAuth client_credentials authentication failed with error: (invalid_client)'
 
@@ -39,6 +44,7 @@ def test_should_fail_init_authorization_invalid_credentials_and_host():
                                 client_secret='fake_secret') \
             .authorization_uri('https://test.checkout.com') \
             .environment(Environment.sandbox()) \
+            .environment_subdomain('123domain') \
             .scopes([OAuthScopes.GATEWAY, OAuthScopes.VAULT]) \
             .build()
     except CheckoutException as err:
diff --git a/tests/payments/request_apm_payments_integration_test.py b/tests/payments/request_apm_payments_integration_test.py
index 445d5621..14458eba 100644
--- a/tests/payments/request_apm_payments_integration_test.py
+++ b/tests/payments/request_apm_payments_integration_test.py
@@ -1,5 +1,6 @@
 from __future__ import absolute_import
 
+import warnings
 import os
 
 import pytest
@@ -10,7 +11,8 @@
 from checkout_sdk.exception import CheckoutApiException
 from checkout_sdk.payments.payment_apm import RequestIdealSource, RequestTamaraSource, \
     PaymentRequestWeChatPaySource, RequestAlipayPlusSource, RequestP24Source, RequestKnetSource, \
-    RequestBancontactSource, RequestMultiBancoSource, RequestPostFinanceSource, RequestStcPaySource, RequestAlmaSource, \
+    RequestBancontactSource, RequestMultiBancoSource, RequestPostFinanceSource, RequestStcPaySource, \
+    RequestAlmaSource, \
     RequestKlarnaSource, RequestFawrySource, RequestTrustlySource, RequestCvConnectSource, RequestIllicadoSource, \
     RequestSepaSource, RequestGiropaySource, RequestEpsSource, RequestBizumSource, RequestOctopusSource, \
     RequestPlaidSource, RequestSequraSource
@@ -143,12 +145,16 @@ def test_should_request_tamara_payment():
     payment_request.reference = 'ORD-5023-4E89'
     payment_request.items = [product]
 
-    preview_api = CheckoutSdk \
+    preview_builder = CheckoutSdk \
         .builder() \
         .oauth() \
         .client_credentials(client_id=os.environ.get('CHECKOUT_PREVIEW_OAUTH_CLIENT_ID'),
-                            client_secret=os.environ.get('CHECKOUT_PREVIEW_OAUTH_CLIENT_SECRET')) \
-        .build()
+                            client_secret=os.environ.get('CHECKOUT_PREVIEW_OAUTH_CLIENT_SECRET'))
+    # 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 warnings.catch_warnings():
+        warnings.simplefilter('ignore', DeprecationWarning)
+        preview_api = preview_builder.use_legacy_domain().build()
 
     payment_response = retriable(callback=preview_api.payments.request_payment,
                                  payment_request=payment_request)