diff --git a/pyproject.toml b/pyproject.toml
index 007101b4..d183e1a4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "sap-cloud-sdk"
-version = "0.55.0"
+version = "0.55.1"
 description = "SAP Cloud SDK for Python"
 readme = "README.md"
 license = "Apache-2.0"
diff --git a/src/sap_cloud_sdk/core/protocol/http/models.py b/src/sap_cloud_sdk/core/protocol/http/models.py
index af6e69b3..e8e68014 100644
--- a/src/sap_cloud_sdk/core/protocol/http/models.py
+++ b/src/sap_cloud_sdk/core/protocol/http/models.py
@@ -13,6 +13,8 @@
 from oauthlib.oauth2 import BackendApplicationClient
 from requests_oauthlib import OAuth2Session
 
+from sap_cloud_sdk.core._tenant import _validate_tenant_subdomain
+
 logger = logging.getLogger(__name__)
 
 _TOKEN_EXPIRY_BUFFER_SECONDS = 60
@@ -109,6 +111,7 @@ def _fetch_token(self, tenant_subdomain: Optional[str]) -> OAuth2Session:
             and identityzone is not None
             and token_url is not None
         ):
+            _validate_tenant_subdomain(tenant_subdomain)
             token_url = str(token_url).replace(str(identityzone), tenant_subdomain)
 
         client = BackendApplicationClient(client_id=str(self._config.client_id))
diff --git a/tests/core/unit/test_http_client.py b/tests/core/unit/test_http_client.py
index af8f9e30..46c7fd4d 100644
--- a/tests/core/unit/test_http_client.py
+++ b/tests/core/unit/test_http_client.py
@@ -217,3 +217,70 @@ def test_close_no_auth_closes_plain_session(self):
         client.close()
         mock_session.close.assert_called_once()
         assert client._plain_session is None
+
+
+class TestXsuaaAuthProviderSubdomainValidation:
+
+    def _make_config(self, identityzone: str = "provider-zone") -> MagicMock:
+        cfg = MagicMock()
+        cfg.token_url = f"https://{identityzone}.authentication.region/oauth/token"
+        cfg.identityzone = identityzone
+        cfg.client_id = "cid"
+        cfg.client_secret = "csecret"
+        return cfg
+
+    def _make_provider(self, identityzone: str = "provider-zone") -> XsuaaAuthProvider:
+        cfg = self._make_config(identityzone)
+        factory = MagicMock(return_value=cfg)
+        factory.has_changed = MagicMock(return_value=False)
+        return XsuaaAuthProvider(factory)
+
+    @patch("sap_cloud_sdk.core.protocol.http.models._validate_tenant_subdomain")
+    def test_validator_called_with_tenant_subdomain(self, mock_validate):
+        provider = self._make_provider()
+        with patch.object(provider, "_fetch_token") as mock_fetch:
+            mock_fetch.return_value = MagicMock()
+            provider.get_session("tenant-123")
+        mock_fetch.assert_called_once_with("tenant-123")
+
+    def test_invalid_subdomain_raises_value_error_before_fetch_token(self):
+        provider = self._make_provider()
+        with patch("sap_cloud_sdk.core.protocol.http.models.OAuth2Session") as mock_oauth_cls:
+            with pytest.raises(ValueError, match="Invalid tenant_subdomain"):
+                provider._fetch_token("evil.example/oauth/token?ignore=")
+            mock_oauth_cls.return_value.fetch_token.assert_not_called()
+
+    @pytest.mark.parametrize("bad_subdomain", [
+        "evil.example/oauth/token?ignore=",
+        "has.dot",
+        "-leading-hyphen",
+        "trailing-hyphen-",
+        "has space",
+        "has/slash",
+        "a" * 64,
+    ])
+    def test_delimiter_bearing_subdomains_rejected(self, bad_subdomain):
+        provider = self._make_provider()
+        with pytest.raises(ValueError, match="Invalid tenant_subdomain"):
+            provider._fetch_token(bad_subdomain)
+
+    def test_none_subdomain_skips_validation_and_uses_provider_url(self):
+        provider = self._make_provider()
+        cfg = provider._config
+        with patch("sap_cloud_sdk.core.protocol.http.models.OAuth2Session") as mock_oauth_cls:
+            mock_session = MagicMock()
+            mock_oauth_cls.return_value = mock_session
+            mock_session.fetch_token.return_value = {"access_token": "tok", "expires_in": 3600}
+            provider._fetch_token(None)
+        call_kwargs = mock_session.fetch_token.call_args[1]
+        assert call_kwargs["token_url"] == cfg.token_url
+
+    def test_valid_subdomain_replaces_identityzone_in_token_url(self):
+        provider = self._make_provider("provider-zone")
+        with patch("sap_cloud_sdk.core.protocol.http.models.OAuth2Session") as mock_oauth_cls:
+            mock_session = MagicMock()
+            mock_oauth_cls.return_value = mock_session
+            mock_session.fetch_token.return_value = {"access_token": "tok", "expires_in": 3600}
+            provider._fetch_token("tenant-123")
+        call_kwargs = mock_session.fetch_token.call_args[1]
+        assert call_kwargs["token_url"] == "https://tenant-123.authentication.region/oauth/token"