fix(auth): handle missing client-credentials scopes safely

src/google/adk/auth/auth_handler.py

@@ -23,6 +23,7 @@
 from .auth_schemes import OpenIdConnectWithConfig
 from .auth_tool import AuthConfig
 from .exchanger.oauth2_credential_exchanger import OAuth2CredentialExchanger
+from .oauth2_credential_util import _normalize_oauth_scopes
 
 if TYPE_CHECKING:
   from ..sessions.state import State
@@ -161,7 +162,7 @@ def generate_auth_uri(
 
     if isinstance(auth_scheme, OpenIdConnectWithConfig):
       authorization_endpoint = auth_scheme.authorization_endpoint
-      scopes = auth_scheme.scopes
+      scopes = _normalize_oauth_scopes(auth_scheme.scopes)
     else:
       authorization_endpoint = (
           auth_scheme.flows.implicit
@@ -173,17 +174,20 @@ def generate_auth_uri(
           or auth_scheme.flows.password
           and auth_scheme.flows.password.tokenUrl
       )
-      scopes = (
-          auth_scheme.flows.implicit
-          and auth_scheme.flows.implicit.scopes
-          or auth_scheme.flows.authorizationCode
-          and auth_scheme.flows.authorizationCode.scopes
-          or auth_scheme.flows.clientCredentials
-          and auth_scheme.flows.clientCredentials.scopes
-          or auth_scheme.flows.password
-          and auth_scheme.flows.password.scopes
-      )
-      scopes = list(scopes.keys())
+      if auth_scheme.flows.implicit:
+        scopes = _normalize_oauth_scopes(auth_scheme.flows.implicit.scopes)
+      elif auth_scheme.flows.authorizationCode:
+        scopes = _normalize_oauth_scopes(
+            auth_scheme.flows.authorizationCode.scopes
+        )
+      elif auth_scheme.flows.clientCredentials:
+        scopes = _normalize_oauth_scopes(
+            auth_scheme.flows.clientCredentials.scopes
+        )
+      elif auth_scheme.flows.password:
+        scopes = _normalize_oauth_scopes(auth_scheme.flows.password.scopes)
+      else:
+        scopes = []
 
     client = OAuth2Session(
         auth_credential.oauth2.client_id,

src/google/adk/auth/oauth2_credential_util.py

@@ -30,6 +30,15 @@
 logger = logging.getLogger("google_adk." + __name__)
 
 
+def _normalize_oauth_scopes(scopes: Optional[dict[str, str] | list[str]]) -> list[str]:
+  """Normalize OAuth scopes into the list shape expected by authlib."""
+  if not scopes:
+    return []
+  if isinstance(scopes, dict):
+    return list(scopes.keys())
+  return list(scopes)
+
+
 @experimental
 def create_oauth2_session(
     auth_scheme: AuthScheme,
@@ -49,21 +58,25 @@ def create_oauth2_session(
       logger.warning("OpenIdConnect scheme missing token_endpoint")
       return None, None
     token_endpoint = auth_scheme.token_endpoint
-    scopes = auth_scheme.scopes or []
+    scopes = _normalize_oauth_scopes(auth_scheme.scopes)
   elif isinstance(auth_scheme, OAuth2):
     # Support both authorization code and client credentials flows
     if (
         auth_scheme.flows.authorizationCode
         and auth_scheme.flows.authorizationCode.tokenUrl
     ):
       token_endpoint = auth_scheme.flows.authorizationCode.tokenUrl
-      scopes = list(auth_scheme.flows.authorizationCode.scopes.keys())
+      scopes = _normalize_oauth_scopes(
+          auth_scheme.flows.authorizationCode.scopes
+      )
     elif (
         auth_scheme.flows.clientCredentials
         and auth_scheme.flows.clientCredentials.tokenUrl
     ):
       token_endpoint = auth_scheme.flows.clientCredentials.tokenUrl
-      scopes = list(auth_scheme.flows.clientCredentials.scopes.keys())
+      scopes = _normalize_oauth_scopes(
+          auth_scheme.flows.clientCredentials.scopes
+      )
     else:
       logger.warning(
           "OAuth2 scheme missing required flow configuration. Expected either"

tests/unittests/auth/test_auth_handler.py

@@ -22,6 +22,7 @@
 from fastapi.openapi.models import APIKeyIn
 from fastapi.openapi.models import OAuth2
 from fastapi.openapi.models import OAuthFlowAuthorizationCode
+from fastapi.openapi.models import OAuthFlowClientCredentials
 from fastapi.openapi.models import OAuthFlows
 from google.adk.auth.auth_credential import AuthCredential
 from google.adk.auth.auth_credential import AuthCredentialTypes
@@ -271,6 +272,35 @@ def test_generate_auth_uri_openid(
     assert "client_id=mock_client_id" in result.oauth2.auth_uri
     assert result.oauth2.state == "mock_state"
 
+  @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
+  def test_generate_auth_uri_client_credentials_with_missing_scopes(
+      self, oauth2_credentials
+  ):
+    """Test client credentials flow tolerates missing scopes."""
+    auth_scheme = OAuth2(
+        flows=OAuthFlows(
+            clientCredentials=OAuthFlowClientCredentials(
+                tokenUrl="https://example.com/oauth2/token"
+            )
+        )
+    )
+    auth_scheme.flows.clientCredentials.scopes = None
+
+    config = AuthConfig(
+        auth_scheme=auth_scheme,
+        raw_auth_credential=oauth2_credentials,
+        exchanged_auth_credential=oauth2_credentials.model_copy(deep=True),
+    )
+
+    handler = AuthHandler(config)
+    result = handler.generate_auth_uri()
+
+    assert (
+        result.oauth2.auth_uri
+        == "https://example.com/oauth2/token?client_id=mock_client_id&scope="
+    )
+    assert result.oauth2.state == "mock_state"
+
 
 class TestGenerateAuthRequest:
   """Tests for the generate_auth_request method."""

tests/unittests/auth/test_oauth2_credential_util.py

@@ -19,6 +19,7 @@
 from authlib.oauth2.rfc6749 import OAuth2Token
 from fastapi.openapi.models import OAuth2
 from fastapi.openapi.models import OAuthFlowAuthorizationCode
+from fastapi.openapi.models import OAuthFlowClientCredentials
 from fastapi.openapi.models import OAuthFlows
 from google.adk.auth.auth_credential import AuthCredential
 from google.adk.auth.auth_credential import AuthCredentialTypes
@@ -207,6 +208,30 @@ def test_create_oauth2_session_oauth2_scheme_with_token_endpoint_auth_method(
     assert token_endpoint == "https://example.com/token"
     assert client.token_endpoint_auth_method == "client_secret_jwt"
 
+  def test_create_oauth2_session_client_credentials_with_missing_scopes(self):
+    """Test client credentials flow tolerates missing scopes."""
+    flows = OAuthFlows(
+        clientCredentials=OAuthFlowClientCredentials(
+            tokenUrl="https://example.com/token"
+        )
+    )
+    flows.clientCredentials.scopes = None
+    scheme = OAuth2(type_="oauth2", flows=flows)
+    credential = AuthCredential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        oauth2=OAuth2Auth(
+            client_id="test_client_id",
+            client_secret="test_client_secret",
+            redirect_uri="https://example.com/callback",
+        ),
+    )
+
+    client, token_endpoint = create_oauth2_session(scheme, credential)
+
+    assert client is not None
+    assert token_endpoint == "https://example.com/token"
+    assert client.scope == ""
+
   def test_update_credential_with_tokens(self):
     """Test update_credential_with_tokens function."""
     credential = AuthCredential(