diff --git a/src/google/adk/tools/openapi_tool/auth/auth_helpers.py b/src/google/adk/tools/openapi_tool/auth/auth_helpers.py
index 2c8ae5bb430..78175e6b8ce 100644
--- a/src/google/adk/tools/openapi_tool/auth/auth_helpers.py
+++ b/src/google/adk/tools/openapi_tool/auth/auth_helpers.py
@@ -26,6 +26,8 @@
 from fastapi.openapi.models import HTTPBase
 from fastapi.openapi.models import HTTPBearer
 from fastapi.openapi.models import OAuth2
+from fastapi.openapi.models import OAuthFlowClientCredentials
+from fastapi.openapi.models import OAuthFlows
 from fastapi.openapi.models import OpenIdConnect
 from fastapi.openapi.models import Schema
 import httpx
@@ -152,13 +154,40 @@ def token_to_scheme_credential(
     raise ValueError(f"Invalid security scheme type: {type}")
 
 
+def _service_account_auth_scheme() -> OAuth2:
+  """Auth scheme for Google Service Account credentials.
+
+  CredentialManager only auto-loads raw non-interactive credentials when the
+  scheme is an OAuth2/OIDC client-credentials flow. An HTTPBearer scheme makes
+  ``_is_client_credentials_flow`` return False, so ``get_auth_credential``
+  returns None and the tool falls back to ``adk_request_credential`` instead of
+  exchanging the service account for a token.
+
+  The token URL is unused by ServiceAccountCredentialExchanger (ADC / JWT
+  assertion), but is required by the OAuth2 client-credentials model.
+  """
+  return OAuth2(
+      flows=OAuthFlows(
+          clientCredentials=OAuthFlowClientCredentials(
+              # Placeholder only; SA exchange does not call this endpoint.
+              # Use the mTLS host form for compliance with Google API endpoint
+              # requirements.
+              tokenUrl="https://oauth2.mtls.googleapis.com/token",
+              scopes={},
+          )
+      )
+  )
+
+
 def service_account_dict_to_scheme_credential(
     config: Dict[str, Any],
     scopes: List[str],
 ) -> Tuple[AuthScheme, AuthCredential]:
   """Creates AuthScheme and AuthCredential for Google Service Account.
 
-  Returns a bearer token scheme, and a service account credential.
+  Returns an OAuth2 client-credentials scheme (so CredentialManager can
+  exchange the service account) and a service account credential. After
+  exchange the credential is an HTTP bearer token.
 
   Args:
       config: A ServiceAccount object containing the Google Service Account
@@ -168,7 +197,6 @@ def service_account_dict_to_scheme_credential(
   Returns:
       Tuple: (AuthScheme, AuthCredential)
   """
-  auth_scheme = HTTPBearer(bearerFormat="JWT")
   service_account = ServiceAccount(
       service_account_credential=ServiceAccountCredential.model_construct(
           **config
@@ -179,7 +207,7 @@ def service_account_dict_to_scheme_credential(
       auth_type=AuthCredentialTypes.SERVICE_ACCOUNT,
       service_account=service_account,
   )
-  return auth_scheme, auth_credential
+  return _service_account_auth_scheme(), auth_credential
 
 
 def service_account_scheme_credential(
@@ -187,7 +215,9 @@ def service_account_scheme_credential(
 ) -> Tuple[AuthScheme, AuthCredential]:
   """Creates AuthScheme and AuthCredential for Google Service Account.
 
-  Returns a bearer token scheme, and a service account credential.
+  Returns an OAuth2 client-credentials scheme (so CredentialManager can
+  exchange the service account) and a service account credential. After
+  exchange the credential is an HTTP bearer token.
 
   Args:
       config: A ServiceAccount object containing the Google Service Account
@@ -196,11 +226,10 @@ def service_account_scheme_credential(
   Returns:
       Tuple: (AuthScheme, AuthCredential)
   """
-  auth_scheme = HTTPBearer(bearerFormat="JWT")
   auth_credential = AuthCredential(
       auth_type=AuthCredentialTypes.SERVICE_ACCOUNT, service_account=config
   )
-  return auth_scheme, auth_credential
+  return _service_account_auth_scheme(), auth_credential
 
 
 def openid_dict_to_scheme_credential(
diff --git a/tests/unittests/tools/openapi_tool/auth/test_auth_helper.py b/tests/unittests/tools/openapi_tool/auth/test_auth_helper.py
index 3f5e8f07b55..8a569f1937e 100644
--- a/tests/unittests/tools/openapi_tool/auth/test_auth_helper.py
+++ b/tests/unittests/tools/openapi_tool/auth/test_auth_helper.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest.mock import AsyncMock
+from unittest.mock import Mock
 from unittest.mock import patch
 
 from fastapi.openapi.models import APIKey
@@ -28,6 +30,8 @@
 from google.adk.auth.auth_credential import ServiceAccountCredential
 from google.adk.auth.auth_schemes import AuthSchemeType
 from google.adk.auth.auth_schemes import OpenIdConnectWithConfig
+from google.adk.auth.auth_tool import AuthConfig
+from google.adk.auth.credential_manager import CredentialManager
 from google.adk.tools.openapi_tool.auth.auth_helpers import credential_to_param
 from google.adk.tools.openapi_tool.auth.auth_helpers import dict_to_auth_scheme
 from google.adk.tools.openapi_tool.auth.auth_helpers import INTERNAL_AUTH_PREFIX
@@ -133,8 +137,10 @@ def test_service_account_dict_to_scheme_credential():
 
   scheme, credential = service_account_dict_to_scheme_credential(config, scopes)
 
-  assert isinstance(scheme, HTTPBearer)
-  assert scheme.bearerFormat == "JWT"
+  assert isinstance(scheme, OAuth2)
+  assert scheme.flows is not None
+  assert scheme.flows.clientCredentials is not None
+  assert scheme.flows.clientCredentials.tokenUrl
   assert credential.auth_type == AuthCredentialTypes.SERVICE_ACCOUNT
   assert credential.service_account.scopes == scopes
   assert (
@@ -163,12 +169,54 @@ def test_service_account_scheme_credential():
 
   scheme, credential = service_account_scheme_credential(config)
 
-  assert isinstance(scheme, HTTPBearer)
-  assert scheme.bearerFormat == "JWT"
+  assert isinstance(scheme, OAuth2)
+  assert scheme.flows is not None
+  assert scheme.flows.clientCredentials is not None
+  assert scheme.flows.clientCredentials.tokenUrl
   assert credential.auth_type == AuthCredentialTypes.SERVICE_ACCOUNT
   assert credential.service_account == config
 
 
+@pytest.mark.asyncio
+async def test_service_account_helper_scheme_allows_credential_manager_exchange():
+  """SA helpers must yield a client-credentials scheme (#6656).
+
+  With HTTPBearer, CredentialManager treated the SA as needing interactive
+  auth and returned None (adk_request_credential) instead of exchanging it.
+  """
+  scheme, credential = service_account_scheme_credential(
+      ServiceAccount(
+          use_default_credential=True,
+          scopes=["https://www.googleapis.com/auth/cloud-platform"],
+      )
+  )
+  manager = CredentialManager(
+      AuthConfig(auth_scheme=scheme, raw_auth_credential=credential)
+  )
+  assert manager._is_client_credentials_flow()  # pylint: disable=protected-access
+
+  exchanged = AuthCredential(
+      auth_type=AuthCredentialTypes.HTTP,
+      http=HttpAuth(
+          scheme="bearer",
+          credentials=HttpCredentials(token="sa-access-token"),
+      ),
+  )
+  manager._load_existing_credential = AsyncMock(return_value=None)  # pylint: disable=protected-access
+  manager._exchange_credential = AsyncMock(return_value=(exchanged, True))  # pylint: disable=protected-access
+  manager._refresh_credential = AsyncMock(return_value=(exchanged, False))  # pylint: disable=protected-access
+  manager._save_credential = AsyncMock()  # pylint: disable=protected-access
+
+  ctx = Mock()
+  ctx.get_auth_response = Mock(return_value=None)
+  result = await manager.get_auth_credential(ctx)
+
+  assert result is not None
+  assert result.auth_type == AuthCredentialTypes.HTTP
+  assert result.http.credentials.token == "sa-access-token"
+  manager._exchange_credential.assert_awaited_once()  # pylint: disable=protected-access
+
+
 def test_openid_dict_to_scheme_credential():
   config_dict = {
       "authorization_endpoint": "auth_url",