Introduce normalize_oauth2_tokens

src/google/adk/auth/exchanger/oauth2_credential_exchanger.py

@@ -25,6 +25,7 @@
 from google.adk.auth.auth_schemes import OAuthGrantType
 from google.adk.auth.auth_schemes import OpenIdConnectWithConfig
 from google.adk.auth.oauth2_credential_util import create_oauth2_session
+from google.adk.auth.oauth2_credential_util import normalize_oauth2_tokens
 from google.adk.auth.oauth2_credential_util import update_credential_with_tokens
 from google.adk.utils.feature_decorator import experimental
 from typing_extensions import override
@@ -193,19 +194,35 @@ async def _exchange_authorization_code(
       return ExchangeResult(auth_credential, False)
 
     try:
-      tokens = client.fetch_token(
-          token_endpoint,
-          authorization_response=self._normalize_auth_uri(
+      token_auth_method = (
+          auth_credential.oauth2.token_endpoint_auth_method
+          if auth_credential.oauth2
+          else None
+      )
+      fetch_token_kwargs = {
+          'authorization_response': self._normalize_auth_uri(
               auth_credential.oauth2.auth_response_uri
           ),
-          code=auth_credential.oauth2.auth_code,
-          grant_type=OAuthGrantType.AUTHORIZATION_CODE,
-          client_id=auth_credential.oauth2.client_id,
-      )
+          'code': auth_credential.oauth2.auth_code,
+          'grant_type': OAuthGrantType.AUTHORIZATION_CODE,
+      }
+      # For client_secret_post, Authlib already includes client_id in POST body
+      # from OAuth2Session. Passing client_id again can duplicate parameters.
+      if token_auth_method != 'client_secret_post':
+        fetch_token_kwargs['client_id'] = auth_credential.oauth2.client_id
+
+      tokens = client.fetch_token(token_endpoint, **fetch_token_kwargs)
+      tokens = normalize_oauth2_tokens(tokens)
       update_credential_with_tokens(auth_credential, tokens)
       logger.debug("Successfully exchanged authorization code for access token")
     except Exception as e:
-      logger.error("Failed to exchange authorization code: %s", e)
+      logger.error(
+          "Failed to exchange authorization code (token_endpoint_auth_method=%s): %s",
+          auth_credential.oauth2.token_endpoint_auth_method
+          if auth_credential.oauth2
+          else None,
+          e,
+      )
       return ExchangeResult(auth_credential, False)
 
     return ExchangeResult(auth_credential, True)

src/google/adk/auth/oauth2_credential_util.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Mapping
 import logging
 from typing import Optional
 from typing import Tuple
@@ -97,6 +98,17 @@ def create_oauth2_session(
   )
 
 
+@experimental
+def normalize_oauth2_tokens(tokens: object) -> OAuth2Token:
+  """Validates and normalizes token payload returned by OAuth libraries."""
+  if not isinstance(tokens, Mapping):
+    raise ValueError(
+        "OAuth2 token response must be a dict-like object, got "
+        f"{type(tokens).__name__}."
+    )
+  return tokens  # type: ignore[return-value]
+
+
 @experimental
 def update_credential_with_tokens(
     auth_credential: AuthCredential, tokens: OAuth2Token
@@ -107,6 +119,7 @@ def update_credential_with_tokens(
       auth_credential: The authentication credential to update.
       tokens: The OAuth2Token object containing new token information.
   """
+  tokens = normalize_oauth2_tokens(tokens)
   auth_credential.oauth2.access_token = tokens.get("access_token")
   auth_credential.oauth2.refresh_token = tokens.get("refresh_token")
   auth_credential.oauth2.expires_at = (

src/google/adk/auth/refresher/oauth2_credential_refresher.py

@@ -23,6 +23,7 @@
 from google.adk.auth.auth_credential import AuthCredential
 from google.adk.auth.auth_schemes import AuthScheme
 from google.adk.auth.oauth2_credential_util import create_oauth2_session
+from google.adk.auth.oauth2_credential_util import normalize_oauth2_tokens
 from google.adk.auth.oauth2_credential_util import update_credential_with_tokens
 from google.adk.utils.feature_decorator import experimental
 from google.auth.transport.requests import Request
@@ -115,6 +116,7 @@ async def refresh(
               url=token_endpoint,
               refresh_token=auth_credential.oauth2.refresh_token,
           )
+          tokens = normalize_oauth2_tokens(tokens)
           update_credential_with_tokens(auth_credential, tokens)
           logger.debug("Successfully refreshed OAuth2 tokens")
         except Exception as e: