fix(auth): Support fallback OAuth token and prefixless credential lookups in session state

Session state might store authentication responses as raw string tokens instead of AuthCredential objects, or under custom credential keys without the standard "temp:" prefix.

Add robust fallback handling to resolve raw token strings, check for prefixless keys, and scan state values for any Google OAuth access tokens starting with "ya29."

Author: tonycoco

src/google/adk/auth/auth_handler.py

@@ -73,9 +73,43 @@ def _validate(self) -> None:
     if not self.auth_scheme:
       raise ValueError("auth_scheme is empty.")
 
-  def get_auth_response(self, state: State) -> AuthCredential:
+  def get_auth_response(self, state: State) -> AuthCredential | None:
+    # 1. Try reading the temp credential key (standard ADK flow)
     credential_key = "temp:" + self.auth_config.credential_key
-    return state.get(credential_key, None)
+    val = state.get(credential_key, None)
+    if val is not None:
+      if isinstance(val, AuthCredential):
+        return val
+      if isinstance(val, str) and val:
+        return self._build_oauth2_credential(val)
+
+    # 2. Try reading the credential key without the 'temp:' prefix
+    val = state.get(self.auth_config.credential_key, None)
+    if val is not None:
+      if isinstance(val, AuthCredential):
+        return val
+      if isinstance(val, str) and val:
+        return self._build_oauth2_credential(val)
+
+    # 3. Fallback: scan the state for any active Google OAuth access token (ya29.*)
+    try:
+      state_dict = state.to_dict() if hasattr(state, "to_dict") else state
+      for k, v in state_dict.items():
+        if isinstance(v, str) and v.startswith("ya29."):
+          return self._build_oauth2_credential(v)
+    except Exception:  # pylint: disable=broad-except
+      pass
+
+    return None
+
+  def _build_oauth2_credential(self, token: str) -> AuthCredential:
+    from .auth_credential import AuthCredentialTypes
+    from .auth_credential import OAuth2Auth
+
+    return AuthCredential(
+        auth_type=AuthCredentialTypes.OAUTH2,
+        oauth2=OAuth2Auth(access_token=token),
+    )
 
   def generate_auth_request(self) -> AuthConfig:
     if not isinstance(

tests/unittests/auth/test_auth_handler.py

@@ -503,6 +503,57 @@ def test_get_auth_response_not_exists(self, auth_config):
     result = handler.get_auth_response(state)
     assert result is None
 
+  def test_get_auth_response_temp_prefix_str_token(self, auth_config):
+    """Test retrieving a string token stored under temp prefix in state."""
+    handler = AuthHandler(auth_config)
+    state = MockState()
+    credential_key = auth_config.credential_key
+    state["temp:" + credential_key] = "ya29.mock_token"
+
+    result = handler.get_auth_response(state)
+
+    assert result is not None
+    assert result.auth_type == AuthCredentialTypes.OAUTH2
+    assert result.oauth2.access_token == "ya29.mock_token"
+
+  def test_get_auth_response_no_prefix_credential(
+      self, auth_config, oauth2_credentials_with_auth_uri
+  ):
+    """Test retrieving a credential stored under the key without prefix."""
+    handler = AuthHandler(auth_config)
+    state = MockState()
+    credential_key = auth_config.credential_key
+    state[credential_key] = oauth2_credentials_with_auth_uri
+
+    result = handler.get_auth_response(state)
+
+    assert result == oauth2_credentials_with_auth_uri
+
+  def test_get_auth_response_no_prefix_str_token(self, auth_config):
+    """Test retrieving a string token stored under the key without prefix."""
+    handler = AuthHandler(auth_config)
+    state = MockState()
+    credential_key = auth_config.credential_key
+    state[credential_key] = "ya29.mock_token_no_prefix"
+
+    result = handler.get_auth_response(state)
+
+    assert result is not None
+    assert result.auth_type == AuthCredentialTypes.OAUTH2
+    assert result.oauth2.access_token == "ya29.mock_token_no_prefix"
+
+  def test_get_auth_response_fallback_google_token(self, auth_config):
+    """Test retrieving fallback Google token from state via scanning."""
+    handler = AuthHandler(auth_config)
+    state = MockState()
+    state["some_other_key"] = "ya29.fallback_google_token"
+
+    result = handler.get_auth_response(state)
+
+    assert result is not None
+    assert result.auth_type == AuthCredentialTypes.OAUTH2
+    assert result.oauth2.access_token == "ya29.fallback_google_token"
+
 
 class TestParseAndStoreAuthResponse:
   """Tests for the parse_and_store_auth_response method."""