def _handle_existing_credential(
      self,
  ) -> Optional[AuthPreparationResult]:
    """Checks for and returns an existing, exchanged credential."""
    if self.credential_store:
      existing_credential = self.credential_store.get_credential(
          self.auth_scheme, self.auth_credential
      )
      if existing_credential:
        return AuthPreparationResult(
            state="done",
            auth_scheme=self.auth_scheme,
            auth_credential=existing_credential,
        )
    return None
Checks for and returns an existing, exchanged credential.
src/google/adk/tools/openapi_tool/openapi_spec_parser/tool_auth_handler.py