def nlg_response_format_spec():
    """Expected response schema for an NLG endpoint.

    Used for validation of the response returned from the NLG endpoint."""
    return {'type': 'object', 'properties': {'text': {'type': 'string'},
        'buttons': {'type': ['array', 'null'], 'items': {'type': 'object'}},
        'elements': {'type': ['array', 'null'], 'items': {'type': 'object'}
        }, 'attachment': {'type': ['object', 'null']}, 'image': {'type': [
        'string', 'null']}}}
