How to clear recveive buffer after read when buffer is reused.

I am using REST protocol with BEAST https. I try to reuse the beast::buffer::flat_buffer.

But I found each time I send a request and read from http::response. the buffer didn't clear the result from previous request.
For example, first time the server send back "[]", the second time I read is "[][next time result]".

I called response->clear() and buff->consume(0); it does'nt work.

How to clear the buffer after I have processed the result and prepare the buffer for next time read?