The HTTP headers appear to not be written to WARC correctly. The request headers are constructed independently of what Scrapy actually sends to the server; the response's original status line, as mentioned in a code comment, is discarded entirely; and the headers get normalised by Scrapy (in scrapy.http.headers.Headers). Instead, the exact bytes sent to and received from the server (on the HTTP layer) should be written to the WARC file.