I'm running rails application with Puma server and every response has some hex numbers at the start (looks like body length) and the end (always zero) of each response.
HTTP/1.1 200 OK
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
X-Content-Type-Options: nosniff
X-UA-Compatible: chrome=1
Content-Type: application/json; charset=utf-8
ETag: "8e6d795d26a3839c23c0b28f7b2e3c08"
Cache-Control: max-age=0, private, must-revalidate
Set-Cookie: [skip]; path=/; HttpOnly
X-Request-Id: 741cdbde-39a8-4b99-9af7-15a7e65ff1ac
X-Runtime: 0.202731
Transfer-Encoding: chunked
f <-- THIS
{"response":{}}
0 <-- AND THIS
What is this?
That's the way chunked transfer encoding works. It sends the body in pieces, preceded by a line containing the length of the piece in hexadecimal. The end of the body is indicated by a chunk with 0 length.
So what you're showing is that the first chunk is 15 characters long, and there's no second chunk.
A proper HTTP client or API should strip these chunk headers out, and just return the contents.
See Wikipedia for more details, or RFC 2616 for the official spec.