Reading about how curl handle and encode parameters to understand what happened on requests with values containing spaces, like following

curl-encodeurl.md

curl -vvv -X POST "http://localhost:4000/set?somekey=spaced value"

Running this command, we have:

POST /set?key=k ey HTTP/1.1 Host: localhost:4000 User-Agent: curl/7.61.0 Accept: /

< HTTP/1.1 400 Bad Request

In this example, curl doesn't encode URL data automatically and we have the following error on server log:

INFO:tornado.general:Malformed HTTP message from ::1: Malformed HTTP request line

To send this as valid requests, as expected from the begining, it should be send with two main changes:

1. Split the querystring part and move to option --data-urlencode. Using this option, curl automatically consider this request as POST;
2. Add -G option to send all --data-urlencode value on querystring, as GET. It can still be combined with -XPOST option to send as POST.

The final command is:

curl -vvv -G -X POST "http://localhost:4000/set" --data-urlencode "somekey=spaced value"

Running this, we have the encoded data being sent on the query string part:

POST /set?key=k%20%20y HTTP/1.1 Host: localhost:4000 User-Agent: curl/7.61.0 Accept: /

< HTTP/1.1 201 Created

On server LOG, now, we receive:

INFO:tornado.access:201 POST /set?key=k%20%20y (::1) 0.91ms

The reference where I found this helpful info, was the following link: https://www.systutorials.com/241492/how-to-encode-spaces-in-curl-get-request-url-on-linux/