Minimal reproduction (requires a webserver running on localhost:9292):

require "rubygems"
require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "async-http-faraday", require: false
end

require "faraday"
require "async/http/faraday"

Faraday.default_adapter = :async_http

conn = Faraday.new(url: "http://localhost:9292")

conn.post("/")

Here's the raw request I get, notice the double content-length (although with different capitalization):

POST / HTTP/1.1\r\n
host: localhost:9200\r\n
User-Agent: Faraday v2.12.2\r\n
Content-Length: 0\r\n
content-length: 0\r\n
\r\n

Changing the Faraday adapter to :net_http doesn't have this problem.

The capitalized Content-Length seems to be set by Faraday itself at Faraday::Env#clear_body. Async::HTTP seems to be adding the lower-case one.

I also noticed that setting the body to empty string doesn't suffer from this issue, but I can't do that in my case since I'm using a 3rd party library:

conn.post("/") do |req|
  req.body = ""
end

Results in:

POST / HTTP/1.1\r\n
host: localhost:9200\r\n
User-Agent: Faraday v2.12.2\r\n
Content-Type: application/x-www-form-urlencoded\r\n
content-length: 0\r\n
\r\n