As @jswanner said (thanks!), XML is decoded only when you use the s3:// scheme. I just released v0.2.2 that adds an :aws_endpoint_url_s3 option so you can use s3:// with a custom URL. This should now work:

def new_req(options \\ []) when is_list(options) do
  access_key = System.fetch_env!("MINIO_ACCESS_KEY")
  secret_access_key = System.fetch_env!("MINIO_SECRET_KEY")
  endpoint_url = System.fetch_env!("MINIO_URL")
  Req.new(
    base_url: "s3://justrunit",
    aws_sigv4: [access_key_id: access_key, secret_access_key: secret_access_key],
    aws_endpoint_url_s3: endpoint_url,
    retry: :transient
  )
  |> ReqS3.attach()
  |> Req.merge(options)
end

(I assume justrunit is the name of the bucket.)

Btw, if you set AWS_ENDPOINT_URL_S3, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, the following would work too:

def new_req(options \\ []) when is_list(options) do
  Req.new(
    base_url: "s3://justrunit"
    retry: :transient
  )
  |> ReqS3.attach()
  |> Req.merge(options)
end

And here’s a version that doesn’t rely on global things:

# docker run -d -p 9000:9000 -p 9001:9001 minio/minio server /data --console-address :9001
Mix.install([
  {:req_s3, github: "wojtekmach/req_s3", ref: "81ea8f6"}
])

req =
  Req.new()
  |> ReqS3.attach()
  |> Req.merge(
    aws_sigv4: [
      access_key_id: "minioadmin",
      secret_access_key: "minioadmin"
    ],
    aws_endpoint_url_s3: "http://localhost:9000"
  )

# create bucket
%{status: status} = Req.put!(req, url: "s3://bucket1")
true = status in [200, 409]

# create object
%{status: 200} = Req.put!(req, url: "s3://bucket1/object1", body: "value1")

# list objects
%{status: 200, body: body} = Req.get!(req, url: "s3://bucket1")
dbg(body)

@Wojciech, just to confirm: your endpoint URL starts with “s3://” correct?

When using ReqS3 plugin, s3:// gets special treatment but here you should use http://. Apologies if this is confusing, I’ll try to improve docs.

Seems you are getting econnrefused so perhaps port 9000 is not bound, there are problems with connectivity between containers etc.

Could you create a minimal reproduction, eg a command to run minio and a single file .exs that uses Mix.install?