diff --git a/Gemfile b/Gemfile
index 8a2a449..41411dd 100644
--- a/Gemfile
+++ b/Gemfile
@@ -15,7 +15,7 @@ gem 'net-http-persistent', '~> 4.0',    :require => false
 gem 'http',                             :require => false
 
 # adapter extensions
-gem 'rack', '< 3'
+gem 'rack'
 gem 'socksify'
 
 # coverage
diff --git a/httpi.gemspec b/httpi.gemspec
index 6abbba4..8930694 100644
--- a/httpi.gemspec
+++ b/httpi.gemspec
@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
 
   s.license = 'MIT'
 
-  s.add_dependency 'rack', '< 3'
+  s.add_dependency 'rack', '>= 2.0', '< 3.1'
   s.add_dependency 'nkf'
   s.add_dependency 'base64'
   s.add_dependency 'mutex_m'
@@ -30,7 +30,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake',     '~> 13.0'
   s.add_development_dependency 'rspec',    '~> 3.5'
   s.add_development_dependency 'mocha',    '~> 0.13'
-  s.add_development_dependency 'puma',     '~> 5.0'
+  s.add_development_dependency 'puma',     '~> 6.0'
   s.add_development_dependency 'webmock'
 
   s.metadata["rubygems_mfa_required"] = "true"
diff --git a/lib/httpi/request.rb b/lib/httpi/request.rb
index 8eebf9a..786e14c 100644
--- a/lib/httpi/request.rb
+++ b/lib/httpi/request.rb
@@ -64,12 +64,12 @@ def ssl?
 
     # Returns a Hash of HTTP headers. Defaults to return an empty Hash.
     def headers
-      @headers ||= Rack::Utils::HeaderHash.new
+      @headers ||= Rack::Headers.new
     end
 
     # Sets the Hash of HTTP headers.
     def headers=(headers)
-      @headers = Rack::Utils::HeaderHash.new(headers)
+      @headers = Rack::Headers.new.merge(headers)
     end
 
     # Adds a header information to accept gzipped content.
diff --git a/lib/httpi/response.rb b/lib/httpi/response.rb
index ca1162e..f9b7c4f 100644
--- a/lib/httpi/response.rb
+++ b/lib/httpi/response.rb
@@ -20,7 +20,7 @@ class Response
     # Initializer expects an HTTP response +code+, +headers+ and +body+.
     def initialize(code, headers, body)
       self.code = code.to_i
-      self.headers = Rack::Utils::HeaderHash.new(headers)
+      self.headers = Rack::Headers.new.merge(headers)
       self.raw_body = body
     end
 
diff --git a/spec/httpi/request_spec.rb b/spec/httpi/request_spec.rb
index 3c9c730..efd88ae 100644
--- a/spec/httpi/request_spec.rb
+++ b/spec/httpi/request_spec.rb
@@ -92,11 +92,11 @@
         after { HTTPI.query_builder = :flat }
 
         it "lets you specify query parameter as Hash" do
-          expect(request.url.to_s).to eq("http://example.com?q[]=nested&q[]=query")
+          expect(request.url.to_s).to eq("http://example.com?q%5B%5D=nested&q%5B%5D=query")
         end
 
         it "getter return String for query parameter as Hash" do
-          expect(request.query).to eq("q[]=nested&q[]=query")
+          expect(request.query).to eq("q%5B%5D=nested&q%5B%5D=query")
         end
       end
     end
@@ -149,7 +149,7 @@
   describe "#headers" do
     it "lets you specify a Hash of HTTP request headers" do
       request.headers = { "Accept-Encoding" => "gzip" }
-      expect(request.headers).to eq({ "Accept-Encoding" => "gzip" })
+      expect(request.headers).to eq Rack::Headers.new.merge({ "Accept-Encoding" => "gzip" })
     end
 
     it "defaults to return an empty Hash" do
@@ -235,7 +235,7 @@ def response_with_cookie(cookie)
         end
         it "request body using a Hash with Array" do
           request.body = {:foo => :bar, :baz => [:foo, :tst]}
-          expect(request.body.split("&")).to match_array(["foo=bar", "baz[]=foo", "baz[]=tst"])
+          expect(request.body.split("&")).to match_array(["foo=bar", "baz%5B%5D=foo", "baz%5B%5D=tst"])
         end
       end
     end
diff --git a/spec/httpi/response_spec.rb b/spec/httpi/response_spec.rb
index 2941575..d797bb3 100644
--- a/spec/httpi/response_spec.rb
+++ b/spec/httpi/response_spec.rb
@@ -88,7 +88,7 @@
 
     describe "#headers" do
       it "returns the HTTP response headers" do
-        expect(response.headers).to eq({ "Content-Encoding" => "gzip" })
+        expect(response.headers).to eq Rack::Headers.new.merge({ "Content-Encoding" => "gzip" })
       end
     end
 
@@ -116,7 +116,7 @@
 
     describe "#headers" do
       it "returns the HTTP response headers" do
-        expect(response.headers).to eq({ "Content-Type" => "application/dime" })
+        expect(response.headers).to eq Rack::Headers.new.merge({ "Content-Type" => "application/dime" })
       end
     end
 
diff --git a/spec/integration/support/server.rb b/spec/integration/support/server.rb
index 368e27a..a7ae442 100644
--- a/spec/integration/support/server.rb
+++ b/spec/integration/support/server.rb
@@ -57,7 +57,7 @@ def stop
   private
 
   def events
-    Puma::Events.new($stdout, $stderr)
+    Puma::Events.new
   end
 
   def add_tcp_listener