diff --git a/lib/httpx/transcoder/json.rb b/lib/httpx/transcoder/json.rb
index 1b6bb8e2..2ab82b93 100644
--- a/lib/httpx/transcoder/json.rb
+++ b/lib/httpx/transcoder/json.rb
@@ -42,17 +42,17 @@ module HTTPX::Transcoder
     # rubocop:disable Style/SingleLineMethods
     if defined?(MultiJson)
       def json_load(*args); MultiJson.load(*args); end
-      def json_dump(*args); MultiJson.dump(*args); end
+      def json_dump(obj); MultiJson.dump(obj); end
     elsif defined?(Oj)
       def json_load(response, *args); Oj.load(response.to_s, *args); end
-      def json_dump(*args); Oj.dump(*args); end
+      def json_dump(obj); Oj.dump(obj, mode: :compat); end
     elsif defined?(Yajl)
       def json_load(response, *args); Yajl::Parser.new(*args).parse(response.to_s); end
-      def json_dump(*args); Yajl::Encoder.encode(*args); end
+      def json_dump(obj); Yajl::Encoder.encode(obj); end
     else
       require "json"
       def json_load(*args); ::JSON.parse(*args); end
-      def json_dump(*args); ::JSON.dump(*args); end
+      def json_dump(obj); ::JSON.dump(obj); end
     end
     # rubocop:enable Style/SingleLineMethods
   end
diff --git a/sig/transcoder/json.rbs b/sig/transcoder/json.rbs
index a705960d..34dfd31c 100644
--- a/sig/transcoder/json.rbs
+++ b/sig/transcoder/json.rbs
@@ -6,7 +6,7 @@ module HTTPX::Transcoder
     def self?.decode: (HTTPX::Response response) -> _Decoder
 
     def self?.json_load: (string source, ?json_options) -> untyped
-    def self?.json_dump: (_ToJson obj, *untyped) -> String
+    def self?.json_dump: (_ToJson obj) -> String
 
     class Encoder
       extend Forwardable
diff --git a/standalone_tests/response_json_multi_json_test.rb b/standalone_tests/json_multi_json_test.rb
similarity index 70%
rename from standalone_tests/response_json_multi_json_test.rb
rename to standalone_tests/json_multi_json_test.rb
index 3d1e8b3a..aed74ceb 100644
--- a/standalone_tests/response_json_multi_json_test.rb
+++ b/standalone_tests/json_multi_json_test.rb
@@ -15,10 +15,15 @@ class ResponseYajlTest < Minitest::Test
     assert_raises(MultiJson::ParseError) { json_response.json }
   end
 
+  def test_request_encoders
+    json_request = request json: { :a => "b" }
+    assert json_request.body.to_s == %({"a":"b"})
+  end
+
   private
 
-  def request(verb = "GET", uri = "http://google.com")
-    Request.new(verb, uri, Options.new)
+  def request(verb = "GET", uri = "http://google.com", **args)
+    Request.new(verb, uri, Options.new, **args)
   end
 
   def response(*args)
diff --git a/standalone_tests/response_json_oj_test.rb b/standalone_tests/json_oj_test.rb
similarity index 69%
rename from standalone_tests/response_json_oj_test.rb
rename to standalone_tests/json_oj_test.rb
index a2e7bea8..b3c70cb8 100644
--- a/standalone_tests/response_json_oj_test.rb
+++ b/standalone_tests/json_oj_test.rb
@@ -15,10 +15,15 @@ class ResponseOjTest < Minitest::Test
     assert_raises(Oj::ParseError) { json_response.json }
   end
 
+  def test_request_encoders
+    json_request = request json: { :a => "b" }
+    assert json_request.body.to_s == %({"a":"b"})
+  end
+
   private
 
-  def request(verb = "GET", uri = "http://google.com")
-    Request.new(verb, uri, Options.new)
+  def request(verb = "GET", uri = "http://google.com", **args)
+    Request.new(verb, uri, Options.new, **args)
   end
 
   def response(*args)
diff --git a/standalone_tests/response_json_yajl_test.rb b/standalone_tests/json_yajl_test.rb
similarity index 70%
rename from standalone_tests/response_json_yajl_test.rb
rename to standalone_tests/json_yajl_test.rb
index 99011074..efc45eda 100644
--- a/standalone_tests/response_json_yajl_test.rb
+++ b/standalone_tests/json_yajl_test.rb
@@ -15,10 +15,15 @@ class ResponseYajlTest < Minitest::Test
     assert_raises(Yajl::ParseError) { json_response.json }
   end
 
+  def test_request_encoders
+    json_request = request json: { :a => "b" }
+    assert json_request.body.to_s == %({"a":"b"})
+  end
+
   private
 
-  def request(verb = "GET", uri = "http://google.com")
-    Request.new(verb, uri, Options.new)
+  def request(verb = "GET", uri = "http://google.com", **args)
+    Request.new(verb, uri, Options.new, **args)
   end
 
   def response(*args)