diff --git a/elasticsearch/Gemfile b/elasticsearch/Gemfile
index 21c38fe23..9fd65ae5b 100644
--- a/elasticsearch/Gemfile
+++ b/elasticsearch/Gemfile
@@ -31,3 +31,6 @@ if ENV['TRANSPORT_VERSION'] == 'main'
 end
 
 gem 'opentelemetry-sdk', require: false if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.0')
+if defined?(JRUBY_VERSION)
+  gem 'manticore', platform: :jruby
+end
diff --git a/elasticsearch/lib/elasticsearch.rb b/elasticsearch/lib/elasticsearch.rb
index 3bf2e7930..e27c79cd4 100644
--- a/elasticsearch/lib/elasticsearch.rb
+++ b/elasticsearch/lib/elasticsearch.rb
@@ -189,10 +189,10 @@ def set_user_agent!(arguments)
     def set_content_type!(arguments)
       headers = {}
       user_headers = arguments&.[](:transport_options)&.[](:headers)
-      unless user_headers&.keys&.detect { |h| h =~ /content-?_?type/ }
+      unless user_headers&.keys&.detect { |h| h =~ /content-?_?type/i }
         headers['content-type'] = 'application/vnd.elasticsearch+json; compatible-with=9'
       end
-      unless user_headers&.keys&.detect { |h| h =~ /accept/ }
+      unless user_headers&.keys&.detect { |h| h =~ /accept/i }
         headers['accept'] = 'application/vnd.elasticsearch+json; compatible-with=9'
       end
       set_header(headers, arguments) unless headers.empty?
diff --git a/elasticsearch/spec/unit/headers_spec.rb b/elasticsearch/spec/unit/headers_spec.rb
index 3751c7448..f8d516fc2 100644
--- a/elasticsearch/spec/unit/headers_spec.rb
+++ b/elasticsearch/spec/unit/headers_spec.rb
@@ -73,20 +73,18 @@
 
       expect_any_instance_of(Faraday::Connection)
         .to receive(:run_request)
-              .with(:get, 'http://localhost:9200/_search', nil, connection_headers) { OpenStruct.new(body: '') }
+          .with(:get, 'http://localhost:9200/_search', nil, connection_headers) { OpenStruct.new(body: '') }
       client.search
     end
   end
 
   context 'when content-type header is changed' do
     let!(:client) do
-      described_class.new(
-        host: 'http://localhost:9200',
-        transport_options: { headers: instance_headers }
-      ).tap do |client|
+      described_class.new(transport_options: { headers: instance_headers }).tap do |client|
         client.instance_variable_set('@verified', true)
       end
     end
+
     let(:instance_headers) do
       { content_type: 'application/json' }
     end
@@ -98,11 +96,88 @@
 
       expect_any_instance_of(Faraday::Connection)
         .to receive(:run_request)
-              .with(:get, 'http://localhost:9200/_search', nil, connection_headers) { OpenStruct.new(body: '') }
+          .with(:get, 'http://localhost:9200/_search', nil, connection_headers) { OpenStruct.new(body: '') }
+      client.search
+    end
+  end
+
+  context 'when Content-Type header is used' do
+    let(:client) do
+      described_class.new(transport_options: { headers: instance_headers }).tap do |client|
+        client.instance_variable_set('@verified', true)
+      end
+    end
+
+    let(:instance_headers) do
+      { 'Content-Type' => 'application/text' }
+    end
+
+    it 'performs the request with the header' do
+      connection_headers = client.transport.connections.connections.first.connection.headers
+      expect(connection_headers['Accept']).to eq 'application/vnd.elasticsearch+json; compatible-with=9'
+      expect(connection_headers['Content-Type']).to eq 'application/text'
+
+      expect_any_instance_of(Faraday::Connection)
+        .to receive(:run_request)
+          .with(:get, 'http://localhost:9200/_search', nil, connection_headers) { OpenStruct.new(body: '') }
       client.search
     end
   end
 
+  if defined?(JRUBY_VERSION)
+    context 'Using Manticore with JRuby' do
+      let(:client) do
+        require 'elastic/transport/transport/http/manticore'
+        described_class.new(
+          transport_class: Elastic::Transport::Transport::HTTP::Manticore,
+          transport_options: { headers: instance_headers }
+        ).tap do |client|
+          client.instance_variable_set('@verified', true)
+        end
+      end
+
+      context 'when Content-Type header is used' do
+        let(:instance_headers) do
+          { 'Content-Type' => 'application/text' }
+        end
+
+        it 'does not duplicate content-type the header' do
+          connection_headers = client.transport.connections.connections.first.connection.instance_variable_get('@options')[:headers]
+          expect(connection_headers['accept']).to eq 'application/vnd.elasticsearch+json; compatible-with=9'
+          expect(connection_headers['content-type']).to be nil
+          expect(connection_headers.fetch('Content-Type')).to eq 'application/text'
+
+          expect_any_instance_of(Manticore::Client)
+            .to receive(:get).with(
+                  'http://localhost:9200/_search',
+                  { headers: connection_headers }
+                ) { OpenStruct.new(body: '') }
+          client.search
+        end
+
+        context 'when content-type header is used' do
+          let(:instance_headers) do
+            { 'content-type' => 'application/text' }
+          end
+
+          it 'does not duplicate Content-Type the header' do
+            connection_headers = client.transport.connections.connections.first.connection.instance_variable_get('@options')[:headers]
+            expect(connection_headers['accept']).to eq 'application/vnd.elasticsearch+json; compatible-with=9'
+            expect(connection_headers['content-type']).to be nil
+            expect(connection_headers.fetch('Content-Type')).to eq 'application/text'
+
+            expect_any_instance_of(Manticore::Client)
+              .to receive(:get).with(
+                'http://localhost:9200/_search',
+                { headers: connection_headers }
+              ) { OpenStruct.new(body: '') }
+            client.search
+          end
+        end
+      end
+    end
+  end
+
   context 'when no header is set, uses v9 content-type and accept' do
     let!(:client) do
       described_class.new(host: 'http://localhost:9200').tap do |client|
@@ -117,7 +192,7 @@
 
       expect_any_instance_of(Faraday::Connection)
         .to receive(:run_request)
-              .with(:get, 'http://localhost:9200/_search', nil, expected_headers) { OpenStruct.new(body: '') }
+          .with(:get, 'http://localhost:9200/_search', nil, expected_headers) { OpenStruct.new(body: '') }
       client.search
     end
   end