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_jruby_spec.rb b/elasticsearch/spec/unit/headers_jruby_spec.rb
new file mode 100644
index 000000000..0255acf02
--- /dev/null
+++ b/elasticsearch/spec/unit/headers_jruby_spec.rb
@@ -0,0 +1,111 @@
+# Licensed to Elasticsearch B.V. under one or more contributor
+# license agreements. See the NOTICE file distributed with
+# this work for additional information regarding copyright
+# ownership. Elasticsearch B.V. licenses this file to you under
+# the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+return unless defined?(JRUBY_VERSION)
+
+require 'spec_helper'
+require 'ostruct'
+
+describe Elasticsearch::Client do
+  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
+
+    let(:client_headers) do
+      client.transport.connections.connections.first.connection.instance_variable_get('@options')[:headers]
+    end
+
+    context 'when Content-Type header is used' do
+      let(:instance_headers) do
+        { 'Content-Type' => 'application/text' }
+      end
+
+      it 'does not duplicate the content-type header' do
+        expect(client_headers['accept']).to eq 'application/vnd.elasticsearch+json; compatible-with=9'
+        expect(client_headers['content-type']).to be nil
+        expect(client_headers.fetch('Content-Type')).to eq 'application/text'
+
+        expect_any_instance_of(Manticore::Client)
+          .to receive(:get).with(
+                'http://localhost:9200/_search',
+                { headers: client_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 the Content-Type header' do
+          expect(client_headers['accept']).to eq 'application/vnd.elasticsearch+json; compatible-with=9'
+          expect(client_headers['content-type']).to be nil
+          expect(client_headers.fetch('Content-Type')).to eq 'application/text'
+
+          expect_any_instance_of(Manticore::Client)
+            .to receive(:get).with(
+                  'http://localhost:9200/_search',
+                  { headers: client_headers }
+                ) { OpenStruct.new(body: '') }
+          client.search
+        end
+      end
+
+      context 'when Content-Type is set in request' do
+        let(:instance_headers) do
+          { 'Content-Type' => 'instance_headers' }
+        end
+        let(:request_headers) { { 'Content-Type' => 'request headers' } }
+
+        it 'does not duplicate content-type headers' do
+          expect_any_instance_of(Manticore::Client)
+            .to receive(:get).with(
+                  'http://localhost:9200/_search',
+                  { headers: client_headers.merge(request_headers) }
+                ) { OpenStruct.new(body: '') }
+          client.search(headers: request_headers)
+          puts client_headers.merge(request_headers)
+        end
+      end
+
+      context 'when content-type is set in request' do
+        let(:instance_headers) do
+          { 'Content-Type' => 'instance_headers' }
+        end
+        let(:request_headers) { { 'content-type' => 'request headers' } }
+
+        it 'does not duplicate content-type headers' do
+          expect_any_instance_of(Manticore::Client)
+            .to receive(:get).with(
+                  'http://localhost:9200/_search',
+                  { headers: client_headers.merge(request_headers) }
+                ) { OpenStruct.new(body: '') }
+          client.search(headers: request_headers)
+          puts client_headers.merge(request_headers)
+          
+        end
+      end
+    end
+  end
+end
diff --git a/elasticsearch/spec/unit/headers_spec.rb b/elasticsearch/spec/unit/headers_spec.rb
index 3751c7448..a6155032d 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
@@ -98,7 +96,30 @@
 
       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
@@ -117,7 +138,7 @@
 
       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