diff --git a/.github/workflows/otel.yml b/.github/workflows/otel.yml
index f735fde75..ee3943f29 100644
--- a/.github/workflows/otel.yml
+++ b/.github/workflows/otel.yml
@@ -2,10 +2,10 @@ name: opentelemetry
 on:
   push:
     branches:
-      - main
+      - 9.3
   pull_request:
     branches:
-      - main
+      - 9.3
 jobs:
   test-otel:
     name: 'Test Open Telemetry'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 899a24e1c..628e3520f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 **See the full release notes on the official documentation website: https://www.elastic.co/docs/release-notes/elasticsearch/clients/ruby**
 
+# 9.3.4
+
+* Fixes header conflict when using Elasticsearch Serverless. [Pull Request](https://github.com/elastic/elasticsearch-ruby/pull/2984)
+
 # 9.3.3
 
 * Fixes NoMethodError when `Content-Type` lacks `compatible-with` parameter. [Pull Request](https://github.com/elastic/elasticsearch-ruby/pull/2974)
diff --git a/elasticsearch-api/lib/elasticsearch/api/version.rb b/elasticsearch-api/lib/elasticsearch/api/version.rb
index 3cfc43557..83d5a64a3 100644
--- a/elasticsearch-api/lib/elasticsearch/api/version.rb
+++ b/elasticsearch-api/lib/elasticsearch/api/version.rb
@@ -17,7 +17,7 @@
 
 module Elasticsearch
   module API
-    VERSION = '9.3.3'.freeze
+    VERSION = '9.3.4'.freeze
     ES_SPECIFICATION_COMMIT = '31789f4216047ef7ba7969e31610fa2bbfe696b1'.freeze
   end
 end
diff --git a/elasticsearch-api/spec/spec_helper.rb b/elasticsearch-api/spec/spec_helper.rb
index aa79707a3..0424408c5 100644
--- a/elasticsearch-api/spec/spec_helper.rb
+++ b/elasticsearch-api/spec/spec_helper.rb
@@ -28,12 +28,18 @@
 end
 require 'elasticsearch'
 require 'elasticsearch-api'
-require 'jbuilder'
 require 'jsonify'
 require 'logger'
 require 'openssl'
 require 'yaml'
 
+# TODO: There is a dependency issue with JRuby 9.4.15.0 and JBuilder at the moment:
+def jruby_exception?
+  defined?(JRUBY_VERSION) &&
+    JRUBY_VERSION.between?('9.4', '9.5')
+end
+require 'jbuilder' unless jruby_exception?
+
 tracer = ::Logger.new(STDERR)
 tracer.formatter = lambda { |s, d, p, m| "#{m.gsub(/^.*$/) { |n| '   ' + n } }\n" }
 
diff --git a/elasticsearch-api/spec/unit/actions/json_builders_spec.rb b/elasticsearch-api/spec/unit/actions/json_builders_spec.rb
index 899674590..db97f1a80 100644
--- a/elasticsearch-api/spec/unit/actions/json_builders_spec.rb
+++ b/elasticsearch-api/spec/unit/actions/json_builders_spec.rb
@@ -83,4 +83,5 @@
       expect(client_double.search(body: json)).to be_a Elasticsearch::API::Response
     end
   end
-end
+end unless jruby_exception?
+# TODO: Exception for JRuby v9.4.15.0 ^
diff --git a/elasticsearch/elasticsearch.gemspec b/elasticsearch/elasticsearch.gemspec
index ca6080135..5130dc5b8 100644
--- a/elasticsearch/elasticsearch.gemspec
+++ b/elasticsearch/elasticsearch.gemspec
@@ -46,7 +46,7 @@ Gem::Specification.new do |s|
   s.rdoc_options      = ['--charset=UTF-8']
 
   s.required_ruby_version = '>= 2.6' # For compatibility with JRuby 9.3
-  s.add_dependency 'elasticsearch-api', '9.3.3'
+  s.add_dependency 'elasticsearch-api', '9.3.4'
   s.add_dependency 'elastic-transport', '~> 8.3'
 
   s.add_development_dependency 'base64'
diff --git a/elasticsearch/lib/elasticsearch.rb b/elasticsearch/lib/elasticsearch.rb
index e27c79cd4..e8ea177ce 100644
--- a/elasticsearch/lib/elasticsearch.rb
+++ b/elasticsearch/lib/elasticsearch.rb
@@ -189,15 +189,19 @@ 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/i }
+      unless user_headers&.keys&.detect { |h| h =~ /content-?_?type/i } || api_version_set?(user_headers)
         headers['content-type'] = 'application/vnd.elasticsearch+json; compatible-with=9'
       end
-      unless user_headers&.keys&.detect { |h| h =~ /accept/i }
+      unless user_headers&.keys&.detect { |h| h =~ /accept/i } || api_version_set?(user_headers)
         headers['accept'] = 'application/vnd.elasticsearch+json; compatible-with=9'
       end
       set_header(headers, arguments) unless headers.empty?
     end
 
+    def api_version_set?(headers)
+      !!headers&.keys&.find { |a| a.match?(/elastic-api-version/i) }
+    end
+
     def set_header(header, arguments)
       arguments[:transport_options] ||= {}
       arguments[:transport_options][:headers] ||= {}
diff --git a/elasticsearch/lib/elasticsearch/version.rb b/elasticsearch/lib/elasticsearch/version.rb
index 799a015d8..a5879b66a 100644
--- a/elasticsearch/lib/elasticsearch/version.rb
+++ b/elasticsearch/lib/elasticsearch/version.rb
@@ -16,5 +16,5 @@
 # under the License.
 
 module Elasticsearch
-  VERSION = '9.3.3'.freeze
+  VERSION = '9.3.4'.freeze
 end
diff --git a/elasticsearch/spec/unit/headers_spec.rb b/elasticsearch/spec/unit/headers_spec.rb
index a6155032d..da072651d 100644
--- a/elasticsearch/spec/unit/headers_spec.rb
+++ b/elasticsearch/spec/unit/headers_spec.rb
@@ -142,4 +142,26 @@
       client.search
     end
   end
+
+  context 'when Elastic-Api-Version header is set' do
+    let!(:client) do
+      described_class.new(
+        host: 'http://localhost:9200',
+        transport_options: { headers: { 'Elastic-Api-Version' => '2023-10-31' } }
+      ).tap do |client|
+        client.instance_variable_set('@verified', true)
+      end
+    end
+
+    it 'does not modify the content-type header' do
+      expected_headers = client.transport.connections.connections.first.connection.headers
+      expect(expected_headers['Content-Type']).to eq 'application/json'
+      expect(expected_headers['Elastic-Api-Version']).to eq '2023-10-31'
+
+      expect_any_instance_of(Faraday::Connection)
+        .to receive(:run_request)
+              .with(:get, 'http://localhost:9200/_search', nil, expected_headers) { OpenStruct.new(body: '') }
+      client.search
+    end
+  end
 end