diff --git a/Gemfile.lock b/Gemfile.lock
index 104afbd..96ee7e6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    leopard (0.1.6)
+    leopard (0.1.7)
       concurrent-ruby (~> 1.1)
       dry-configurable (~> 1.3)
       dry-monads (~> 1.9)
@@ -28,7 +28,7 @@ GEM
       dry-core (~> 1.1)
       zeitwerk (~> 2.6)
     io-console (0.8.1)
-    json (2.13.1)
+    json (2.13.2)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
     logger (1.7.0)
@@ -53,10 +53,10 @@ GEM
     racc (1.8.1)
     rainbow (3.1.1)
     rake (13.3.0)
-    regexp_parser (2.10.0)
+    regexp_parser (2.11.0)
     reline (0.6.2)
       io-console (~> 0.5)
-    rubocop (1.79.0)
+    rubocop (1.79.2)
       json (~> 2.3)
       language_server-protocol (~> 3.17.0.2)
       lint_roller (~> 1.1.0)
@@ -66,7 +66,6 @@ GEM
       regexp_parser (>= 2.9.3, < 3.0)
       rubocop-ast (>= 1.46.0, < 2.0)
       ruby-progressbar (~> 1.7)
-      tsort (>= 0.2.0)
       unicode-display_width (>= 2.4.0, < 4.0)
     rubocop-ast (1.46.0)
       parser (>= 3.3.7.2)
@@ -92,7 +91,6 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.13.2)
     simplecov_json_formatter (0.1.4)
-    tsort (0.2.0)
     unicode-display_width (3.1.4)
       unicode-emoji (~> 4.0, >= 4.0.4)
     unicode-emoji (4.0.4)
diff --git a/examples/echo_endpoint.rb b/examples/echo_endpoint.rb
index 9774389..8d605ce 100755
--- a/examples/echo_endpoint.rb
+++ b/examples/echo_endpoint.rb
@@ -7,7 +7,7 @@
 class EchoService
   include Rubyists::Leopard::NatsApiServer
 
-  def initialize(a_var = 1)
+  def initialize(a_var: 1)
     logger.info "EchoService initialized with a_var: #{a_var}"
   end
 
@@ -21,7 +21,7 @@ def initialize(a_var = 1)
     service_opts: {
       name: 'example.echo',
       version: '1.0.0',
-      instance_args: [2],
+      instance_args: { a_var: 2 },
     },
     instances: 1,
   )
diff --git a/lib/leopard/message_wrapper.rb b/lib/leopard/message_wrapper.rb
index e1638ff..3f1279b 100644
--- a/lib/leopard/message_wrapper.rb
+++ b/lib/leopard/message_wrapper.rb
@@ -10,10 +10,11 @@ class MessageWrapper
       #
       # @!attribute [r] data
       # @return [Object] The parsed data from the NATS message.
+      attr_reader :raw, :data
       #
-      # @!attribute [r] headers
+      # @!attribute [w] headers
       # @return [Hash] The headers from the NATS message.
-      attr_reader :raw, :data, :headers
+      attr_accessor :headers
 
       # @param nats_msg [NATS::Message] The NATS message to wrap.
       def initialize(nats_msg)
@@ -26,6 +27,7 @@ def initialize(nats_msg)
       #
       # @return [void]
       def respond(payload)
+        raw.header = headers unless headers.empty?
         raw.respond(serialize(payload))
       end
 
diff --git a/lib/leopard/nats_api_server.rb b/lib/leopard/nats_api_server.rb
index 582ccdc..b165dd1 100644
--- a/lib/leopard/nats_api_server.rb
+++ b/lib/leopard/nats_api_server.rb
@@ -98,6 +98,8 @@ def spawn_instances(url, opts, count, workers, blocking)
           pool = Concurrent::FixedThreadPool.new(count)
           @instance_args = opts.delete(:instance_args) || nil
           logger.info "Building #{count} workers with options: #{opts.inspect}, instance_args: #{@instance_args}"
+          raise ArgumentError, 'instance_args must be a Hash' if @instance_args && !@instance_args.is_a?(Hash)
+
           count.times do
             pool.post { build_worker(url, opts, workers, blocking) }
           end
@@ -106,18 +108,19 @@ def spawn_instances(url, opts, count, workers, blocking)
 
         # Builds a worker instance and sets it up with the NATS server.
         #
-        # @param url [String] The URL of the NATS server.
-        # @param opts [Hash] Options for the NATS service.
+        # @param nats_url [String] The URL of the NATS server.
+        # @param service_opts [Hash] Options for the NATS service.
         # @param workers [Array] The array to store worker instances.
         # @param blocking [Boolean] If true, blocks the current thread until the worker is set up.
         #
         # @return [void]
-        def build_worker(url, opts, workers, blocking)
-          worker = @instance_args ? new(*@instance_args) : new
+        def build_worker(nats_url, service_opts, workers, blocking)
+          worker = @instance_args ? new(**@instance_args) : new
           workers << worker
-          return worker.setup_worker!(nats_url: url, service_opts: opts) if blocking
+          args = { nats_url:, service_opts: }
+          return worker.setup_worker!(**args) if blocking
 
-          worker.setup_worker(nats_url: url, service_opts: opts)
+          worker.setup_worker(**args)
         end
 
         # Shuts down the NATS API server gracefully.
diff --git a/test/lib/message_wrapper.rb b/test/lib/message_wrapper.rb
index d4c85c9..7c94d88 100644
--- a/test/lib/message_wrapper.rb
+++ b/test/lib/message_wrapper.rb
@@ -4,7 +4,8 @@
 require 'leopard/message_wrapper'
 
 class FakeMsg
-  attr_reader :data, :header, :responded_payload, :error_args
+  attr_reader :data, :responded_payload, :error_args
+  attr_accessor :header
 
   def initialize(data, header = {})
     @data = data