diff --git a/.gitignore b/.gitignore
index d256830..ff9adbc 100755
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 .env
 coverage
 Gemfile.lock
+pkg
 spec/examples.txt
 spec/vcr_cassettes
diff --git a/.rubocop.yml b/.rubocop.yml
index 8b2cb66..0d91bb7 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -4,6 +4,8 @@ require:
 
 AllCops:
   TargetRubyVersion: 2.6.4
+  Exclude:
+    - bin/**/*
 
 Layout/AlignParameters:
   EnforcedStyle: with_fixed_indentation
diff --git a/Rakefile b/Rakefile
index b7e9ed5..b6ae734 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,6 +1,8 @@
+# frozen_string_literal: true
+
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 
 RSpec::Core::RakeTask.new(:spec)
 
-task :default => :spec
+task default: :spec
diff --git a/eml.gemspec b/eml.gemspec
index c9cd53a..209a4a7 100644
--- a/eml.gemspec
+++ b/eml.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.homepage = "https://github.com/MorningCoffeeDev/eml_ruby"
   spec.license = "MIT"
 
-  spec.files = Dir.chdir(File.expand_path('..', __FILE__)) do
+  spec.files = Dir.chdir(File.expand_path(__dir__)) do
     `git ls-files -z`.split("\x0").
       reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
diff --git a/lib/eml.rb b/lib/eml.rb
index 22c3358..6d2dbc7 100644
--- a/lib/eml.rb
+++ b/lib/eml.rb
@@ -30,6 +30,7 @@ module EML
 require "eml/lib/basic_auth/verify"
 require "eml/lib/constant_time_compare"
 require "eml/lib/endpoint_class"
+require "eml/lib/model_hash"
 
 require "eml/model"
 require "eml/parameters"
diff --git a/lib/eml/error/rest.rb b/lib/eml/error/rest.rb
index 47ee025..1988415 100644
--- a/lib/eml/error/rest.rb
+++ b/lib/eml/error/rest.rb
@@ -16,27 +16,27 @@ class RESTError < Error
     def initialize(message = nil, response = nil)
       super(message)
 
-      @response = T.let(response, EML::Response)
+      @response = T.let(response, T.nilable(EML::Response))
     end
 
-    sig { returns(String) }
+    sig { returns(T.nilable(String)) }
     def http_body
-      @response.error || @response.body.to_s
+      @response&.error || @response&.body&.to_s
     end
 
-    sig { returns(T::Hash[Symbol, String]) }
+    sig { returns(T.nilable(T::Hash[Symbol, String])) }
     def http_headers
-      @response.headers
+      @response&.headers
     end
 
-    sig { returns(Integer) }
+    sig { returns(T.nilable(Integer)) }
     def http_status
-      @response.status
+      @response&.http_status
     end
 
-    sig { returns(String) }
+    sig { returns(T.nilable(String)) }
     def url
-      @response.url.to_s
+      @response&.url&.to_s
     end
   end
 end
diff --git a/lib/eml/lib/basic_auth/generate.rb b/lib/eml/lib/basic_auth/generate.rb
index 4bdb2b9..552c9fd 100644
--- a/lib/eml/lib/basic_auth/generate.rb
+++ b/lib/eml/lib/basic_auth/generate.rb
@@ -16,7 +16,7 @@ class Generate
         ).returns(String)
       end
       def self.call(username, password, prefix: "")
-        new(username, password, prefix).call
+        new(username, password, prefix: prefix).call
       end
 
       sig do
@@ -26,10 +26,10 @@ def self.call(username, password, prefix: "")
           prefix: String
         ).void
       end
-      def initialize(username, password, prefix)
-        @username = username
-        @password = password
-        @prefix = prefix
+      def initialize(username, password, prefix: "")
+        @username = T.let(username, String)
+        @password = T.let(password, String)
+        @prefix = T.let(prefix, String)
       end
 
       sig { returns(String) }
diff --git a/lib/eml/lib/basic_auth/verify.rb b/lib/eml/lib/basic_auth/verify.rb
index cd10366..c0454ed 100644
--- a/lib/eml/lib/basic_auth/verify.rb
+++ b/lib/eml/lib/basic_auth/verify.rb
@@ -8,7 +8,7 @@ class Verify
 
       sig do
         params(
-          auth_token: String,
+          auth_token: T.nilable(String),
           username: String,
           password: String
         ).returns(T::Boolean)
@@ -19,15 +19,15 @@ def self.call(auth_token, username, password)
 
       sig do
         params(
-          auth_token: String,
+          auth_token: T.nilable(String),
           username: String,
           password: String
         ).void
       end
       def initialize(auth_token, username, password)
-        @auth_token = auth_token
-        @username = username
-        @password = password
+        @auth_token = T.let(auth_token || "", String)
+        @username = T.let(username, String)
+        @password = T.let(password, String)
       end
 
       sig { returns(T::Boolean) }
@@ -40,6 +40,7 @@ def call
 
       private
 
+      sig { returns(String) }
       def parse_auth_token
         @auth_token.sub(/^[^\s]+\s/, "")
       end
diff --git a/lib/eml/lib/constant_time_compare.rb b/lib/eml/lib/constant_time_compare.rb
index f69fbe8..b09778f 100644
--- a/lib/eml/lib/constant_time_compare.rb
+++ b/lib/eml/lib/constant_time_compare.rb
@@ -12,8 +12,8 @@ def self.call(comparison, expected)
 
     sig { params(comparison: String, expected: String).void }
     def initialize(comparison, expected)
-      @comparison = comparison
-      @expected = expected
+      @comparison = T.let(comparison, String)
+      @expected = T.let(expected, String)
     end
 
     sig { returns(T::Boolean) }
@@ -22,8 +22,8 @@ def call
 
       result = 0
 
-      Hash[[@comparison.bytes, @expected.bytes].transpose].
-        each { |x, y| result |= x ^ y }
+      transposed = [@comparison.bytes, @expected.bytes].transpose
+      Hash[transposed].each { |x, y| result |= x ^ y }
 
       result.zero?
     end
diff --git a/lib/eml/lib/endpoint_class.rb b/lib/eml/lib/endpoint_class.rb
index d5237bc..d609762 100644
--- a/lib/eml/lib/endpoint_class.rb
+++ b/lib/eml/lib/endpoint_class.rb
@@ -5,6 +5,10 @@ module EML
   class EndpointClass
     extend T::Sig
 
+    sig do
+      params(class_type: String, resource_class: T.untyped, endpoint: String).
+        returns(T.untyped)
+    end
     def self.call(class_type:, resource_class:, endpoint:)
       new(
         class_type: class_type,
@@ -13,18 +17,25 @@ def self.call(class_type:, resource_class:, endpoint:)
       ).call
     end
 
+    sig do
+      params(class_type: String, resource_class: T.untyped, endpoint: String).
+        void
+    end
     def initialize(class_type:, resource_class:, endpoint:)
-      @class_type = class_type
-      @resource_class = resource_class
-      @endpoint = endpoint
+      @class_type = T.let(class_type, String)
+      @resource_class = T.let(resource_class, T.untyped)
+      @endpoint = T.let(endpoint, String)
+      @class_name = T.let(nil, T.nilable(String))
     end
 
+    sig { returns(T.untyped) }
     def call
       Object.const_get(class_name) if Object.const_defined?(class_name)
     end
 
     private
 
+    sig { returns(String) }
     def class_name
       @class_name ||= begin
         name_parts = @resource_class.name.split("::")
@@ -35,6 +46,7 @@ def class_name
       end
     end
 
+    sig { returns(String) }
     def action_class_name
       @endpoint.capitalize.sub(/s$/, "")
     end
diff --git a/lib/eml/lib/model_hash.rb b/lib/eml/lib/model_hash.rb
new file mode 100644
index 0000000..1a7d675
--- /dev/null
+++ b/lib/eml/lib/model_hash.rb
@@ -0,0 +1,54 @@
+# typed: strict
+# frozen_string_literal: true
+
+module EML
+  class ModelHash
+    extend T::Sig
+
+    sig { params(model: EML::Model).returns(T::Hash[Symbol, T.untyped]) }
+    def self.call(model)
+      new(model).call
+    end
+
+    sig { params(model: EML::Model).void }
+    def initialize(model)
+      @model = T.let(model, EML::Model)
+      @hash = T.let({}, T::Hash[Symbol, T.untyped])
+    end
+
+    sig { returns(T::Hash[Symbol, T.untyped]) }
+    def call
+      @model.class.enumerate_fields do |_, local_name|
+        value = @model.public_send(local_name)
+        add_value(local_name, value)
+      end
+
+      @hash
+    end
+
+    private
+
+    sig { params(name: Symbol, value: T.untyped).void }
+    def add_value(name, value)
+      @hash[name] = stored_value(value)
+    end
+
+    sig { params(value: T.untyped).returns(T.untyped) }
+    def stored_value(value)
+      if value.is_a?(Array)
+        array_value(value)
+      elsif value.respond_to?(:to_h)
+        value.to_h
+      else
+        value
+      end
+    end
+
+    sig { params(value: T::Array[T.untyped]).returns(T::Array[T.untyped]) }
+    def array_value(value)
+      value.each_with_object([]) do |item, array|
+        array << stored_value(item)
+      end
+    end
+  end
+end
diff --git a/lib/eml/model.rb b/lib/eml/model.rb
index 4eb26fb..06abbf2 100644
--- a/lib/eml/model.rb
+++ b/lib/eml/model.rb
@@ -1,27 +1,52 @@
 # typed: true
 # frozen_string_literal: true
 
+require "json"
+
 module EML
   class Model
     extend T::Sig
+    extend T::Helpers
+    abstract!
 
+    sig do
+      params(
+        fields: T.any(T::Hash[String, T.untyped], T::Array[String])
+      ).void
+    end
     def self.fields(fields)
       const_set(:FIELDS, fields.freeze)
-      fields.each do |response_name, local_name|
-        local_name ||= response_name
+      enumerate_fields do |_, local_name|
         __send__(:attr_reader, :"#{local_name}")
       end
     end
 
+    sig { void }
+    def self.enumerate_fields
+      const_get(:FIELDS).each do |response_name, local_name|
+        local_name ||= response_name.to_sym
+        yield(response_name, local_name)
+      end
+    end
+
     sig { params(raw_values: T::Hash[String, T.untyped]).void }
     def initialize(raw_values)
-      self.class::FIELDS.each do |response_name, local_name|
-        local_name ||= response_name
+      self.class.enumerate_fields do |response_name, local_name|
         value = field_value(response_name, raw_values[response_name])
         instance_variable_set(:"@#{local_name}", value)
       end
     end
 
+    sig { returns(T::Hash[Symbol, T.untyped]) }
+    def to_h
+      ModelHash.(self)
+    end
+
+    sig { params(_args: T.nilable(Array)).returns(String) }
+    def to_json(*_args)
+      to_h.to_json
+    end
+
     protected
 
     sig { params(name: String, raw_value: T.untyped).returns(T.untyped) }
diff --git a/lib/eml/parameters.rb b/lib/eml/parameters.rb
index 4eab247..196b36c 100644
--- a/lib/eml/parameters.rb
+++ b/lib/eml/parameters.rb
@@ -24,7 +24,7 @@ class << self
           params: T::Hash[Symbol, T.untyped]
         ).returns(T.untyped)
       end
-      def self.convert(resource_class, endpoint, params)
+      def convert(resource_class, endpoint, params)
         endpoint_class = EML::EndpointClass.(
           class_type: ENDPOINT_CLASS_TYPE, resource_class: resource_class,
           endpoint: endpoint
@@ -112,6 +112,7 @@ def validate_max_length(param_name, param_value, length)
 
     private
 
+    sig { params(array: T::Array[String]).returns(String) }
     def array_as_string(array)
       array.dup.tap { |vals| vals[-1] = "or #{vals.last}" }.join(", ")
     end
diff --git a/lib/eml/response.rb b/lib/eml/response.rb
index f26c27f..5d55e84 100644
--- a/lib/eml/response.rb
+++ b/lib/eml/response.rb
@@ -84,7 +84,7 @@ def raise_error(id: nil)
 
     sig { returns(T.nilable(EML::RESTError)) }
     def check_for_incorrectly_categorised_errors
-      if error.match?(/is not in (the proper|a valid) status/)
+      if error&.match?(/is not in (the proper|a valid) status/)
         raise T.unsafe(EML::REST::UnprocessableEntityError).new(error, self)
       end
     end
diff --git a/lib/eml/uk.rb b/lib/eml/uk.rb
index a8ffce8..95f07c4 100644
--- a/lib/eml/uk.rb
+++ b/lib/eml/uk.rb
@@ -12,8 +12,9 @@ module UK
 require "eml/uk/lib/endpoint_class"
 require "eml/uk/lib/parse_date"
 
-require "eml/uk/models/tns_card"
-require "eml/uk/models/tns_transaction"
+require "eml/uk/models/tns/card"
+require "eml/uk/models/tns/message"
+require "eml/uk/models/tns/transaction"
 require "eml/uk/models/transaction"
 
 require "eml/uk/parameters"
diff --git a/lib/eml/uk/api_resource.rb b/lib/eml/uk/api_resource.rb
index a7e5e67..57063cb 100644
--- a/lib/eml/uk/api_resource.rb
+++ b/lib/eml/uk/api_resource.rb
@@ -15,6 +15,7 @@ class APIResource
       def initialize(id: nil)
         @id = T.let(id, T.nilable(String))
         @headers = T.let(nil, T.nilable(T::Hash[String, String]))
+        @credentials = T.let(nil, T.nilable(T::Hash[Symbol, String]))
       end
 
       sig do
@@ -50,13 +51,13 @@ def resource_url(endpoint)
 
       private
 
-      sig { returns(T::Hash[Symbol, T.nilable(String)]) }
+      sig { returns(T::Hash[Symbol, String]) }
       def credentials
         @credentials ||= begin
           config = EML::UK.config
           {
-            username: config.rest_username,
-            password: config.rest_password,
+            username: config.rest_username || "",
+            password: config.rest_password || "",
           }
         end
       end
@@ -74,8 +75,8 @@ def domain
       def headers
         @headers ||= {
           "Authorization" => ::EML::BasicAuth::Generate.(
-            credentials[:username],
-            credentials[:password],
+            T.must(credentials[:username]),
+            T.must(credentials[:password]),
             prefix: "Basic "
           ),
         }
diff --git a/lib/eml/uk/models/tns/card.rb b/lib/eml/uk/models/tns/card.rb
new file mode 100644
index 0000000..5eb894a
--- /dev/null
+++ b/lib/eml/uk/models/tns/card.rb
@@ -0,0 +1,22 @@
+# typed: true
+# frozen_string_literal: true
+
+module EML
+  module UK
+    module Models
+      module TNS
+        class Card < ::EML::Model
+          fields(
+            "AvailableBalance" => :availableBalance,
+            "CarrierNumber" => :carrierNumber,
+            "ClientTrackingId" => :clientTrackingId,
+            "Currency" => :currency,
+            "ExternalId" => :externalId,
+            "Program" => :program,
+            "MerchantGroup" => :merchantGroup
+          )
+        end
+      end
+    end
+  end
+end
diff --git a/lib/eml/uk/models/tns/message.rb b/lib/eml/uk/models/tns/message.rb
new file mode 100644
index 0000000..7cc8ca2
--- /dev/null
+++ b/lib/eml/uk/models/tns/message.rb
@@ -0,0 +1,25 @@
+# typed: true
+# frozen_string_literal: true
+
+module EML
+  module UK
+    module Models
+      module TNS
+        class Message < ::EML::Model
+          extend T::Sig
+
+          fields("Transactions" => :transactions)
+
+          sig { params(raw_values: T::Hash[Symbol, T.untyped]).void }
+          def initialize(raw_values)
+            @transactions = raw_values[:Transactions].
+              each_with_object([]) do |raw_transaction, transactions|
+                transactions << EML::UK::Models::TNS::Transaction.
+                  new(raw_transaction)
+              end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/eml/uk/models/tns/transaction.rb b/lib/eml/uk/models/tns/transaction.rb
new file mode 100644
index 0000000..211ea94
--- /dev/null
+++ b/lib/eml/uk/models/tns/transaction.rb
@@ -0,0 +1,51 @@
+# typed: true
+# frozen_string_literal: true
+
+module EML
+  module UK
+    module Models
+      module TNS
+        class Transaction < ::EML::Model
+          extend T::Sig
+
+          fields(
+            "AuthorizationRequestId" => :authorizationRequestId,
+            "Card" => :card,
+            "Cards" => :cards,
+            "EmlId" => :emlId,
+            "MerchantCategoryCode" => :merchantCategoryCode,
+            "MerchantCountry" => :merchantCountry,
+            "Note" => :note,
+            "OriginalTransactionDate" => :originalDate,
+            "PosTransactionTime" => :posTime,
+            "Reason" => :reason,
+            "Result" => :result,
+            "RetrievalReferenceNumber" => :retrievalReferenceNumber,
+            "TransactionAmount" => :amount,
+            "TransactionCurrency" => :currency,
+            "TransactionDescription" => :description,
+            "TransactionId" => :transactionId,
+            "TransactionLocation" => :location,
+            "TransactionTime" => :time
+          )
+
+          sig { params(raw_values: T::Hash[String, T.untyped]).void }
+          def initialize(raw_values)
+            super
+            initialize_cards
+          end
+
+          private
+
+          sig { void }
+          def initialize_cards
+            @card = Card.new(@card) unless @card.nil?
+            @cards = (@cards || []).each_with_object([]) do |card, cards|
+              cards << Card.new(card)
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/eml/uk/models/tns_card.rb b/lib/eml/uk/models/tns_card.rb
deleted file mode 100644
index fb3e8f3..0000000
--- a/lib/eml/uk/models/tns_card.rb
+++ /dev/null
@@ -1,20 +0,0 @@
-# typed: true
-# frozen_string_literal: true
-
-module EML
-  module UK
-    module Models
-      class TNSCard < ::EML::Model
-        fields(
-          "AvailableBalance" => :availableBalance,
-          "CarrierNumber" => :carrierNumber,
-          "ClientTrackingId" => :clientTrackingId,
-          "Currency" => :currency,
-          "ExternalId" => :externalId,
-          "Program" => :program,
-          "MerchantGroup" => :merchantGroup,
-        )
-      end
-    end
-  end
-end
diff --git a/lib/eml/uk/models/tns_transaction.rb b/lib/eml/uk/models/tns_transaction.rb
deleted file mode 100644
index 690958a..0000000
--- a/lib/eml/uk/models/tns_transaction.rb
+++ /dev/null
@@ -1,49 +0,0 @@
-# typed: true
-# frozen_string_literal: true
-
-module EML
-  module UK
-    module Models
-      class TNSTransaction < ::EML::Model
-        extend T::Sig
-
-        fields(
-          "AuthorizationRequestId" => :authorizationRequestId,
-          "Card" => :card,
-          "Cards" => :cards,
-          "EmlId" => :emlId,
-          "MerchantCategoryCode" => :merchantCategoryCode,
-          "MerchantCountry" => :merchantCountry,
-          "Note" => :note,
-          "OriginalTransactionDate" => :originalDate,
-          "PosTransactionTime" => :posTime,
-          "Reason" => :reason,
-          "Result" => :result,
-          "RetrievalReferenceNumber" => :retrievalReferenceNumber,
-          "TransactionAmount" => :amount,
-          "TransactionCurrency" => :currency,
-          "TransactionDescription" => :description,
-          "TransactionId" => :transactionId,
-          "TransactionLocation" => :location,
-          "TransactionTime" => :time
-        )
-
-        sig { params(raw_values: T::Hash[String, T.untyped]).void }
-        def initialize(raw_values)
-          super
-          initialize_cards
-        end
-
-        private
-
-        sig { void }
-        def initialize_cards
-          @card = TNSCard.new(@card) unless @card.nil?
-          @cards = (@cards || []).each_with_object([]) do |card, cards|
-            cards << TNSCard.new(card)
-          end
-        end
-      end
-    end
-  end
-end
diff --git a/lib/eml/uk/models/transaction.rb b/lib/eml/uk/models/transaction.rb
index b2a4c58..99a675d 100644
--- a/lib/eml/uk/models/transaction.rb
+++ b/lib/eml/uk/models/transaction.rb
@@ -38,7 +38,7 @@ class Transaction < ::EML::Model
         sig { params(field: String, raw_value: T.untyped).returns(T.untyped) }
         def field_value(field, raw_value)
           if field.match?(/date|time/)
-            EML::UK::ParseDate.(raw_value)
+            ::EML::UK::ParseDate.(raw_value)
           else
             raw_value
           end
diff --git a/lib/eml/uk/parameters.rb b/lib/eml/uk/parameters.rb
index ee460cf..6184602 100644
--- a/lib/eml/uk/parameters.rb
+++ b/lib/eml/uk/parameters.rb
@@ -27,7 +27,7 @@ class << self
         end
         def convert(resource_class, endpoint, params)
           endpoint_class = EML::UK::EndpointClass.(
-            class_type: self::ENDPOINT_CLASS_TYPE,
+            class_type: const_get(:ENDPOINT_CLASS_TYPE),
             resource_class: resource_class, endpoint: endpoint
           )
 
@@ -40,10 +40,13 @@ def convert(resource_class, endpoint, params)
 
       protected
 
-      SEARCH_PARAMETER_OPTIONS = %w[
-        ActualCardNumber CarrierNumber ClientTrackingId ExternalId
-        PaymentTrackingID PrintText
-      ].freeze
+      SEARCH_PARAMETER_OPTIONS = T.let(
+        %w[
+          ActualCardNumber CarrierNumber ClientTrackingId ExternalId
+          PaymentTrackingID PrintText
+        ].freeze,
+        T::Array[String]
+      )
 
       sig { params(value: String).void }
       def validate_search_parameter(value)
diff --git a/lib/eml/uk/payload.rb b/lib/eml/uk/payload.rb
index 16d1905..459cac3 100644
--- a/lib/eml/uk/payload.rb
+++ b/lib/eml/uk/payload.rb
@@ -26,7 +26,7 @@ class << self
         end
         def convert(resource_class, endpoint, payload)
           endpoint_class = EML::UK::EndpointClass.(
-            class_type: self::ENDPOINT_CLASS_TYPE,
+            class_type: const_get(:ENDPOINT_CLASS_TYPE),
             resource_class: resource_class,
             endpoint: endpoint
           )
diff --git a/lib/eml/uk/responses/card/transaction.rb b/lib/eml/uk/responses/card/transaction.rb
index 071a48e..83626c8 100644
--- a/lib/eml/uk/responses/card/transaction.rb
+++ b/lib/eml/uk/responses/card/transaction.rb
@@ -10,6 +10,16 @@ class Transaction < ::EML::UK::Response
 
           field :count
 
+          sig { params(response: HTTP::Response, id: T.nilable(String)).void }
+          def initialize(response, id: nil)
+            super
+
+            @transactions = T.let(
+              nil,
+              T.nilable(T::Array[::EML::UK::Models::Transaction])
+            )
+          end
+
           sig { returns(T::Array[::EML::UK::Models::Transaction]) }
           def transactions
             @transactions ||= body["transactions"].
diff --git a/lib/eml/uk/tns/process_request.rb b/lib/eml/uk/tns/process_request.rb
index 6994107..a485d0d 100644
--- a/lib/eml/uk/tns/process_request.rb
+++ b/lib/eml/uk/tns/process_request.rb
@@ -11,7 +11,7 @@ class ProcessRequest
           params(
             auth_token: String,
             parameters: T::Hash[Symbol, T.untyped]
-          ).returns(EML::UK::TNS::Response)
+          ).returns(EML::UK::Models::TNS::Message)
         end
         def self.call(auth_token, parameters)
           new(auth_token, parameters).call
@@ -24,14 +24,15 @@ def self.call(auth_token, parameters)
           ).void
         end
         def initialize(auth_token, parameters)
-          @auth_token = auth_token
-          @parameters = parameters
+          @auth_token = T.let(auth_token, String)
+          @parameters = T.let(parameters, T::Hash[Symbol, T.untyped])
+          @credentials = T.let(nil, T.nilable(T::Hash[Symbol, T.untyped]))
         end
 
-        sig { returns(EML::UK::TNS::Response) }
+        sig { returns(EML::UK::Models::TNS::Message) }
         def call
           verify_auth_token
-          EML::UK::TNS::Response.new(@parameters)
+          EML::UK::Models::TNS::Message.new(@parameters)
         end
 
         private
diff --git a/lib/eml/uk/tns/response.rb b/lib/eml/uk/tns/response.rb
index a8d062e..a738069 100644
--- a/lib/eml/uk/tns/response.rb
+++ b/lib/eml/uk/tns/response.rb
@@ -7,13 +7,27 @@ module TNS
       class Response
         extend T::Sig
 
+        sig { returns(T::Array[EML::UK::Models::TNS::Transaction]) }
         attr_reader :transactions
 
         sig { params(response: T::Hash[Symbol, T.untyped]).void }
         def initialize(response)
-          @transactions = response[:Transactions].
+          @transactions = T.let(
+            model_transactions(response),
+            T::Array[EML::UK::Models::TNS::Transaction]
+          )
+        end
+
+        private
+
+        sig do
+          params(response: T::Hash[Symbol, T.untyped]).
+            returns(T::Array[EML::UK::Models::TNS::Transaction])
+        end
+        def model_transactions(response)
+          response[:Transactions].
             each_with_object([]) do |raw_transaction, transactions|
-              transactions << EML::UK::Models::TNSTransaction.
+              transactions << EML::UK::Models::TNS::Transaction.
                 new(raw_transaction)
             end
         end
diff --git a/lib/eml/version.rb b/lib/eml/version.rb
index 352f607..3c69a81 100644
--- a/lib/eml/version.rb
+++ b/lib/eml/version.rb
@@ -2,5 +2,5 @@
 # frozen_string_literal: true
 
 module EML
-  VERSION = "2.0.0"
+  VERSION = "2.1.0"
 end
diff --git a/spec/helpers/tns_uk_helper.rb b/spec/helpers/tns_uk_helper.rb
index 575ddb1..ce1f120 100644
--- a/spec/helpers/tns_uk_helper.rb
+++ b/spec/helpers/tns_uk_helper.rb
@@ -6,37 +6,41 @@ module Helpers
     module TNSUK
       extend T::Sig
 
-      TRANSACTION = {
-        "TransactionId" => 123_456_789,
-        "EmlId" => nil,
-        "TransactionTime" => "2020-01-01T00:00:00.1234567-06:00",
-        "TransactionAmount" => 1.0,
-        "TransactionDescription" => "Unload",
-        "TransactionCurrency" => "GBP",
-        "TransactionLocation" => "",
-        "Reason" => "Active",
-        "Result" => "Unlocked",
-        "Note" => "Testing",
-        "MerchantCategoryCode" => "",
-        "MerchantCountry" => "",
-        "PosTransactionTime" => nil,
-        "OriginalTransactionDate" => nil,
-        "AuthorizationRequestId" => "",
-        "RetrievalReferenceNumber" => "",
-        "Card" => {
-          "AvailableBalance" => 1.23,
-          "ExternalId" => "ABCDEFGHIJKLMNOP",
-          "ClientTrackingId" => "ABCDEFGHIJ",
-          "Program" => "Program123",
-          "MerchantGroup" => "Merchant12",
-          "Currency" => "GBP",
-          "CarrierNumber" => nil,
-        },
-      }.freeze
+      TRANSACTION = T.let(
+        {
+          "TransactionId" => 123_456_789,
+          "EmlId" => nil,
+          "TransactionTime" => "2020-01-01T00:00:00.1234567-06:00",
+          "TransactionAmount" => 1.0,
+          "TransactionDescription" => "Unload",
+          "TransactionCurrency" => "GBP",
+          "TransactionLocation" => "",
+          "Reason" => "Active",
+          "Result" => "Unlocked",
+          "Note" => "Testing",
+          "MerchantCategoryCode" => "",
+          "MerchantCountry" => "",
+          "PosTransactionTime" => nil,
+          "OriginalTransactionDate" => nil,
+          "AuthorizationRequestId" => "",
+          "RetrievalReferenceNumber" => "",
+          "Card" => {
+            "AvailableBalance" => 1.23,
+            "ExternalId" => "ABCDEFGHIJKLMNOP",
+            "ClientTrackingId" => "ABCDEFGHIJ",
+            "Program" => "Program123",
+            "MerchantGroup" => "Merchant12",
+            "Currency" => "GBP",
+            "CarrierNumber" => nil,
+          },
+        }.freeze,
+        T::Hash[String, T.untyped]
+      )
 
-      RESPONSE = {
-        Transactions: [TRANSACTION],
-      }.freeze
+      RESPONSE = T.let(
+        { Transactions: [TRANSACTION] }.freeze,
+        T::Hash[Symbol, T::Array[T::Hash[String, T.untyped]]]
+      )
     end
   end
 end
diff --git a/spec/lib/basic_auth/verify_spec.rb b/spec/lib/basic_auth/verify_spec.rb
index bfab15b..ea70cef 100644
--- a/spec/lib/basic_auth/verify_spec.rb
+++ b/spec/lib/basic_auth/verify_spec.rb
@@ -2,12 +2,12 @@
 # frozen_string_literal: true
 
 RSpec.describe EML::BasicAuth::Verify do
+  subject { described_class.(token, username, password) }
+
   let(:expected_token) { "dXNlcm5hbWU6cGFzc3dvcmQ=" }
   let(:username) { "username" }
   let(:password) { "password" }
 
-  subject { described_class.(token, username, password) }
-
   context "when the token is valid for the username and password" do
     let(:token) { expected_token }
 
diff --git a/spec/lib/constant_time_compare_spec.rb b/spec/lib/constant_time_compare_spec.rb
index 8f5d9ff..c9973cf 100644
--- a/spec/lib/constant_time_compare_spec.rb
+++ b/spec/lib/constant_time_compare_spec.rb
@@ -2,7 +2,7 @@
 # frozen_string_literal: true
 
 RSpec.describe EML::ConstantTimeCompare do
-  subject { described_class.(expected, comparison) }
+  subject(:compare) { described_class.(expected, comparison) }
 
   let(:expected) { "expected" }
 
@@ -29,13 +29,13 @@
       let(:expected) { 1 }
       let(:comparison) { "expected" }
 
-      specify { expect { subject }.to raise_error TypeError }
+      specify { expect { compare }.to raise_error TypeError }
     end
 
     context "when the comparison value is an integer" do
       let(:comparison) { 1 }
 
-      specify { expect { subject }.to raise_error TypeError }
+      specify { expect { compare }.to raise_error TypeError }
     end
   end
 end
diff --git a/spec/model_spec.rb b/spec/model_spec.rb
new file mode 100644
index 0000000..784cc77
--- /dev/null
+++ b/spec/model_spec.rb
@@ -0,0 +1,59 @@
+# typed: ignore
+# frozen_string_literal: true
+
+RSpec.describe EML::Model do
+  before do
+    test_model = Class.new(::EML::Model) do
+      fields(
+        "First" => :first,
+        "Second" => :second,
+        "Third" => :third,
+        "Fourth" => :fourth,
+        "Fifth" => :fifth
+      )
+    end
+    stub_const("TestModel", test_model)
+  end
+
+  let(:params) do
+    {
+      "First" => "A",
+      "Second" => 1,
+      "Third" => Time.now,
+      "Fourth" => { a: "a" },
+      "Fifth" => %w[a b],
+    }
+  end
+
+  describe "#to_h" do
+    let(:expected_hash) do
+      {
+        first: params["First"],
+        second: params["Second"],
+        third: params["Third"],
+        fourth: params["Fourth"],
+        fifth: params["Fifth"],
+      }
+    end
+
+    it "converts the model to a hash" do
+      expect(TestModel.new(params).to_h).to eq(expected_hash)
+    end
+  end
+
+  describe "#to_json" do
+    let(:expected_string) do
+      {
+        first: params["First"],
+        second: params["Second"],
+        third: params["Third"],
+        fourth: params["Fourth"],
+        fifth: params["Fifth"],
+      }.to_json
+    end
+
+    it "converts the model to a JSON string" do
+      expect(TestModel.new(params).to_json).to eq(expected_string)
+    end
+  end
+end