diff --git a/CHANGELOG.md b/CHANGELOG.md
index b1b59ed..a12ad50 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,31 +4,48 @@ All changes to `grape-idempotency` will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.0] - (Next)
+
+### Fix
+
+* Your contribution here.
+
+### Changed
+
+* [#11](https://github.com/jcagarcia/grape-idempotency/pull/11): Changing error response formats - [@Flip120](https://github.com/Flip120).
+* Your contribution here.
+
+### Feature
+
+* [#15](https://github.com/jcagarcia/grape-idempotency/pull/15): Allow to mark the idempotent header as required - [@jcagarcia](https://github.com/jcagarcia).
+* [#11](https://github.com/jcagarcia/grape-idempotency/pull/11): Return 409 conflict when a request is still being processed - [@Flip120](https://github.com/Flip120).
+* Your contribution here.
+
 ## [0.1.3] - 2023-01-07
 
 ### Fix
 
-- Second calls were returning `null` when the first response was generated inside a `rescue_from`.
-- Conflict response had invalid format.
+* [#9](https://github.com/jcagarcia/grape-idempotency/pull/9): Second calls were returning `null` when the first response was generated inside a `rescue_from`. - [@jcagarcia](https://github.com/jcagarcia).
+- [#9](https://github.com/jcagarcia/grape-idempotency/pull/9): Conflict response had invalid format. - [@jcagarcia](https://github.com/jcagarcia).
 
 ## [0.1.2] - 2023-01-06
 
 ### Fix
 
-- Return correct original response when the endpoint returns a hash in the body
+* [#5](https://github.com/jcagarcia/grape-idempotency/pull/5): Return correct original response when the endpoint returns a hash in the body - [@jcagarcia](https://github.com/jcagarcia).
 
 ## [0.1.1] - 2023-01-06
 
 ### Fix
 
-- Return `409 - Conflict` response if idempotency key is provided for same query and body parameters BUT different endpoints.
-- Use `nx: true` when storing the original request in the Redis storage for only setting the key if it does not already exist.
+* [#4](https://github.com/jcagarcia/grape-idempotency/pull/4): Return `409 - Conflict` response if idempotency key is provided for same query and body parameters BUT different endpoints. - [@jcagarcia](https://github.com/jcagarcia).
+* [#4](https://github.com/jcagarcia/grape-idempotency/pull/4): Use `nx: true` when storing the original request in the Redis storage for only setting the key if it does not already exist. - [@jcagarcia](https://github.com/jcagarcia).
 
 ### Changed
 
-- Include `idempotency-key` in the response headers
-  - In the case of a concurrency error when storing the request into the redis storage (because now `nx: true`), a new idempotency key will be generated, so the consumer can check the new one seeing the headers.
+* [#4](https://github.com/jcagarcia/grape-idempotency/pull/4): Include `idempotency-key` in the response headers - [@jcagarcia](https://github.com/jcagarcia).
+  * In the case of a concurrency error when storing the request into the redis storage (because now `nx: true`), a new idempotency key will be generated, so the consumer can check the new one seeing the headers.
 
 ## [0.1.0] - 2023-01-03
 
-- Initial version
+* [#1](https://github.com/jcagarcia/grape-idempotency/pull/1): Initial version - [@jcagarcia](https://github.com/jcagarcia).
diff --git a/README.md b/README.md
index 4d66e73..40fd264 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,7 @@ Topics covered in this README:
 - [Installation](#installation-)
 - [Basic Usage](#basic-usage-)
 - [How it works](#how-it-works-)
+- [Making idempotency key header mandatory](#making-idempotency-key-header-mandatory-)
 - [Configuration](#configuration-)
 - [Changelog](#changelog)
 - [Contributing](#contributing)
@@ -65,7 +66,7 @@ That's all! 🚀
 
 ## How it works 🤔
 
-Once you've set up the gem and enclosed your endpoint code within the `idempotent` method, your endpoint will exhibit idempotent behavior, but this will only occur if the consumer of the endpoint includes an idempotency key in their request.
+Once you've set up the gem and enclosed your endpoint code within the `idempotent` method, your endpoint will exhibit idempotent behavior, but this will only occur if the consumer of the endpoint includes an idempotency key in their request. (If you want to make the idempotency key header mandatory for your endpoint, check [How to make idempotency key header mandatory](#making-idempotency-key-header-mandatory-) section)
 
 This key allows your consumer to make the same request again in case of a connection error, without the risk of creating a duplicate object or executing the update twice.
 
@@ -80,6 +81,38 @@ Results are only saved if an API endpoint begins its execution. If incoming para
 
 Additionally, this gem automatically appends the `Original-Request` header and the `Idempotency-Key` header to your API's response, enabling you to trace back to the initial request that generated that specific response.
 
+## Making idempotency key header mandatory ⚠️
+
+For some endpoints, you want to enforce your consumers to provide idempotency key. So, when wrapping the code inside the `idempotent` method, you can mark it as `required`:
+
+```ruby
+require 'grape'
+require 'grape-idempotency'
+
+class API < Grape::API
+    post '/payments' do
+      idempotent(required: true) do
+        status 201
+        Payment.create!({
+          amount: params[:amount]
+        })
+      end
+    end
+  end
+end
+```
+
+If the Idempotency-Key request header is missing for a idempotent operation requiring this header, the gem will reply with an HTTP 400 status code with the following body:
+
+```json
+{
+  "title": "Idempotency-Key is missing",
+  "detail": "This operation is idempotent and it requires correct usage of Idempotency Key.",
+}
+```
+
+If you want to change the error message returned in this scenario, check [How to configure idempotency key missing error message](#mandatory_header_response) section.
+
 ## Configuration 🪚
 
 In addition to the storage aspect, you have the option to supply additional configuration details to tailor the gem to the specific requirements of your project.
@@ -153,9 +186,11 @@ Grape::Idempotency.configure do |c|
 end
 ```
 
+In the configuration above, the error is following the [RFC-7807](https://datatracker.ietf.org/doc/html/rfc7807) format.
+
 ### processing_response
 
-When a request with a `Idempotency-Key: <key>` header is performed while a previous one still on going with the same idempotency value, this gem returns a `409 - Conflict` status. Thre response body returned by the gem looks like:
+When a request with a `Idempotency-Key: <key>` header is performed while a previous one still on going with the same idempotency value, this gem returns a `409 - Conflict` status. The response body returned by the gem looks like:
 
 ```json
 {
@@ -181,6 +216,33 @@ end
 
 In the configuration above, the error is following the [RFC-7807](https://datatracker.ietf.org/doc/html/rfc7807) format.
 
+### mandatory_header_response
+
+If the Idempotency-Key request header is missing for a idempotent operation requiring this header, the gem will reply with an HTTP 400 status code with the following body:
+
+```json
+{
+  "title": "Idempotency-Key is missing",
+  "detail": "This operation is idempotent and it requires correct usage of Idempotency Key.",
+}
+```
+
+You have the option to specify the desired response body to be returned to your users when this error occurs. This allows you to align the error format with the one used in your application.
+
+```ruby
+Grape::Idempotency.configure do |c|
+  c.storage = @storage
+  c.mandatory_header_response = {
+    "type": "about:blank",
+    "status": 400,
+    "title": "Idempotency-Key is missing",
+    "detail": "Please, provide a valid idempotent key in the headers for performing this operation"
+}
+end
+```
+
+In the configuration above, the error is following the [RFC-7807](https://datatracker.ietf.org/doc/html/rfc7807) format.
+
 ## Changelog
 
 If you're interested in seeing the changes and bug fixes between each version of `grape-idempotency`, read the [Changelog](https://github.com/jcagarcia/grape-idempotency/blob/main/CHANGELOG.md).
@@ -223,4 +285,5 @@ Open issues on the GitHub issue tracker with clear information.
 
 ### Contributors
 
-*   Juan Carlos García - Creator - https://github.com/jcagarcia
\ No newline at end of file
+* Juan Carlos García - Creator - https://github.com/jcagarcia
+* Carlos Cabanero - Contributor - https://github.com/Flip120
\ No newline at end of file
diff --git a/lib/grape/idempotency.rb b/lib/grape/idempotency.rb
index 8467907..8bfa3d6 100644
--- a/lib/grape/idempotency.rb
+++ b/lib/grape/idempotency.rb
@@ -21,19 +21,19 @@ def restore_configuration
         @configuration = clean_configuration
       end
 
-      def idempotent(grape, &block)
+      def idempotent(grape, required: false, &block)
         validate_config!
 
         idempotency_key = get_idempotency_key(grape.request.headers)
-        return block.call unless idempotency_key
+
+        grape.error!(configuration.mandatory_header_response, 400) if required && !idempotency_key
+        return block.call if !idempotency_key
 
         cached_request = get_from_cache(idempotency_key)
         if cached_request && (cached_request["params"] != grape.request.params || cached_request["path"] != grape.request.path)
-          grape.status 409
-          return configuration.conflict_error_response
+          grape.error!(configuration.conflict_error_response, 409)
         elsif cached_request && cached_request["processing"] == true
-          grape.status 409
-          return configuration.processing_response.to_json
+          grape.error!(configuration.processing_response, 409)
         elsif cached_request
           grape.status cached_request["status"]
           grape.header(ORIGINAL_REQUEST_HEADER, cached_request["original_request"])
@@ -213,7 +213,7 @@ def configuration
     end
 
     class Configuration
-      attr_accessor :storage, :expires_in, :idempotency_key_header, :request_id_header, :conflict_error_response, :processing_response
+      attr_accessor :storage, :expires_in, :idempotency_key_header, :request_id_header, :conflict_error_response, :processing_response, :mandatory_header_response
 
       class Error < StandardError; end
 
@@ -230,6 +230,10 @@ def initialize
           "title" => "A request is outstanding for this Idempotency-Key",
           "detail" => "A request with the same idempotent key for the same operation is being processed or is outstanding."
         }
+        @mandatory_header_response = {
+          "title" => "Idempotency-Key is missing",
+          "detail" => "This operation is idempotent and it requires correct usage of Idempotency Key."
+        }
       end
     end
   end
diff --git a/lib/grape/idempotency/helpers.rb b/lib/grape/idempotency/helpers.rb
index f56396e..3acba23 100644
--- a/lib/grape/idempotency/helpers.rb
+++ b/lib/grape/idempotency/helpers.rb
@@ -1,8 +1,8 @@
 module Grape
   module Idempotency
     module Helpers
-      def idempotent(&block)
-        Grape::Idempotency.idempotent(self) do
+      def idempotent(required: false, &block)
+        Grape::Idempotency.idempotent(self, required: required) do
           block.call
         end
       end
diff --git a/spec/idempotent_spec.rb b/spec/idempotent_spec.rb
index 20e26d7..2d1a7b9 100644
--- a/spec/idempotent_spec.rb
+++ b/spec/idempotent_spec.rb
@@ -162,7 +162,7 @@
                 header "idempotency-key", idempotency_key
                 post 'payments?locale=en', { amount: 800_00 }.to_json
                 expect(last_response.status).to eq(409)
-                expect(last_response.body).to eq("{\"title\"=>\"Idempotency-Key is already used\", \"detail\"=>\"This operation is idempotent and it requires correct usage of Idempotency Key. Idempotency Key MUST not be reused across different payloads of this operation.\"}")
+                expect(last_response.body).to eq("{\"title\":\"Idempotency-Key is already used\",\"detail\":\"This operation is idempotent and it requires correct usage of Idempotency Key. Idempotency Key MUST not be reused across different payloads of this operation.\"}")
               end
             end
 
@@ -192,7 +192,7 @@
                 header "idempotency-key", idempotency_key
                 post 'refunds?locale=es', { amount: 100_00 }.to_json
                 expect(last_response.status).to eq(409)
-                expect(last_response.body).to eq("{\"title\"=>\"Idempotency-Key is already used\", \"detail\"=>\"This operation is idempotent and it requires correct usage of Idempotency Key. Idempotency Key MUST not be reused across different payloads of this operation.\"}")
+                expect(last_response.body).to eq("{\"title\":\"Idempotency-Key is already used\",\"detail\":\"This operation is idempotent and it requires correct usage of Idempotency Key. Idempotency Key MUST not be reused across different payloads of this operation.\"}")
               end
             end
           end
@@ -340,6 +340,24 @@
             post 'payments', { amount: 100_00 }.to_json
             expect(last_response.body).to eq({ amount_to: 2 }.to_json)
           end
+
+          context 'BUT the idempotency key header was mandatory' do
+            it 'returns 400 bad request response' do
+              app.post('/payments') do
+                idempotent(required: true) do
+                  status 201
+                  { amount_to: SecureRandom.random_number }.to_json
+                end
+              end
+
+              post 'payments', { amount: 100_00 }.to_json
+              expect(last_response.body).to eq({
+                title: "Idempotency-Key is missing",
+                detail: "This operation is idempotent and it requires correct usage of Idempotency Key."
+              }.to_json)
+              expect(last_response.status).to eq(400)
+            end
+          end
         end
       end
     end
@@ -513,7 +531,7 @@
         header "idempotency-key", idempotency_key
         post 'payments?locale=en', { amount: 800_00 }.to_json
         expect(last_response.status).to eq(409)
-        expect(last_response.body).to eq("{:error=>\"An error wadus with conflict\", :status=>409, :message=>\"You are using the same idempotency key for two different requests\"}")
+        expect(last_response.body).to eq("{\"error\":\"An error wadus with conflict\",\"status\":409,\"message\":\"You are using the same idempotency key for two different requests\"}")
       end
     end