How to Customize Existing API Routes in Open Source Medusa Project

[Abdul1Mateen]

Hi team,

I'm currently working with the open-source Medusa backend and would like to understand the best way to customize the existing APIs provided by Medusa—for example, to modify the logic of core endpoints like product creation, cart handling, or customer authentication etc.

I’ve gone through the docs and seen the approach for creating custom endpoints via custom routes and services. However, my goal is to extend or override the functionality of Medusa’s core API endpoints, not just add new ones.

Specifically, I’m looking for guidance on:

What is the recommended way to override the logic of a default Medusa endpoint (e.g., POST /store/customers, POST /admin/products)?

Should I clone the Medusa core packages and modify them directly, or is there an extension mechanism available (e.g., decorators, plugins) for these cases?

How do I ensure that my changes remain maintainable and upgrade-safe as Medusa continues to evolve?

If there's a standard practice (like middleware injection or handler replacement), could you point to an example or pattern to follow?

Any help or references to examples (repos, plugins, etc.) would be much appreciated. Thanks in advance for your support.

Best,
Abdul Mateen

[Vallabh-Mahanta]

You can customize Medusa's core endpoints without modifying the core packages.

In medusa/integration-tests/modules/medusa-config.js

1. Use Service Overrides:

   // In your project's services directory
   export class CustomProductService extends ProductService {
     async create(data: CreateProductInput): Promise<Product> {
       // Add your custom logic before
       const product = await super.create(data)
       // Add your custom logic after
       return product
     }
   }

2. Register Custom Services:

   // In your medusa-config.js
   module.exports = {
     modules: {
       services: {
         productService: {
           resolve: CustomProductService,
           options: {}
         }
       }
     }
   }

3. Use Middleware for Request/Response Modification:

   // In your project's middleware
   export const customMiddleware = async (req, res, next) => {
     // Modify request/response
     await next()
   }

What I think is always extend existing services rather than replacing them, for request/response modifications I prefer to use middleware.

[Vallabh-Mahanta]

Hope this helped.