Error Unmergable intersection when reusing schema

[angel-luminos]

Description

I need to transform an id param to a MongoDb's ObjectId for an input property which follows the example posted here https://ez.robintail.cz/v24.7.0/transformations. This works fine until a middleware is added and the following error is returned Unmergable intersection. Error path: [\"id\"].

Expected

I would expect this to work with a middleware.

Reproduction

I've tried both z.preprocess and .transform to transform the input string to an ObjectId.

export const objectIdPreprocess = z.preprocess((val) => {
    if (typeof val === 'string') {
        return new ObjectId(val);
    }
    return val;
}, z.instanceof(ObjectId));

// export const objectIdTransform = z.string().transform((val) => {
//   return new ObjectId(val);
// })

const params = z.object({
  id: objectIdPreprocess,
});

const helloWorldEndpoint = defaultEndpointsFactory
  .addMiddleware({
    input: params,
    handler: async ({ input }) => {
      // logs an ObjectId instance as expected
      console.log({ input })
      return {}
    }
  })
  .build({
    method: 'post',
  
    input: params,
    output: z.object({
      value: z.string(),
    }),
    handler: async ({ input: { id }, options, logger }) => {
      // never gets to this point
      return { value: id.toHexString()};
    },
});

Context

• Node.js version: 22.15.0
• express-zod-api version: 24.7.1

[RobinTail]

Hello @angel-luminos ,

the problem is that you have used params schema twice:

1. in .addMiddleware()
2. and in .build().

You should not be doing that, because the final endpoint input schema consists of the input schemas of all the middlewares attached, plus the one you specify in .build(). Because you used params schema twice it can not merge those object-schemas due to meeting the same id property twice. This is zod's limitation on intersecting schemas — Unmergable intersection.

So, it should be enough in your case just to delete the input within .build(). The id still will be available for you in all handlers.