moshi adapter toJson converts to string instead of object

Question

Below is my code

val itemsInCart = viewModelScope.async {
    cartItemRepository = CartItemRepository()
    cartItemRepository?.getItemsInCartBySellerSuspend(email, sellerPK)
}

val getItemsInCart = itemsInCart.await()
val moshi = Moshi.Builder().add(BigDecimalAdapter).add(KotlinJsonAdapterFactory()).build()
val tempListItemsInCart = Types.newParameterizedType(List::class.java, CartItem::class.java)
val moshiAdapterForItemsInCart = moshi.adapter<List<CartItem>>(tempListItemsInCart)
val itemsInCartJson = moshiAdapterForItemsInCart.toJson(getItemsInCart)

Code above retrieves list of CartItem from db. CartItem contains BigDecimal, hence the usage of BigDecimalAdapter.

I passed itemsInCartJson to params["items"] = itemsInCartJson. Here I thought itemsInCartJson is supposed to be json object, but actually it was passed as String.

Here's how items was passed, as per retrofit's log

{"items":"[{\"id\":1,\"email\":\"[email protected]\",\"productId\":4,\"productPK\":\"BUSINESS#[email protected]\",\"productSK\":\"PRODUCT#f89tlW1ihl\",\"sku\":\"SK67\",\"name\":\"dekonh\",\"unitPrice\":\"20\",\"quantity\":3,\"totalPrice\":\"60\",\"hoursNeededToPrepare\":0,\"discountCode\":\"\"},{\"id\":2,\"email\":\"[email protected]\",\"productId\":5,\"productPK\":\"BUSINESS#[email protected]\",\"productSK\":\"PRODUCT#yt0TjnnCdD\",\"sku\":\"SKP8\",\"name\":\"First+product\",\"unitPrice\":\"1.7\",\"quantity\":2,\"totalPrice\":\"3.4\",\"hoursNeededToPrepare\":3,\"discountCode\":\"\"}]","pickUpBy":"fr"}

If it helps, here's how I build my retrofit

private val moshi = Moshi.Builder()
    .add(BigDecimalAdapter)
    .add(KotlinJsonAdapterFactory())
    .build()
private val retrofit = Retrofit.Builder()
    .addConverterFactory(ScalarsConverterFactory.create())
    .addConverterFactory(MoshiConverterFactory.create(moshi))
    .addCallAdapterFactory(NetworkResponseAdapterFactory())
    .baseUrl(BASE_URL)
    .client(client)
    .build()

Where did I do wrong? items in my backend expected object and not string.