MANDATORY_FIELDS_MISS creating Invoice via Vtiger CRM webservices — LineItems capitalisation bug

I am trying to create an Invoice via the Vtiger CRM Webservices API. I keep getting MANDATORY_FIELDS_MISS even though I am sending all the fields that the UI shows as required. My payload includes lineitems. Vtiger 8.3.0.

The field name lineitems is wrong. The Vtiger CRM webservices API for inventory modules is case-sensitive and the correct key is LineItems — capital L, capital I.

Working payload:

json

{

  "subject": "INV-2025-001",

  "account_id": "3x12",

  "invoicedate": "2025-06-01",

  "LineItems": [

    {

      "productid": "14x55",

      "quantity": "2",

      "listprice": "150.00",

      "comment": "",

      "deleted": "0"

    }

  ]

}

Other case-sensitive inventory field names to watch:

| Wrong | Correct |
|---|---|
| lineitems | LineItems |
| hdnproductid | productid |
| productdescription | productDescription |

Debug tip: The error message MANDATORY_FIELDS_MISS does not tell you which field is missing. Enable PHP debug logging and look for the SQL INSERT that fails.