[400] Client error: `POST https://api.hubapi.com/crm/v3/objects/contacts/batch/create` resulted in a `400 Bad Request` response: {"status":"error","message":"Invalid input JSON on line 1, column 75: Cannot deserialize value of type `java.lang.String (truncated...)
Private App Key Migration: Cannot deserialize value of type `java.lang.String (truncated...
The JSON is different in V3 than V1. On the same documentation page that is snapped above, scroll down and look at the Response example. It shows properly formatted JSON. It no longer has property & value. So, skimura was on the right track.