How to set and add items to an array using HTTP API

No, the approach you suggested won't work for setting a specific array element by index. In Sanity's HTTP API patch operations, you cannot directly target array elements by numeric index in the set operation like that.

To modify a specific array element, you have a few options:

1. Use JSONMatch path syntax with the array index:

The correct way is to use the path notation in your set operation:

{
  "patch": {
    "id": "document-id",
    "set": {
      "data[34].field": "value"
    }
  }
}

This targets the element at index 34 in the data array and sets its field property.

2. Target by a unique identifier (recommended):

If your array objects have a _key field (which they should for arrays of objects), it's better to target by that:

{
  "patch": {
    "id": "document-id", 
    "set": {
      "data[_key==\"unique-key-here\"].field": "value"
    }
  }
}

This is more reliable because array indices can change if items are added or removed, but _key values remain stable.

3. Replace the entire array element:

You can also replace the whole element at a specific index:

{
  "patch": {
    "id": "document-id",
    "set": {
      "data[34]": {
        "_key": "some-key",
        "field": "value",
        "otherField": "otherValue"
      }
    }
  }
}

The key takeaway is that you use the path notation (like "data[34].field") in the keys of your set object, not as nested objects like in your example. The Sanity documentation on patches explains this uses JSONMatch syntax for targeting specific values in documents.