Add a Stat Only If It Is Missing

Deep merge and + always win: whatever your patch writes, lands. Sometimes you want the opposite. You are shipping a default and you only want it to apply when the target does NOT already define it, so an item (or another mod) that already set its own value keeps it. That is what the ? fill-if-absent suffix does. The real case: hexcode wants every armor piece to grant some Mana and Volatility, but armor that already carries its own Mana from another mod must be left exactly as the author tuned it.

Goal

• Give Armor_Bronze_Hands a Volatility modifier, because it has none.
• Do NOT overwrite its Mana modifier, because the item already defines one.
• Touch nothing else.

The intention here is to have a "soft add" - so that if "Mana" is NOT defined, it can be added here. But if it is, to NOT override it.

The Asset

Server/Item/Items/Armor/Bronze/Armor_Bronze_Hands.json

{
  "TranslationProperties": {
    "Name": "server.items.Armor_Bronze_Hands.name"
  },
  "Quality": "Common",
  "ItemLevel": 8,
  "PlayerAnimationsId": "Block",
  "ItemSoundSetId": "ISS_Armor_Light",
  "Categories": [
    "Items.Armors"
  ],
  "Icon": "Icons/ItemsGenerated/Armor_Bronze_Hands.png",
  "Model": "Items/Armors/Bronze/Hands.blockymodel",
  "Texture": "Items/Armors/Bronze/Hands_Texture.png",
  "Armor": {
    "ArmorSlot": "Hands",
    "BaseDamageResistance": 0,
    "StatModifiers": {
      "Mana": [  // already present
        {
          "Amount": 50,
          "CalculationType": "Additive"
        }
      ]
    }
  },
  "MaxDurability": 80,
  "DurabilityLossOnHit": 0.5,
  "Tags": {
    "Type": [
      "Armor"
    ],
    "Family": [
      "Bronze"
    ]
  }
}

The patch

Server/Item/Items/Armor/Bronze/Armor_Bronze_Hands.patch

{
  "Armor": {
    "StatModifiers": {
      "Mana?": [
        { "Amount": 200, "CalculationType": "Additive" }
      ],
      "Volatility?": [
        { "Amount": 26, "CalculationType": "Additive" }
      ]
    }
  }
}

Mana? and Volatility? each resolve independently against the base. StatModifiers already has Mana, so the base wins and the patch's 200 is dropped. It has no Volatility, so that one is written.

Resulting Asset

• Mana is still the item's own 50. The patch's 200 never landed, because the key was already there.
• Volatility is now 26, added because the item had none.

{
  "Armor": {
    "ArmorSlot": "Hands",
    "BaseDamageResistance": 0,
    "StatModifiers": {
      "Mana": [
        {
          "Amount": 50,
          "CalculationType": "Additive"
        }
      ],
      "Volatility": [
        {
          "Amount": 26,
          "CalculationType": "Additive"
        }
      ]
    }
  }
}

Notes

• ? checks key presence, not value type. It works the same whether the value is an array, a scalar, or an object: present means base wins and the patch value is dropped; absent means the patch value is written.
• It is per key, so you opt each field in independently. Drop the ? from a key (e.g. plain Mana) and that key reverts to normal deep-merge / replace and overwrites the base.
• Contrast with $Priority: priority only orders patches against EACH OTHER, never against the base. A low $Priority still overwrites the base. ? is the only way to let the base win.
• Ship the same ? patch across a whole armor set: pieces that lack the stat get your default, pieces that already tune it keep theirs. See the array rules in the syntax reference.