minecraft_lzstools/data/lzstools/predicate/hammer/mainhand.json

{
    "condition": "minecraft:entity_properties",
    "entity": "this",
    "predicate": {
        "equipment": {
            "mainhand": {
                "predicates": {
                    "minecraft:custom_data": {
                        "lzstools": [
                            "hammer"
                        ]
                    }
                }
            }
        }
    }
}