Here is my datapack strucutre
.
└── tetra/
└── stamina_cost/
└── modular_sword.json

{
  "type": "melee_weapon",
  "stamina_cost": 10
}

However, it doesn't work. This works fine on vanilla minecraft weapons, but not with tetra.

Have you tried this with other mods? I've tested my datapack system with other mods like Simply Swords & Cataclysm, but not Tetra. I'll test that mod out and see if it has any weird outliers that might be causing the issue, and update here if I find anything.

not effect the tetra weapon with tetratic combat mod
but while adding a datapack, it works fine
here is my datapack strucutre
── data/
── ── tetra/
── ── ── stamina_cost/
── ── ── ── modular_sword.json

Have found a fix for this issue & the new update will have built in support for almost any mod that modifies the attack damage attribute of weapons. This means full support of Tetra and Truly Modular.