What is the default Casus Belli Costs game rule in CK3?

Default: Casus Bellis cost resources to declare, such as Prestige or Piety. (patch 1.20.1.2)

Does changing the Casus Belli Costs rule disable achievements?

Yes for “No Costs”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

Game rule · Difficulty · Game Modes

Casus Belli Costs

Default
No Costs ✕

Casus Belli Costs is a game rule in Crusader Kings III (Difficulty and Game Modes) with 2 settings: Default (the default) and No Costs. “No Costs” disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Casus Belli Costs game rule in CK3?: Default: Casus Bellis cost resources to declare, such as Prestige or Piety. (patch 1.20.1.2)
Does changing the Casus Belli Costs rule disable achievements?: Yes for “No Costs”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

2 settings

Default
Default
Casus Bellis cost resources to declare, such as Prestige or Piety.
default_casus_belli_costs
No Costs
Disables achievements
Casus Bellis are free to use, without any associated costs.
The script checks it once, in 1 place
Script values 1
- common_cb_cost_multiplier_core
Every has_game_rule = no_cost_casus_belli_costs in game/common and game/events, by the top-level object it sits in.
no_cost_casus_belli_costs

Source: game/common/game_rules/00_game_rules.txt line 845 (key casus_belli_costs), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.