Model-Based Testing for Dungeons & Dragons

Summary

Last time, I modeled a single D&D character. One creature, standing alone in the void, tracking hit points, conditions, and spell slots. The Quint spec (a formal modeling language) for that was around 6,000 lines. Now I added combat. Not just “a Fighter hits a Goblin with a sword” kind of combat, but the hard part: Counterspell chains, Shield interrupting mid-attack, readied spells holding concentration, Legendary Resistance flipping saves. Thus, I did the hard part first. Without it, you can’t prove full viability. A spec that handles the simple cases proves nothing, and is akin to a “Todo App project”; the real blockers live in the interactions. If Counterspell chains can deadlock the state machine, or readied-spell concentration can orphan active effects, I needed to know now, not after building three more layers on top. The battle spec is less than half the creature code. You’d think it amounts to less than half the complexity, but that is far from true. A single creature’s state space is a flat reducer. A glorified character sheet. Level up, take damage, gain a condition, lose a condition. The state space is large but flat. Combat makes it profound. Two creatures interacting means every action can trigger reactions, every reaction can trigger counter-reactions, and the whole thing nests before anyone’s taken damage. This is where beginner and even proficient players often get confused. Here’s what the spec covers now: all the character classes (12 of them,) 14 conditions that modify what you can and can’t do. Legendary creatures that act on other creatures’ turns. Counterspell that interrupts spellcasting. Opportunity attacks that fire mid-movement and trigger full attack resolution chains. Both player characters and monsters, with character lists and stat blocks. The demo replays a scripted arcane battle step by step: Fireball, Counterspell chains, AoE damage, death saves. Step forward, step back, or hit play and watch the interrupt windows open and resolve. L...