Combat

Yes, in code rather than in prose. Hit points, armour class, spell slots, concentration, conditions, the action economy, experience and death saves are all tracked and enforced from SRD 5.2.1. The Dungeon Master narrates what the rules produced; it does not decide it.

• Hit points, damage, resistance, vulnerability and immunity, applied per damage type.
• Temporary hit points, which do not stack, and are spent before your real ones.
• Armour class as a computed number: armour, your dexterity within its cap, a shield, a fighting style, magic bonuses. Tap your AC in the app and you can read every line that produced it.
• Spell slots, prepared and known spells, and whether you actually have the action or bonus action to cast.
• Concentration, at the real save DC, broken by the real triggers, on one spell at a time.
• Weapon masteries as live mechanics: a miss with Graze still deals your ability modifier, and a hit with Vex gives advantage on your next attack against that target.

All of the numbers that matter. A creature arrives with its actual armour class, hit points, speed, senses, ability scores, saving throws, resistances and immunities. If it has legendary actions, it has them, they are counted, and they refill.

Multiattack is read from the creature's own description rather than assumed, so something that makes one bite and three claw attacks makes four attacks. At the top of the range, a tarrasque arrives at AC 25 and 697 hit points, with six uses of Legendary Resistance a day and the ability to swallow up to six creatures at once.

• A natural 20 on a death save puts you back up at 1 hit point, conscious, with the counters cleared.
• A natural 1 costs two failures.
• Being hit while you are down costs one failure, and two if the hit was a critical.
• Damage reduced to zero by resistance costs you no failure at all.
• Three successes stabilises you, and you come back at 1 hit point after 1d4 hours unless someone heals you sooner.
• Take damage while stable and the wound reopens: death saves resume on your next turn.
• A single hit at or above your hit point maximum kills outright.

And a Champion Fighter at level 18 gets to break the rule: Survivor treats an 18 or a 19 as a natural 20, and the game says so on the card when it happens.

There is no grid, no map, no measured distances, no facing, no cover and no line of sight. This is theatre of the mind, deliberately. If you want a battle map with counted squares, this is the wrong tool, and we would rather tell you now than have you find out in your third fight.

Encumbrance is not tracked. Multiclassing is not supported. Neither is respeccing a character after the fact. Those are real limits, and knowing them now is better than planning a build around one.

When a fight does go wrong, and software being software it sometimes will, the wrench beside the composer is the way out rather than a restart.