What is CB20?

July 9, 2026 · 5 min read

CB20 is the original tabletop RPG system that powers CharacterBug's Dungeon Mode. It was designed specifically for AI-driven play — which means it had to solve problems that traditional tabletop systems don't have.

Why not just use D&D rules?

Dungeons & Dragons is the most well-known tabletop RPG, and for good reason. But using D&D rules in a software product creates two problems:

Trademark and licensing. D&D and its associated terms are owned by Wizards of the Coast. Using them commercially requires navigating the Open Gaming License, which has been the subject of significant controversy and legal uncertainty. CB20 avoids this entirely — every term, mechanic, and rule is original.

Complexity. D&D 5th Edition has hundreds of pages of rules, edge cases, and optional systems. That's great for a human DM who can make rulings on the fly, but it's problematic for an AI that needs to apply rules consistently. CB20 is streamlined: enough depth to be interesting, but simple enough for a deterministic engine to run without ambiguity.

The core mechanics

Six abilities

Every character has six ability scores: Might (strength and melee), Finesse (agility and ranged), Vitality (health and endurance), Insight (magic and perception), Awareness (alertness and reflexes), and Presence (force of personality).

These map to familiar archetypes but use original names to avoid trademark issues. Ability modifiers are calculated the same way as most d20 systems: (score - 10) / 2, rounded down.

d20 core resolution

Like most tabletop RPGs, CB20 uses a 20-sided die as the core mechanic. To attack, you roll d20 + attack modifier and compare to the target's Defense (armor class). Equal or higher hits. Natural 20 is a critical success (double damage dice). Natural 1 is a critical failure.

Combat

Combat follows a familiar structure: roll for initiative, take turns, attack or use abilities, roll damage. Each character has HP (hit points), Defense (armor class), and a damage dice based on their weapon. When HP reaches 0, the character is down — and must make recovery saves (death saves) to avoid dying.

Progression

Characters earn XP from defeating monsters and completing quests. Level thresholds follow a standard curve (100 XP for level 2, 300 for level 3, etc.). Each level grants increased max HP (based on the class's hit die), and new class abilities unlock at specific levels. At levels 4, 8, and 12, characters gain ability score improvements.

11 classes

CB20 has 11 classes, each with a unique playstyle:

Warrior — tanky melee fighter
Skirmisher — agile striker
Mage — arcane spellcaster
Tracker — ranged hunter
Adept — support caster
Templar — holy warrior
Berserker — reckless damage
Minstrel — buff/support
Warden — nature defender
Ascetic — disciplined monk
Pactblade — eldritch knight

Why CB20 works for AI play

The key design principle is deterministic state tracking with AI narration. The AI never decides game outcomes — it only narrates what the deterministic engine has already resolved. This means:

The AI can't cheat (let you win when you should have lost)
The AI can't lose track of your HP or inventory
Combat outcomes are reproducible and fair
The AI focuses on what it's good at: storytelling and world-building

This is why CB20 was built from scratch rather than adapting an existing system. Traditional tabletop RPGs assume a human brain is tracking state and making rulings. CB20 assumes a deterministic engine is tracking state, and the AI is purely a narrator. That architectural difference shapes every rule.