Blades in the Dark

The @randsum/games/blades subpath provides mechanics for Blades in the Dark, a tabletop RPG about a crew of daring scoundrels seeking their fortunes on the haunted streets of an industrial-fantasy city.

Official Site Blades in the Dark by Evil Hat

Installation

bun add @randsum/games

npm install @randsum/games

Usage

import { roll } from '@randsum/games/blades'

// Roll with a dice pool of 3
const { result } = roll(3)
console.log(result) // 'critical' | 'success' | 'partial' | 'failure'

import { roll } from '@randsum/games/blades'

roll(0)  // Zero dice: rolls 2d6, keeps lowest
roll(1)  // One die
roll(2)  // Two dice
roll(3)  // Three dice
roll(4)  // Four dice (e.g., with assistance)
roll(5)  // Five dice (e.g., with assistance + push)
roll(6)  // Six dice (maximum pool)

import { roll } from '@randsum/games/blades'

const { result } = roll(2)

switch (result) {
case 'critical':
  // Multiple 6s - exceptional outcome
  break
case 'success':
  // Highest die is 6 - full success
  break
case 'partial':
  // Highest die is 4-5 - success with complication
  break
case 'failure':
  // Highest die is 1-3 - things go badly
  break
}

API

`roll(rating?: number)`

Input:

Parameter	Type	Description
`rating`	`number` (optional)	Action rating / dice pool size (0-6)

Returns: GameRollResult with:

Property	Type	Description
`result`	`BladesRollResult`	`'critical' \| 'success' \| 'partial' \| 'failure'`
`total`	`number`	Sum of kept dice after modifiers
`rolls`	`RollRecord[]`	Raw dice data from the core roller

Outcomes

Outcome	Condition	Description
`critical`	Multiple 6s	Critical success — exceptional outcome
`success`	Highest die is 6	Full success — you do it
`partial`	Highest die is 4-5	Partial success — you do it, but with a complication
`failure`	Highest die is 1-3	Bad outcome — things go wrong

Zero Dice Pool

When the dice pool is 0, the system rolls 2d6 and keeps only the lowest die. This makes failure much more likely and prevents critical results.

About this implementation

This package handles the dice roll outcome only. Position (controlled, risky, desperate) and Effect (limited, standard, great) are narrative mechanics in Blades in the Dark that a caller manages separately — they are not part of the roll output. Use result to determine the mechanical outcome, then apply position and effect in your own game logic.

Error handling

Game roll() can throw two types of errors:

ValidationError (from @randsum/roller) — numeric input out of range or not finite (e.g., rating of 7 when max is 6)
SchemaError (from @randsum/games/blades) — game-specific issues like unmatched outcome tables

import { roll, SchemaError } from '@randsum/games/blades'
import { ValidationError } from '@randsum/roller'

try {
roll(99)
} catch (error) {
if (error instanceof ValidationError) {
  // Out-of-range rating (must be 0–6)
  console.log(error.code)    // 'VALIDATION_ERROR'
} else if (error instanceof SchemaError) {
  // Game-specific error
  console.log(error.code)    // 'NO_TABLE_MATCH'
}
}

Types

import type { BladesRollResult, GameRollResult, RollRecord } from '@randsum/games/blades'
import { SchemaError } from '@randsum/games/blades'

Schema

This game is powered by a .randsum.json spec that defines the dice pool, outcome tiers, and validation rules. The TypeScript code is generated from this spec at build time. See Schema Overview for how game specs work.