Developer Tools

npm: npmjs.com/package/@playbykey/theory
Documentation: theory-engine.docs.playbykey.com - interactive playgrounds
Changelog: GitHub Releases

Packages for PlayByKey - see the table below for what's here.

Install

npm install @playbykey/theory
pnpm add @playbykey/theory
yarn add @playbykey/theory
bun add @playbykey/theory

Quickstart and API examples: packages/theory/README.md

Foundation music theory engine: music theory as code. Modes, keys, intervals, and scale relationships resolved deterministically; one source of truth every app, notation tool, or AI agent can build on.
Zero dependencies: no transitive baggage; safe for agents, edge runtimes, and tight bundles
Strictly typed: every function's types exported alongside it, no any
Agent-ready: copy-paste LLM context prompt and interactive docs
Sharps-only notation: one canonical spelling per note (C#, not Db) keeps the Note type safe and unambiguous. Flats are fully supported for both input (parseNote/parseNoteToken accept Db, Eb, Gb, Ab, Bb) and output (getFlats/getEnharmonicLabels), without extending Note itself.

Package	Description	Links
`@playbykey/theory`	Zero-dependency music theory engine - scales, modes, intervals, chords, progressions, key relationships	npm · docs · changelog
`@playbykey/theory-mcp`	MCP server that exposes the theory engine as AI-callable tools	npm · README
`theory-docs`	Starlight documentation site (private workspace package)	live site · README

MIT for the repository tooling and @playbykey/theory. See LICENSE. Individual packages may declare their own license in their package.json.