principle-type-system-discipline

Use it when:: Designing types, reviewing a signature, or writing code in any statically typed language.
It does:: Brands primitives like UserId and OrderId so they can't be swapped, parses external data into typed models at boundaries, and derives types from the schema that owns the shape.
Aims for:: A compiler that rejects impossible states, swapped primitives and unhandled variants, and errors and special cases defined out of existence rather than handled.
Runs:: Only when you call it.

Skill 10 of 14 in the pstack bundle, after principle-boundary-discipline and before principle-fix-root-causes.

TL;DR

One of pstack's principle skills, in Cursor's plugin repo, for any typed language: a branded intersection in TypeScript, a newtype elsewhere. A branded value is validated once, when it's created, and trusted downstream. RPC payloads, JSON, IPC messages, CLI arguments, config files, environment variables and database rows count as untyped until a parse function turns them into the typed model. When a protocol buffer, OpenAPI spec, GraphQL schema, database migration or design-token file owns a shape, the type comes from it instead of a hand-written copy. For where validation goes it points to pstack's boundary-discipline principle, and for TypeScript syntax to its typescript-best-practices skill.

Summarizes the skill at 8ecfb19.

npx skills add https://github.com/cursor/plugins --skill principle-type-system-discipline

skills.sh page Source on GitHub All skills from cursor/plugins

Total Downloads: 5,300 on skills.sh, Nov 9, 2026
Repo stars: 15,776 cursor/plugins
First seen: May 24, 2026 on skills.sh

Weekly Downloads

334

+9% over the 8 weeks to Nov 9

Weekly values

Week to	Installs
Sep 21	307
Sep 28	344
Oct 5	411
Oct 12	334
Oct 19	461
Oct 26	498
Nov 2	385
Nov 9	334

Category	category:engineering category:software-engineering Test-first work, implementation from specs, git hygiene and scope control. Everything for building software: planning, coding, debugging, design, frameworks, cloud, agents and skills.
Type	type:guidelines Knowledge and rules the agent applies while it works: best practices, design principles, API references.
Effect	effect:none Adds no side effects of its own. It shapes how the agent thinks, writes or reviews. Guidelines and routers are always effect:none, even when the task they support edits code.
Needs	needs:none Instructions only. Works in any agent that supports skills.
Publisher	by:community Everyone else. Unverified vendor claims default to community.
Topics	code-quality domain-modeling

SKILL.md

From cursor/plugins at 8ecfb19. Checked daily, last changed Nov 6, 2026.

Frontmatter

name: principle-type-system-discipline
description: "Apply when designing types, reviewing a function signature, or writing code in any statically-typed language. Make illegal states unrepresentable, brand semantic primitives, parse external data at boundaries, exhaust variants, derive from authoritative schemas."
disable-model-invocation: true

Type System Discipline

Make the compiler reject impossible states, mismatched primitives, and unhandled variants. Prefer defining errors and special cases out of existence over proliferating handlers. For TypeScript syntax, see the typescript-best-practices skill.

Brand semantic primitives. UserId and OrderId are strings underneath but should not be interchangeable. Give each its own type, such as a branded intersection in TypeScript or a newtype elsewhere. Validate once at creation, trust the type downstream. If two arguments share a primitive type but mean different things, brand them.
External data is untyped until parsed. RPC payloads, JSON, IPC messages, CLI args, config files, environment variables, database rows. Have a parse function at every boundary that turns unstructured input into the typed model. The boundary-discipline principle skill says where to put validation.
Derive types from authoritative schemas. When a protocol buffer, OpenAPI spec, GraphQL schema, database migration, design-token file, or another file owns a shape, derive from it instead of hand-rolling a parallel type. See the encode-lessons-in-structure principle skill.