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Run npx skillmds@latest add acaprino/type-safety-rules in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

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Who published Type Safety Rules?

acaprino (@acaprino) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Type Safety Rules by acaprino — Agent Skill

Name: Type Safety Rules
Author: acaprino

Type Safety Rules

20 rules across 7 categories, each with detection and fix guidance. TRIGGER WHEN: reviewing or hardening TypeScript against type-system erosion: `any` leakage, unsound casts, missing boundary validation, assertion abuse, tsconfig strictness, exhaustiveness and generics soundness. DO NOT TRIGGER WHEN: style and naming review (use typescript-write), or dead-code detection (use knip).

acaprino Updated Sep 17, 2026

File contents

TypeScript Type-Safety Rules

Review-oriented rule set for hunting type-system erosion in TypeScript codebases. 20 rules across 7 categories, prioritized by blast radius. Used as the audit checklist by the type-safety-auditor agent and usable standalone while writing or hardening TypeScript.

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Any Erosion	CRITICAL	`any-`
2	Unsound Casts	CRITICAL	`cast-`
3	Boundary Validation	CRITICAL	`boundary-`
4	Assertion Abuse	HIGH	`assert-`
5	Compiler Configuration	HIGH	`config-`
6	Exhaustiveness	MEDIUM-HIGH	`exhaust-`
7	Generics Soundness	MEDIUM	`generics-`

Quick Reference

1. Any Erosion (CRITICAL)

any-explicit - Replace explicit any with unknown plus narrowing
any-implicit-boundary - Type the results of JSON.parse, response.json(), and catch immediately
any-generic-default - Never default a type parameter to any

2. Unsound Casts (CRITICAL)

cast-as-unsound - An as cast that changes the shape needs a runtime check instead
cast-double - as unknown as X is a type-system bypass; write a guard or converter
cast-const-assertion - Use as const for literal narrowing instead of widening annotations

3. Boundary Validation (CRITICAL)

boundary-http - Parse HTTP payloads with a schema at the edge
boundary-queue - Validate queue and event messages before processing
boundary-storage - Validate and version storage reads (localStorage, files, DB JSON)
boundary-env - Access environment variables through one validated config module

4. Assertion Abuse (HIGH)

assert-non-null - ! needs an adjacent invariant justification or a fail-fast check
assert-ts-expect-error - Use @ts-expect-error with a reason, never @ts-ignore

5. Compiler Configuration (HIGH)

config-strict - "strict": true is the baseline
config-unchecked-index - Enable noUncheckedIndexedAccess
config-exact-optional - Enable exactOptionalPropertyTypes
config-skiplibcheck - skipLibCheck may speed up third-party types, never hide first-party errors

6. Exhaustiveness (MEDIUM-HIGH)

exhaust-switch-never - Discriminated unions get a never assertion in the default branch
exhaust-satisfies-record - Use satisfies Record<K, V> for complete lookup tables

7. Generics Soundness (MEDIUM)

generics-constraint - Constrain type parameters on public APIs
generics-type-guards - A type predicate must verify the whole shape it claims

How to Use

Read individual rule files in the rules/ directory for detailed explanations and code examples. Each rule file contains a brief explanation, an incorrect example, a correct example, and a detection hint for reviewers.

---name: type-safety-rulesdescription: 20 rules across 7 categories, each with detection and fix guidance. TRIGGER WHEN: reviewing or hardening TypeScript against type-system erosion: `any` leakage, unsound casts, missing boundary validation, assertion abuse, tsconfig strictness, exhaustiveness and generics soundness. DO NOT TRIGGER WHEN: style and naming review (use typescript-write), or dead-code detection (use knip).---# TypeScript Type-Safety RulesReview-oriented rule set for hunting type-system erosion in TypeScript codebases. 20 rules across 7 categories, prioritized by blast radius. Used as the audit checklist by the `type-safety-auditor` agent and usable standalone while writing or hardening TypeScript.## Rule Categories by Priority| Priority | Category | Impact | Prefix ||----------|----------|--------|--------|| 1 | Any Erosion | CRITICAL | `any-` || 2 | Unsound Casts | CRITICAL | `cast-` || 3 | Boundary Validation | CRITICAL | `boundary-` || 4 | Assertion Abuse | HIGH | `assert-` || 5 | Compiler Configuration | HIGH | `config-` || 6 | Exhaustiveness | MEDIUM-HIGH | `exhaust-` || 7 | Generics Soundness | MEDIUM | `generics-` |## Quick Reference### 1. Any Erosion (CRITICAL)- `any-explicit` - Replace explicit `any` with `unknown` plus narrowing- `any-implicit-boundary` - Type the results of `JSON.parse`, `response.json()`, and `catch` immediately- `any-generic-default` - Never default a type parameter to `any`### 2. Unsound Casts (CRITICAL)- `cast-as-unsound` - An `as` cast that changes the shape needs a runtime check instead- `cast-double` - `as unknown as X` is a type-system bypass; write a guard or converter- `cast-const-assertion` - Use `as const` for literal narrowing instead of widening annotations### 3. Boundary Validation (CRITICAL)- `boundary-http` - Parse HTTP payloads with a schema at the edge- `boundary-queue` - Validate queue and event messages before processing- `boundary-storage` - Validate and version storage reads (localStorage, files, DB JSON)- `boundary-env` - Access environment variables through one validated config module### 4. Assertion Abuse (HIGH)- `assert-non-null` - `!` needs an adjacent invariant justification or a fail-fast check- `assert-ts-expect-error` - Use `@ts-expect-error` with a reason, never `@ts-ignore`### 5. Compiler Configuration (HIGH)- `config-strict` - `"strict": true` is the baseline- `config-unchecked-index` - Enable `noUncheckedIndexedAccess`- `config-exact-optional` - Enable `exactOptionalPropertyTypes`- `config-skiplibcheck` - `skipLibCheck` may speed up third-party types, never hide first-party errors### 6. Exhaustiveness (MEDIUM-HIGH)- `exhaust-switch-never` - Discriminated unions get a `never` assertion in the default branch- `exhaust-satisfies-record` - Use `satisfies Record<K, V>` for complete lookup tables### 7. Generics Soundness (MEDIUM)- `generics-constraint` - Constrain type parameters on public APIs- `generics-type-guards` - A type predicate must verify the whole shape it claims## How to UseRead individual rule files in the `rules/` directory for detailed explanations and code examples. Each rule file contains a brief explanation, an incorrect example, a correct example, and a detection hint for reviewers.

Frequently asked questions