version: "1.0.0" name: use-is-kit description: Create, compose, migrate, and review TypeScript runtime type guards with is-kit. Use when adding reusable guards; replacing typeof, null checks, or handwritten type predicates; narrowing unknown values; validating JSON or API responses; building collection or object schemas with struct or typedStruct; or reviewing existing is-kit usage. Do not use for one-off environment checks or validation that requires data transformation or rich error reporting.

Use is-kit

Build runtime checks that preserve TypeScript control-flow narrowing without inventing project architecture or duplicating guards that already exist.

1. Inspect before editing

1. Read the repository instructions and relevant package scripts.
2. Confirm the installed is-kit version from the package manifest and lockfile.
3. Inspect the installed declarations, package exports, or monorepo source before

relying on an API name. Prefer the installed version over online examples.

1. Search for an equivalent local guard, wrapper, barrel export, schema, or

established import boundary.

1. Reuse an existing guard when it already expresses the required semantics.

Do not add a wrapper layer merely because this skill mentions one. If the project already has an is-kit boundary, respect it; otherwise import from is-kit directly.

2. Choose the smallest correct guard

Read guard-selection.md when choosing between built-ins, logical combinators, custom predicates, equality guards, and collection combinators.

For object or API response validation, always read object-validation.md before choosing struct, typedStruct, optionalKey, or safeParse.

For replacement or cleanup work, read migration-checklist.md before changing manual checks.

Prefer composition over a handwritten type-predicate signature. Keep guards total, deterministic, and side-effect free.

3. Place the guard by responsibility

• Put broadly reusable, domain-free guards in the project's existing shared

guard location, if one exists.

• Keep a schema beside the boundary that consumes the validated value when the

schema exists for one API, file, parser, or integration.

• Keep business decisions and domain state predicates in their owning feature

or domain module.

• Add or update barrel exports only when the project already uses them.
• Follow the repository's naming, documentation, and test-collocation rules.

Avoid creating a global guard from a one-off payload projection. Avoid moving domain knowledge into a generic guard module.

4. Implement without weakening the boundary

• Accept untrusted input as unknown.
• Narrow before reading or returning typed data.
• Remove an as assertion when the new guard makes it unnecessary.
• Store an indexed property in a local constant before narrowing when

TypeScript does not retain the property refinement.

• Use safeParse when the success branch consumes the validated value; call a

guard directly when only a boolean decision is needed.

• Default to forward-compatible object validation. Reject extra keys only when

the contract explicitly requires an exact object.

5. Verify behavior and types

Add focused tests using the project's existing test tools:

• at least one accepted value;
• at least one rejected value;
• at least one value of the wrong runtime type;
• object cases for missing, optional, and extra keys when relevant;
• type-level assertions when the repository already tests inference or

narrowing.

Run the narrowest relevant test, typecheck, lint, and formatting commands. Inspect the final diff for unrelated formatting or generated-file changes.

Boundaries

• Do not replace runtime or platform detection such as

typeof window === 'undefined' merely for stylistic consistency.

• Do not replace a clear one-off primitive check unless reuse, composition, or

narrowing materially improves.

• Do not upgrade is-kit unless the task requires an API unavailable in the

installed version or the user requests the upgrade.

• Recommend a schema validator instead when the requirement depends on data

transformation, coercion, detailed issue paths, or rich validation errors.