Typescript contracts review

TypeScript Contracts Review

Purpose

"The build passes" is only meaningful evidence of type safety if the active tsconfig is actually strict and the code doesn't defeat it with any, unchecked assertions, or broad suppression comments — and TypeScript types are fully erased at compile time, so a type annotation on external data (a parsed JSON response, a third-party SDK payload, a URL parameter) provides zero runtime protection unless paired with an actual runtime validator. This skill exists so those two failure modes — a loose or silently-weakened tsconfig, and type annotations that assert safety no runtime check backs up — get audited explicitly instead of being assumed away by a green checkmark. It also checks exported public-API type surfaces so a "just an internal refactor" diff doesn't silently break every downstream consumer of a published package.

When to use

Use this skill when the user asks to:

• review a TypeScript/TSX diff for type-safety soundness before merge,
• audit a codebase's tsconfig strictness posture against current TypeScript-recommended defaults,
• check any/type-assertion/non-null-assertion usage, especially at external-data boundaries,
• verify a discriminated union or type-guard function actually narrows correctly,
• assess whether a change to an exported/published package breaks its public API type surface.

Do not use this skill for:

• pure runtime/logic bug hunting that has nothing to do with type contracts — that is a general code-review task,
• JavaScript files with no type annotations or JSDoc types — there is no type contract to audit,
• live tsc --noEmit/type-coverage execution results interpretation beyond what this static review can determine from the diff — report that a live run is needed rather than fabricating its result.

Context7 Documentation Protocol

• Resolve the TypeScript library ID with resolve-library-id before ruling on any compiler-flag or strict-family question; do not answer from memory, because recommended defaults change across releases (confirmed via Context7: TypeScript 5.9's tsc --init now emits noUncheckedIndexedAccess: true and exactOptionalPropertyTypes: true as a separate "Stricter Typechecking Options" block alongside strict: true, where earlier tsc --init output only set strict: true).
• Before asserting which individual flags strict: true bundles (strictNullChecks, noImplicitAny, noImplicitThis, alwaysStrict, strictFunctionTypes, strictBindCallApply, strictPropertyInitialization, useUnknownInCatchVariables), call query-docs against the current TypeScript docs rather than reciting a memorized list — the bundle is described as open-ended ("future versions of TypeScript may introduce additional stricter checking under this flag"), so a stale list under-reports what a repo's strict: true actually enables on its installed compiler version.
• Before flagging or endorsing a typescript-eslint rule (e.g. no-explicit-any, no-unsafe-assignment, no-non-null-assertion), verify via query-docs whether that rule is in the repo's active shared config (recommended, recommended-type-checked, strict-type-checked) — rule membership across those tiers has changed between major versions (confirmed via Context7: the v7→v8 recommended-type-checked diff added/removed multiple rules), so do not assume a rule is enabled just because the repo extends a config by name without checking the installed version.
• Read package.json/lockfile first to confirm the installed typescript and typescript-eslint/@typescript-eslint/* major versions before citing version-gated behavior (e.g. exactOptionalPropertyTypes semantics, satisfies operator availability, const type parameters) — do not assume a feature is available because it appears in current docs if the installed major predates it.
• If Context7 is unavailable, fall back to the official_docs URLs in this skill's metadata.json and label every version-sensitive claim documentation-based, unverified against installed compiler version.

Lean operating rules

• Never accept "the build passes" as sufficient evidence of type safety without checking the actual tsconfig strict-family flags in effect — a loose config proves far less than developers assume, and strict: true alone (pre-5.9 tsc --init default) does not imply noUncheckedIndexedAccess or exactOptionalPropertyTypes are on.
• Every new any must carry an adjacent justification comment; flag unjustified any as blocking, especially inside application logic that later consumers trust as validated.
• Every trust-boundary type (parsed JSON, third-party SDK response, postMessage payload, URL/query-param, form input, environment variable) must be paired with an actual runtime validator — a type annotation alone is erased at compile time and enforces nothing at runtime.
• Treat any proposal to loosen existing tsconfig strictness (removing strict, disabling strictNullChecks) as requiring an explicit, separately-reviewed migration plan, not a routine PR change.
• Flag broad @ts-nocheck/file-level suppression as blocking by default; require @ts-ignore/@ts-expect-error to carry an adjacent comment explaining why, weighted higher severity if the suppressed code is security- or trust-boundary-relevant.
• Do not let generic-type complexity become unreadable; a type requiring a comment to explain what it constrains is a design smell worth simplifying, not a badge of sophistication.
• Do not run or assert tsc --noEmit success from memory — invoke it or explicitly flag that CI must, rather than fabricating a compile-success claim.
• When a diff touches an exported/published package's public surface, check for a breaking type change (removed export, narrowed parameter type, widened return type becoming a narrower consumer-facing type, added required property) before approving; a type-only change can still be a semver-breaking change even with zero runtime behavior difference.

References

Load these only when needed:

• Strict-flag posture reference — use when auditing or recommending a tsconfig strict-family flag set, including which flags are bundled under strict vs. separately opt-in (noUncheckedIndexedAccess, exactOptionalPropertyTypes), and how to read a repo's actual effective config (including extends chains).
• Trust-boundary validation patterns — use when auditing external-data ingestion points (API responses, JSON.parse, postMessage, URL parsing, form/env input) for paired runtime validation against their declared types.
• Public API surface diffing — use when a diff touches an exported/published package and a breaking-change check against the previous public .d.ts/export surface is needed.

Response minimum

Return, at minimum:

• the tsconfig strictness posture summary (which strict-family flags are on/off vs. current TypeScript-recommended defaults, and the compiler version that posture was checked against),
• the any/assertion audit for every new any, as, and ! in the diff, each flagged with its justification or lack thereof and file:line evidence,
• the trust-boundary validation audit for every external-data ingestion point touched, naming the missing or present runtime validator,
• the public-API surface diff and any breaking-change flags, when the diff touches an exported package,
• verdict (approve / approve-with-notes / block),
• residual risk notes for anything requiring a live tsc --noEmit/type-coverage run beyond this static diff review.