Apply the language's actual contracts

Use this skill for code changes and language-specific review. Resolve the consuming checkout, instructions and affected files with how. Read the project's manifests, lockfiles, compiler/runtime targets, formatter and existing callers. A file extension is a routing hint, not proof of an application's stack, version, deployment model or permission to migrate it.

For every code change or review, read the code quality baseline first, then the language guide.

Select only the relevant guide

For a polyglot boundary, load the guides on both sides of the actual interface, not every guide in the repository. For an unlisted language, preserve the project's owning conventions and use focused research; do not guess that a neighboring language has the same semantics. The optional dependency-free router maps provided repository-relative paths to local guides; the agent can use it directly. It reads names only and executes no project code. It does not crawl the workspace, identify versions, choose frameworks or guarantee that a header is C rather than C++.

Carry the behavior through its callers

Identify who owns memory/resources, cancellation, mutable state and the committed effect. Preserve input validation, authority, error meaning and cleanup across language/FFI/process boundaries. Static annotations, deserialization and a successful compile do not establish valid domain state or tenant authorization. Define byte encoding, numeric ranges, nullability and error transport explicitly at those boundaries; do not silently truncate identifiers or timestamps.

Use the existing project's idioms and supported version. Add a dependency or new abstraction only when a demonstrated requirement justifies it. When the choice of language/runtime/framework is itself unsettled, use stack selection; routine coding is not a reason to reconsider the whole stack. Domain/API design stays with architect and backend design.

Select authorized checks by the changed contract under CI design: relevant compiler/analyzer/formatter and failure-sensitive behavior checks. Reuse configured commands and disposable fixtures. Do not install every toolchain, run all suites or claim production, platform, race freedom or live-agent qualification from these written examples.

The guides work offline. Their dated references are update pointers, and version notes apply only when the project's own target matches. Report what you could not confirm for the installed version.

Express valid states

Use types to represent valid combinations and handle variants exhaustively where the language permits. Parse external/persisted data at runtime; types and casts prove neither authorization nor freshness. Avoid ceremony that burdens callers. The type-system method supplies a worked check; implementation details stay in the relevant language reference.

Expo runtime

When Expo is actually present or explicitly selected, inspect SDK/runtime and native ownership, then the affected language guide. EAS-only native delivery does not select an Expo runtime migration.