Write Svelte

Svelte Conventions

Load write-frontend first. Read the installed Svelte/SvelteKit versions, compiler configuration, nearby components, and official documentation for the installed major version before choosing an API. New code uses Svelte 5 idioms unless an existing compatibility boundary requires legacy syntax.

Component boundaries

• Give a component one clear semantic responsibility. Split by behavior or ownership, not by an arbitrary line count.
• Keep state in the lowest component that owns it. Pass explicit data and callbacks; use context only for stable, tree-wide capabilities that prop threading would obscure.
• Build small semantic components that compose like bricks. Use props for behavior and stable state; use typed snippets, children, and neighboring components for optional content and arrangement. Split an API that grows presentation booleans or position switches.
• Let one content prop accept text shorthand or a typed snippet when both are valid. Collection items keep a plain text name for accessibility and typeahead and use a separate snippet for visual content.
• Preserve the native element API where practical. Wrapper components must not silently remove form, focus, keyboard, or accessibility behavior.
• Keep route/business orchestration out of reusable visual components. Keep platform-only imports out of publishable packages.
• Let shared layout components own only geometry, responsive behavior, and typed composition points. Keep landmarks, navigation, headings, labels, surfaces, route orchestration, and domain workflow in the consuming application unless one is the explicit semantic responsibility of a smaller primitive.

Svelte 5 reactivity

• Declare a typed props shape and destructure $props(). Give defaults at the boundary.
• Use $bindable only when two-way ownership is part of the public contract; ordinary props remain one-way.
• Use $state for owned mutable state and $derived for values computed from state. Do not mirror a derivation through $effect.
• Reserve $effect for synchronization with an external system. Return cleanup for subscriptions, observers, timers, and listeners.
• Keep effects narrowly dependent and safe to repeat. Never depend on effect ordering to make state valid.
• Use callback props for component events in new Svelte 5 code. Type payloads and callback return values; do not dispatch an event merely to mutate parent state indirectly.
• Use event attributes such as onclick in new Svelte 5 code. Do not mix legacy on: directives with event attributes inside one component.
• Use snippets and {@render ...} for new composition APIs. Follow an existing public legacy-slot contract until a planned breaking change migrates consumers.

Markup and accessibility

• Apply write-frontend native-semantics rules inside every template. Prefer native controls over event handlers on generic elements.
• Treat every Svelte accessibility compiler warning as a defect. Suppress only a verified false positive with the narrow rule name and an explanation of the satisfied behavior.
• Do not use {@html} for untrusted or user-controlled content. A type annotation is not sanitization.
• Use stable IDs for labels, descriptions, and errors. Avoid IDs derived from list indexes when items can move.
• Key each block by stable identity whenever instances can be inserted, removed, or reordered.
• Keep focus transitions intentional for dialogs, menus, route changes, validation, and async content.

Rendering and lifecycle

• Keep module initialization and template expressions free of side effects.
• Guard window, document, storage, observers, and browser-only libraries behind browser-aware lifecycle or environment boundaries so SSR and prerendering remain valid.
• Do not repair hydration mismatches by disabling SSR. Make server and client input deterministic.
• Clean up timers, subscriptions, observers, and global listeners when ownership ends.
• For native events queued around teardown, read the owning element from event.currentTarget inside the handler instead of a component binding that Svelte may already have cleared. Snapshot the element before an await; event dispatch state does not remain live after the synchronous handler. Cover teardown-sensitive native behavior in a real browser.
• Use actions or attachments for reusable DOM integration; keep their setup and teardown symmetric.
• Avoid imperative component APIs unless integration with a non-Svelte host requires them.

Styling

• Keep component styles scoped. Use :global only at an explicit application-shell or third-party integration boundary, with the reason visible beside it.
• Consume design tokens instead of declaring product values. Load write-design-system when a token or reusable component contract changes.
• Prefer class/state attributes and CSS pseudo-classes over inline style strings. CSS custom properties are the supported escape hatch for calculated values.
• Preserve logical properties, responsive content flow, reduced motion, forced colors, and visible focus behavior from write-frontend.

SvelteKit applications

• Put secrets, privileged calls, and server-only dependencies in server-only modules and server load functions/actions. Never import them through a module reachable by browser code.
• Use framework load, form actions, error, and redirect contracts rather than duplicating routing or request lifecycle logic in components.
• Return serializable data across the server/client boundary. Validate URL, form, cookie, and API input at runtime.
• Use progressive-enhancement helpers without making the enhanced path the only functional path.
• Keep layouts responsible for shared page structure and route files responsible for route data; extract reusable presentation and behavior into components/modules.

Published Svelte packages

• Export only intentional public entry points. Keep internal modules unreachable through package exports.
• Add concise JSDoc/TSDoc to exported components, props, snippet contracts, callbacks, and non-obvious invariants. Describe usage and behavior without restating the type; keep maintainer comments for decisions the code cannot make evident.
• Compile or lint documentation examples as Svelte/HTML/TypeScript with the declared language. Keep setup imports separate from illustrative component composition.
• Declare Svelte as a compatible peer dependency and emit the documented svelte export condition.
• Keep application, Storybook, fixtures, tests, and source-only tooling out of the tarball. Inspect the packed archive before release.
• Ship README, license, declarations, and required styles/assets. Test consumption from the package root rather than private source paths.
• Avoid module-level browser state in libraries; multiple consumers and SSR requests must remain isolated.

Validation

Run svelte-check through the repository lint/typecheck script, component tests through a-novel test --type=pnpm -y, and the production/package build through a-novel build --type=pnpm -y. For a published package, also inspect the package archive and verify that every documented import resolves.