Svelte Vitals

svelte-vitals

When to use

Use this whenever you are writing or reviewing SvelteKit route files (+page.svelte, +layout.svelte) or components in this project — svelte-vitals statically checks SEO, performance, correctness, security, and architecture patterns.

Playbook

1. After writing or editing code, run npx svelte-vitals . --diff --reporter agent and fix any findings it reports.
2. Before committing, run npx svelte-vitals . --staged as a pre-commit gate.
3. For a rule's full rationale, configurable options and fix examples, run npx svelte-vitals explain <rule-id> (add --json for a structured object) or open its docs link below.
4. For anything else — reporters, the config file, scoping to a change, CI, monorepos — run npx svelte-vitals docs list and then npx svelte-vitals docs show <name>. Those guides ship inside the CLI, so they match the version installed here; prefer them over searching the web.

Rule digest

SEO

• seo/title-presence — Title presence (critical): A unique, non-empty is the single strongest on-page SEO signal and the text shown in search results and browser tabs. Fix: Add a inside svelte:head (a dynamic title is fine). (docs)
• seo/description-presence — Description presence (critical): A meta description is the snippet search engines show under your title; without one they invent one from page text, often poorly. Fix: Add a inside svelte:head, or set description on your meta component. (docs)
• seo/canonical-url — Canonical URL (warning): A canonical URL tells search engines which URL is authoritative, preventing duplicate-content dilution across query strings and trailing-slash variants. Fix: Add inside svelte:head, or set the canonical prop on your meta component. (docs)
• seo/og-image — Open Graph image (warning): og:image is the preview thumbnail shown when the page is shared on social platforms; without it links render bare and get fewer clicks. Fix: Add , or set openGraph.images on your meta component. (docs)
• seo/og-title — Open Graph title (warning): og:title controls the headline shown when the page is shared on social platforms, independent of the document . Fix: Add , or set openGraph.title on your meta component. (docs)
• seo/robots-txt — robots.txt (warning): robots.txt tells crawlers which paths they may fetch and points them to your sitemap; missing it leaves crawl behaviour to defaults. Fix: Create static/robots.txt (or a src/routes/robots.txt/+server endpoint). (docs)
• seo/sitemap-xml — sitemap.xml (warning): A sitemap.xml lists your URLs so search engines can discover and prioritise them, especially pages not well linked internally. Fix: Create static/sitemap.xml (or a src/routes/sitemap.xml/+server endpoint). (docs)
• seo/json-ld — JSON-LD structured data (info): JSON-LD structured data lets search engines render rich results (breadcrumbs, articles, products) for the page. Fix: Add a JSON-LD inside svelte:head with literal JSON (Svelte emits the script body as-is). (docs)
• seo/html-lang — (warning): The attribute declares the page language for search engines, screen readers, and translation tools. Fix: Set the lang attribute on in src/app.html. (docs)
• seo/indexability — Indexability (info): A noindex directive removes the page from search results; an accidental noindex on a public route silently deindexes it. Fix: If this route should be indexed, drop noindex from its . (docs)
• seo/twitter-card — Twitter Card (info): twitter:card selects how the page renders when shared on X/Twitter; without it the platform falls back to a basic link (Open Graph tags are used as fallbacks for the rest). Fix: Add a twitter:card meta tag in svelte:head. (docs)
• seo/og-description — Open Graph description (warning): og:description is the summary shown under the title in social previews; without it platforms guess or show nothing, lowering click-through. Fix: Add an og:description meta tag in svelte:head. (docs)
• seo/og-url — Open Graph URL (info): og:url tells social platforms the canonical address to attribute shares and likes to, consolidating engagement on one URL. Fix: Add an og:url meta tag in svelte:head. (docs)
• seo/viewport — Viewport (warning): Without a viewport meta tag the page is not mobile-responsive, which Google penalizes under mobile-first indexing. Fix: Add the viewport meta tag (typically in src/app.html ). (docs)
• seo/sitemap-in-robots — Sitemap referenced in robots.txt (info): A Sitemap: line in robots.txt helps crawlers discover your sitemap; without it discovery relies on manual submission. Fix: Add a Sitemap: line to static/robots.txt. (docs)
• seo/json-ld-validity — JSON-LD validity (warning): Invalid JSON-LD — unparseable, or missing @context/@type — is silently ignored by search engines, so the structured data does nothing. Fix: Make the JSON-LD valid: parseable JSON with both @context (schema.org) and @type. (docs)
• seo/json-ld-deprecated-type — Deprecated structured-data type (info): Some schema types no longer produce rich results, so the markup adds weight without the SERP benefit. (docs)
• seo/json-ld-relative-url — JSON-LD relative URL (warning): Search engines need absolute URLs in structured data; a relative URL cannot be resolved reliably. Fix: Replace relative URLs in JSON-LD with absolute URLs. (docs)
• seo/json-ld-date-format — JSON-LD date format (info): Schema.org date properties expect ISO-8601; other formats may be ignored or misparsed. Fix: Format JSON-LD date properties as ISO-8601. (docs)
• seo/json-ld-placeholder — JSON-LD placeholder text (info): Leftover placeholder text (e.g. "Your Company Name", "lorem ipsum") ships misleading structured data. (docs)
• seo/json-ld-required-props — JSON-LD required properties (warning): A recognized @type missing its required properties is ineligible for the corresponding rich result. (docs)
• seo/title-length — Title length (info): A title that is too short wastes the strongest on-page signal; one that is too long is truncated in the SERP. (docs)
• seo/description-length — Description length (info): A description that is too short under-uses the SERP snippet; one that is too long is truncated by search engines. (docs)
• seo/charset — Character encoding (warning): Without a declared character encoding the browser must guess, which can render text as mojibake; is the standard declaration. Fix: Add the charset meta tag (typically the first line of in src/app.html). (docs)
• seo/image-alt — Image alt text (warning): An with no alt attribute is invisible to image search and assistive technology; a descriptive alt is an image-SEO signal. Fix: Add a descriptive alt attribute to the (or alt="" if purely decorative). (docs)
• seo/hreflang — hreflang validity (warning): A malformed hreflang code or a missing x-default breaks international targeting, so search engines may serve the wrong language version. (docs)
• seo/single-h1 — Heading hierarchy (warning): Each page should have exactly one naming its main topic; none leaves the page without a primary heading, and several dilute the topic signal. (docs)
• seo/duplicate-title — Duplicate title (warning): Duplicate titles across pages make them compete in search results and weaken each page’s relevance signal. (docs)
• seo/duplicate-description — Duplicate description (warning): Duplicate meta descriptions give search engines no per-page summary, so they are often ignored or rewritten. (docs)
• seo/heading-level-skip — Heading order (info): Skipping a heading level breaks the document outline that search engines and assistive tech rely on to understand page structure. (docs)
• seo/ssr-disabled — SSR disabled (warning): SvelteKit's SEO guidance is to leave SSR on unless there is a good reason not to: server-rendered content is indexed more frequently and reliably, and SPA mode costs an extra network round trip before anything renders. (docs)

Performance

• performance/image-dimensions — Image dimensions (warning): An without explicit width and height triggers layout shift (CLS) as it loads, hurting Core Web Vitals and visual stability. Fix: Add explicit width and height attributes to the . (docs)
• performance/image-loading-hint — Image loading hint (info): A loading attribute lets the browser defer offscreen images; without it images load eagerly and can delay more important content. Static analysis cannot tell which image is the LCP, so this is advisory. Fix: Add loading="lazy" to offscreen elements (leave the LCP/hero image eager). (docs)
• performance/preload-missing-as — Preload missing as (warning): A <link rel="preload"> without an as attribute is ignored by the browser (or fetched a second time), wasting the preload. Fix: Add an as attribute matching the resource type to the preload link. (docs)
• performance/font-preload-crossorigin — Font preload missing crossorigin (warning): A font preload without crossorigin does not match the actual (CORS) font request, so the preloaded file is never used and the font downloads twice. Fix: Add the crossorigin attribute to the font preload link. (docs)
• performance/lcp-image — LCP image eager loading (warning): Lazy-loading the LCP (first/above-the-fold) image delays the largest paint and hurts Core Web Vitals. The first image is the best static proxy for the LCP candidate. Fix: Remove loading="lazy" from the first/LCP image; consider fetchpriority="high". (docs)
• performance/responsive-image — Responsive image (info): An without srcset ships one fixed-size asset to every device, wasting bytes on small screens. Static analysis cannot measure intended display size, so this is advisory. Fix: Add a srcset (and sizes) to the for responsive delivery. (docs)
• performance/render-blocking-script — Render-blocking script (warning): A synchronous in blocks HTML parsing until it downloads and runs, delaying first paint. defer, async, or type="module" avoids the block. Fix: Add defer (or type="module") / async to the head . (docs)
• performance/preconnect — Preconnect third-party origin (info): Connecting to a third-party origin (DNS + TCP + TLS) is costly; a preconnect/dns-prefetch hint starts it early so the resource arrives sooner. Fix: Add a preconnect hint for the third-party origin. (docs)
• performance/heavy-import — Heavy dependency import (info): Importing a large, non-tree-shakeable package pulls its whole weight into the bundle even when only a fraction is used, slowing load. (docs)
• performance/namespace-import — Namespace import (info): A namespace import (import * as X) is only tree-shakeable while every access to X stays static; passing X around or indexing it dynamically forces the bundler to keep the whole module. Named imports are reliably shakeable and make the dependency surface explicit. (docs)
• performance/minify-disabled — Minification disabled (warning): Disabling minification ships unminified JS/CSS to production, inflating bundle size several-fold and slowing every page load; the override is usually a leftover from debugging. Fix: Remove the minify: false override from vite.config (Vite minifies with esbuild by default), or scope it to non-production builds. (docs)
• performance/load-waterfall — Load waterfall (warning): In a universal load, every await that depends on a previous result costs a full network round trip from the browser on client-side navigation; chains multiply latency on every page visit. A server load runs the same hops server-side. Fix: Move the dependent await chain into a server load (+page.server.ts), where hops run server-to-server. (docs)
• performance/sequential-awaits — Sequential independent awaits (info): Awaits that do not use each other's results still run one after another, adding their latencies; starting them together costs nothing and bounds the wait to the slowest request. Fix: Start the independent requests together and await them with Promise.all. (docs)
• performance/state-raw — Raw state opportunity (info): Objects and arrays in $state are made deeply reactive through proxying, which taxes every property access. A binding that is only ever reassigned — API responses are the canonical case — never uses that machinery; Svelte's own guidance is to use $state.raw for it. Fix: Replace $state(...) with $state.raw(...); keep the same initializer. (docs)

Correctness

• correctness/each-key — Keyed each block (warning): An unkeyed {#each} adds/removes nodes at the end and rewrites the data of the DOM nodes in between when the list reorders, so element state/focus sticks to positions instead of items; a key lets Svelte insert, move, and delete the right nodes instead. (docs)
• correctness/each-index-key — Index used as each key (warning): Svelte's guidance is explicit: the key must uniquely identify the object — do not use the index. An index key gives items position-based identity, so element state (focus, inputs, transitions) sticks to positions when the list reorders or items are inserted or removed, exactly like an unkeyed block — but the visible key masks the problem. (docs)
• correctness/effect-as-derived — Effect used to derive state (warning): An $effect whose body only assigns to $state is the "useEffect → $effect" anti-pattern: it reruns after render and can cause extra passes or loops. $derived expresses the same dependency declaratively. (docs)
• correctness/effect-as-onmount — Effect used as onMount (warning): An $effect that reads no reactive value runs once after mount and never re-runs — it is an onMount in disguise, which obscures intent and misuses the reactivity system. (docs)
• correctness/unmutated-state — Unmutated $state (info): A $state that is never mutated pays for reactivity (deep proxying, tracking) it never uses; const (or $state.raw) is clearer and cheaper. (docs)
• correctness/prop-mutation — Mutated non-bindable prop (warning): Svelte's docs say plainly: don't mutate props unless they are $bindable. A plain-object prop mutation is a silent no-op (the object isn't a state proxy); a reactive-state-proxy prop mutation works but triggers the ownership_invalid_mutation dev warning only when that code path actually runs. In legacy mode, mutating methods like .push()/.splice() never trigger an update on their own — Svelte's reactivity there is based on assignments, not mutations. Neither case is caught by the compiler, so this rule catches both statically. (docs)
• correctness/stale-prop-derivation — Stale prop derivation (warning): Svelte's guidance is to treat props as though they will change: a plain let color = type === 'danger' ? 'red' : 'green' freezes the first render's value, so the UI silently stops tracking the parent when the prop changes. In runes mode, $derived keeps the computation live at no cost; in legacy mode (export let props), a $: reactive statement does the same job. Fix: Wrap the prop-derived computation in $derived(...) (or $derived.by(() => ...) for a function body) in runes mode, or prefix the assignment with $: in legacy mode, keeping the same expression. (docs)
• correctness/nonreactive-builtin-state — Non-reactive built-in in $state (warning): $state deep-proxies plain objects and arrays only; built-in collection, date, and URL instances stay untracked, so property-level changes never reach effects, deriveds, or the template. Svelte's own answer is the drop-in classes in svelte/reactivity. Fix: Import Svelte from 'svelte/reactivity' and replace new (...) with new Svelte(...) — the API is identical. (docs)
• correctness/checkable-bind-value — bind:value on a checkable input (warning): bind:value binds the DOM value property. A checkbox/radio's user interaction toggles checkedness, which bind:value never observes — the bound state is frozen at its initial value. Svelte's checked/grouped bindings (bind:checked, bind:group) are built for exactly this. Fix: For a single checkbox, replace bind:value={x} with bind:checked={x} (x becomes a boolean). For a checkbox list or radio group, replace bind:value={x} with bind:group={x} on every input sharing the group, keeping each input's static value attribute to identify the option. (docs)
• correctness/orphan-effect — Orphan $effect (critical): An $effect created outside component initialisation throws effect_orphan at runtime — the compiler does not catch it, and it typically surfaces as a production 500. (docs)
• correctness/orphan-lifecycle — Lifecycle call outside component initialisation (critical): Svelte lifecycle and context functions require an active component context; called at module scope, in a shared-state class constructor, or in a load/handler they throw lifecycle_outside_component at runtime — the compiler does not catch it, and it surfaces as a production crash. (docs)
• correctness/base-path-navigation — Root-relative navigation under a base path (warning): A root-relative literal resolves against the domain root, not kit.paths.base, so navigation lands outside an app served from a sub-path. The break only appears once the app is deployed under its base — locally base is usually empty, so every such link works. Fix: Import { resolve } from '$app/paths' and wrap the path: href={resolve('/about')}, goto(resolve('/about')), redirect(303, resolve('/login')). (docs)
• correctness/server-browser-global — Browser global in server module code (critical): window, document, localStorage and friends do not exist on the server; a read in module scope or a load/handler crashes SSR with a ReferenceError — the compiler does not catch it, and it surfaces as a production 500. (docs)
• correctness/instance-browser-global — Browser global during component initialisation (warning): A component instance script runs on the server on every SSR render, where window/document/localStorage do not exist. Warning, not critical: a component rendered only behind a parent {#if browser} (or a client-only dynamic import) is a legitimate pattern that static analysis cannot prove cross-file. (docs)

Security

• security/raw-html — Raw HTML render (warning): {@html} renders its value as unescaped HTML; if the value can contain user input and is not sanitized, it is a cross-site-scripting (XSS) vector. (docs)
• security/javascript-url — javascript: URL (warning): A javascript: URL in href/src/action executes arbitrary script on activation — an XSS / unsafe-navigation vector that also breaks under a strict Content-Security-Policy. (docs)
• security/handler-state-write — Handler writes imported state (critical): SvelteKit's docs mark this NEVER-DO-THIS: the server is one long-lived process shared by every user, so module state written during a request is visible to ALL later requests — one user's data can be served to another. (docs)
• security/server-module-state — Server module-scope state (warning): Module scope on the server is one shared, long-lived instance (SvelteKit docs: "Avoid shared state on the server"): a value reassigned during one user's request is served to every other user, and it silently resets on every deploy or restart. (docs)
• security/shared-state-import — Shared runes-state import on the server (warning): A .svelte.ts module with module-scope $state is one shared instance on the server: mutated, it leaks data between users; read-only, every request sees the same boot-time value instead of per-user data. (docs)

Architecture

• architecture/component-size — Component size (info): A very large component is hard to read, test, and reuse, and is a common sign that several responsibilities should be split out. (docs)
• architecture/prop-count — Prop count (info): A component taking many props is usually doing too much; grouping or splitting keeps its API understandable. (docs)
• architecture/private-scope-import — Private-scope import (info): A unit placed inside a private directory is written for one owner; importing it from elsewhere couples two parts of the tree that were meant to move independently, and the unit belongs higher up instead. Fix: Move this unit out of its private scope, to the directory shared by all of its importers, and update this import. (inert until configured) (docs)
• architecture/unit-entry-file — Unit entry file (info): A directory named after a unit but missing that unit's entry file is either an incomplete unit or a grouping wearing the wrong name; either way the tree no longer says what it means, and tooling that resolves by convention starts guessing. Fix: Make the directory and its entry file agree — add the entry file, or stop declaring this directory a unit. (inert until configured) (docs)
• architecture/directory-naming — Directory naming (info): A directory whose name breaks the convention its location declares stops carrying the meaning the convention gave it, and every reader — human or agent — has to open the directory to learn what it is. Fix: Rename the directory to the declared casing, or narrow the declaration that governs it. (inert until configured) (docs)
• architecture/reserved-directory-names — Reserved directory names (info): A closed set of directory names is only worth writing down if it stays closed: one directory outside it and the table stops describing the tree, so every reader has to open a directory to learn what it holds. Fix: Rename the directory to a declared name, move it under one of them, or add its name to the declaration. (inert until configured) (docs)
• architecture/reserved-name-placement — Reserved name placement (info): A name reserved for one kind of place stops carrying that meaning the moment it appears somewhere else: a reader who has met one exception has to open the directory to learn what it holds. Fix: Move the directory to one of the places declared for its name, rename it, or declare this place for the name. (inert until configured) (docs)
• architecture/route-component-import — Route component import (info): A route entry is written on the assumption that SvelteKit renders it: Kit hands a page its data and params, and an error page its page.error and page.status. Imported from somewhere else it receives none of that and renders against nothing, or against the importing page data standing in for its own. (docs)
• architecture/doc-link-target — Documentation link target (info): A documentation link written in a comment is invisible to type checking, module resolution and the test runner, so a convention-driven rename leaves it pointing at nothing and only human review notices. (inert until configured) (docs)