sveltejs/kit · error · Error

${keypath} option must either be the empty string or a root-

Error message

${keypath} option must either be the empty string or a root-relative path that starts but doesn't end with '/'. See https://svelte.dev/docs/kit/configuration#paths

What it means

`paths.base` must be either the empty string or a root-relative path that starts with `/` but does not end with `/`. SvelteKit enforces this so URLs are composed unambiguously. `assert_string` also guarantees it is a string first.

Source

Thrown at packages/kit/src/core/config/options.js:191

		);
	}),

	outDir: string('.svelte-kit'),

	output: object({
		linkHeaderPreload: boolean(false),
		preloadStrategy: removed(
			(keypath) => `\`${keypath}\` has been removed. modulepreload will always be used`
		),
		bundleStrategy: list(['split', 'single', 'inline'])
	}),

	paths: object({
		base: validate('', (input, keypath) => {
			assert_string(input, keypath);

			if (input !== '' && (input.endsWith('/') || !input.startsWith('/'))) {
				throw new Error(
					`${keypath} option must either be the empty string or a root-relative path that starts but doesn't end with '/'. See https://svelte.dev/docs/kit/configuration#paths`
				);
			}

			return input;
		}),
		assets: validate('', (input, keypath) => {
			assert_string(input, keypath);

			if (input) {
				if (!/^[a-z]+:\/\//.test(input)) {
					throw new Error(
						`${keypath} option must be an absolute path, if specified. See https://svelte.dev/docs/kit/configuration#paths`
					);
				}

				if (input.endsWith('/')) {
					throw new Error(

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Use `''` (empty string) if the app is served from the domain root
2. Otherwise use a path starting with `/` and not ending with `/`: `/my-app`
3. Normalize env-derived values: `base.replace(/\/$/, '')` and ensure a leading `/`

Example fix

// before
paths: { base: 'my-app/' }
// after
paths: { base: '/my-app' }

const base = config.paths?.base;
if (base !== undefined && base !== '' && (!base.startsWith('/') || base.endsWith('/'))) {
  throw new Error('paths.base must be "" or start with "/" and not end with "/"');
}

function isValidBasePath(v) {
  return v === undefined || v === '' || (typeof v === 'string' && v.startsWith('/') && !v.endsWith('/'));
}

try {
  assertBasePath(config.paths?.base);
} catch (e) {
  if (String(e.message).includes("paths.base option must either be the empty string")) {
    console.error('Use "" or a path like /my-app (no trailing slash)');
  }
  throw e;
}

When it happens

Trigger: `paths.base: 'app'` (no leading slash), `paths.base: '/app/'` (trailing slash), or a non-string value that passes string assertion only when it is a string but malformed.

Common situations: Deploying under a subpath and writing the subpath without a leading slash; copying GitHub Pages examples with trailing slashes; env vars like `BASE_PATH=app/` with trailing slash.

Related errors

• ${keypath} option must be an absolute path, if specified. Se
• Invalid isr.expiration value: ${JSON.stringify(value)} (${de
• The SvelteKit options from the Vite config must be an object
• ${keypath} should be "fail", "warn", "ignore" or a custom fu
• The SvelteKit Vite plugin ${keypath} should be an object wit

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/90900a1d407cd00a. Report an issue: GitHub.