sveltejs/kit · error · Error

${keypath} must be an array of strings

Error message

${keypath} must be an array of strings

What it means

The `extensions` config option controls which file extensions SvelteKit treats as routable pages. It must be an array whose every member is a string. SvelteKit throws this when the value is not an array or contains non-string entries.

Source

Thrown at packages/kit/src/core/config/options.js:127

		{
			tracing: removed(
				(keypath) =>
					`\`${keypath}\` has been removed. Server-side tracing is now configured via \`tracing.server\``
			),
			instrumentation: removed(
				(keypath) =>
					`\`${keypath}\` has been removed. \`src/instrumentation.server.js\` is now included in the build automatically when it exists; no opt-in is required`
			),
			remoteFunctions: boolean(false),
			forkPreloads: boolean(false),
			handleRenderingErrors: removed()
		},
		true
	),

	extensions: validate(['.svelte'], (input, keypath) => {
		if (!Array.isArray(input) || !input.every((page) => typeof page === 'string')) {
			throw new Error(`${keypath} must be an array of strings`);
		}

		input.forEach((extension) => {
			if (extension[0] !== '.') {
				throw new Error(`Each member of ${keypath} must start with '.' — saw '${extension}'`);
			}

			if (!/^(\.[a-z0-9]+)+$/i.test(extension)) {
				throw new Error(`File extensions must be alphanumeric — saw '${extension}'`);
			}
		});

		return input;
	}),

	files: object({
		src: string('src'),
		assets: string('static'),

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Wrap the value in an array: `extensions: ['.svelte']`
2. Ensure every member is a string — remove non-string entries
3. If building from an env var, split it: `process.env.EXTENSIONS?.split(',') ?? ['.svelte']`

Example fix

// before
const config = { extensions: '.svelte,.js' };
// after
const config = { extensions: ['.svelte', '.js'] };

if (!Array.isArray(config.extensions) || !config.extensions.every((e) => typeof e === 'string')) {
  throw new Error('extensions must be an array of strings');
}

function isStringArray(v) {
  return Array.isArray(v) && v.every((e) => typeof e === 'string');
}

try {
  build({ config });
} catch (e) {
  if (String(e.message).includes('must be an array of strings')) {
    console.error('Wrap extensions in an array of strings');
  }
  throw e;
}

When it happens

Trigger: Setting `extensions` to a string (e.g. `extensions: '.svelte'`), to a non-array value, or to an array containing numbers/objects/null (e.g. `extensions: ['.svelte', null]`).

Common situations: Hand-editing svelte.config.js and forgetting array brackets; generating config from JSON where an env list is comma-joined into a single string; typos like `extensions: ['.svelte', 1]`.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• 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
• ${keypath} should be an object

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/f0955917df48b2bc. Report an issue: GitHub.