What causes "Invalid alias value: ${value}" in sveltejs/kit?

Change the alias value to a simple relative path without a leading './' or trailing slash: 'src/lib' instead of './src/lib/'.

sveltejs/kit · error

Invalid alias value

Error message

Invalid alias value: ${value}

What it means

Each kit.alias value must be a plain relative path, optionally ending in `/*` or a file extension (alias_value = /^(.+?)((\/\*)|(\.\w+))?$/). Values like absolute paths with odd segments, URLs, or values with unsupported suffixes fail this regex, and sync throws before writing the tsconfig paths.

Solutions

Change the alias value to a simple relative path without a leading './' or trailing slash: 'src/lib' instead of './src/lib/'.
For wildcard aliases pair the key and value with `/*`: '$lib/*': 'src/lib/*'.
Remove non-path values (URLs, absolute paths) from kit.alias — aliases must point inside the project.

Example fix

// before (svelte.config.js)
alias: { '$utils': './src/utils/', '$cdn': 'https://cdn.example.com' }
// after
alias: { '$utils': 'src/utils', '$utils/*': 'src/utils/*' }

Defensive patterns

Strategy: validation

Validate before calling

const ALIAS_VALUE = /^(.+?)((\/\*)|(\.\w+))?$/;
for (const value of Object.values(config.kit?.alias ?? {})) {
  if (!ALIAS_VALUE.test(value)) throw new Error(`Invalid kit.alias value: ${value}`);
}

Type guard

function isValidAliasValue(value) {
  return typeof value === 'string' && value.length > 0 && /^(.+?)((\/\*)|(\.\w+))?$/.test(value);
}

Try / catch

try {
  await runSync();
} catch (e) {
  if (e.message.startsWith('Invalid alias value')) {
    console.error('Fix kit.alias in svelte.config.js:', e.message);
  }
  throw e;
}

Prevention

Alias values must be plain relative paths like 'src/lib' — no URLs, absolute paths, './' prefixes, or trailing slashes.
For wildcard mappings, mirror the key's /* in the value.
Validate svelte.config.js in CI before running sync/build.

When it happens

Trigger: kit.alias values such as 'https://example.com/lib', '/abs/path', './src/lib/', or 'src/lib.d' patterns that don't conform — anything not matching a relative path with optional /* or extension suffix.

Common situations: Copy-pasting alias values from webpack configs; accidentally adding trailing slashes; including protocol prefixes or leading './' variants that confuse the validation; typos in the path.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/e25f877ed554c9bb. Report an issue: GitHub.

Appendix: source

Thrown at packages/kit/src/core/sync/write_tsconfig/index.js:256

 *
 * @param {import('types').ValidatedConfig} config
 * @param {string} root
 * @returns {Record<string, string[]>}
 */
function get_paths(config, root) {
	const alias = {
		...config.alias
	};

	/** @type {Record<string, string[]>} */
	const paths = {};

	for (const [key, value] of Object.entries(alias)) {
		const key_match = alias_key.exec(key);
		if (!key_match) throw new Error(`Invalid alias key: ${key}`);

		const value_match = alias_value.exec(value);
		if (!value_match) throw new Error(`Invalid alias value: ${value}`);

		const resolved = path.resolve(root, remove_trailing_slashstar(value));
		const slashstar = key_match[2];

		if (slashstar) {
			paths[key] = [resolved + '/*'];
		} else {
			paths[key] = [resolved];
			const fileending = value_match[4];

			if (!fileending && !(key + '/*' in alias)) {
				paths[key + '/*'] = [resolved + '/*'];
			}
		}
	}

	return paths;
}

View on GitHub (pinned to 03f1687fe6)