sveltejs/kit · error · Error

${keypath} should be a string, if specified

Error message

${keypath} should be a string, if specified

What it means

`assert_string` verifies that a config option expected to be a string is actually a string. SvelteKit throws this when a string-typed option (like an adapter name, paths option, or env dir) receives a number, boolean, object, or undefined-that-isn't-absent.

Source

Thrown at packages/kit/src/core/config/options.js:516

	return validate(fallback, (input, keypath) => {
		if (typeof input !== 'function') {
			throw new Error(`${keypath} should be a function, if specified`);
		}
		return input;
	});
}

function any() {
	return validate(undefined, (input) => input);
}

/**
 * @param {string} input
 * @param {string} keypath
 */
function assert_string(input, keypath) {
	if (typeof input !== 'string') {
		throw new Error(`${keypath} should be a string, if specified`);
	}
}

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Check the option named in the keypath in svelte.config.js.
2. Convert the value to a string (add quotes) or supply the correct string value.
3. For path-like options, ensure you didn't paste a URL or number where a path is required.

Example fix

// before
paths: { base: 3000 }
// after
paths: { base: '/app' }

const stringOptions = [['paths', 'base'], ['paths', 'assets']];
for (const [a, b] of stringOptions) {
  const v = config?.kit?.[a]?.[b];
  if (v !== undefined && typeof v !== 'string') {
    throw new Error(`${a}.${b} must be a string, got ${typeof v}`);
  }
}

function isString(v) {
  return typeof v === 'string';
}

try {
  await build(config);
} catch (e) {
  if (String(e.message).includes('should be a string')) {
    console.error('String config option got a non-string value:', e.message);
  } else throw e;
}

When it happens

Trigger: Setting a string config option to a non-string, e.g. `paths: { base: 3000 }`, `outDir: true`, or passing a number where an appDir string is required.

Common situations: Copying config from docs with placeholder values not replaced; using a numeric port where a string path is expected; YAML/JSON-style config mistakes after converting to JS.

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/32d0308db4aaf146. Report an issue: GitHub.