sveltejs/kit · error

Character escape sequence in ${id} must be lowercase

Error message

Character escape sequence in ${id} must be lowercase

What it means

SvelteKit route folders support character escapes like `[u+1f600]` or `[x41]` to embed Unicode/hex characters in route paths. The escape sequence (including the bracket marker u/x and hex digits) must be entirely lowercase; an uppercase variant such as `[U+1F600]` or `[x+4A]` is rejected during route scanning.

Source

Thrown at packages/kit/src/core/sync/create_manifest_data/index.js:142

	const routes_base = posixify(path.relative(cwd, config.files.routes));

	const valid_extensions = [...config.extensions, ...config.moduleExtensions];

	/** @type {import('types').PageNode[]} */
	const nodes = [];

	// create route data by processing files in `src/routes`
	if (fs.existsSync(config.files.routes)) {
		/**
		 * @param {number} depth
		 * @param {string} id
		 * @param {string} segment
		 * @param {import('types').RouteData | null} parent
		 */
		const walk = (depth, id, segment, parent) => {
			const unescaped = id.replace(/\[([ux])\+([^\]]+)\]/gi, (match, type, code) => {
				if (match !== match.toLowerCase()) {
					throw new Error(`Character escape sequence in ${id} must be lowercase`);
				}

				if (!/[0-9a-f]+/.test(code)) {
					throw new Error(`Invalid character escape sequence in ${id}`);
				}

				if (type === 'x') {
					if (code.length !== 2) {
						throw new Error(`Hexadecimal escape sequence in ${id} must be two characters`);
					}

					return String.fromCharCode(parseInt(code, 16));
				} else {
					if (code.length < 4 || code.length > 6) {
						throw new Error(
							`Unicode escape sequence in ${id} must be between four and six characters`
						);
					}

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Lowercase the entire escape sequence in the directory name, e.g. `[u+1f600]`
2. Lowercase both the type marker (`u`/`x`) and all hex digits
3. Rename the folder and update any links pointing at the old path

Example fix

// before
// src/routes/[U+1F600]/+page.svelte

// after
// src/routes/[u+1f600]/+page.svelte

// validate folder names before committing
const bad = /\[[ux]\+[^\]]*\]/i;
for (const name of routeFolders) {
  const m = name.match(bad);
  if (m && m[0] !== m[0].toLowerCase()) console.error('Lowercase required:', name);
}

try {
  await dev();
} catch (e) {
  if (String(e.message).includes('must be lowercase')) {
    console.error('Lowercase the [u+..]/[x+..] escape in the named route folder');
  }
  throw e;
}

When it happens

Trigger: Naming a route directory with an uppercase escape: `src/routes/[U+1F4A9]/+page.svelte` or mixed case like `[u+1F600]` — detected by walk() when creating manifest data (dev server start or build).

Common situations: Copying emoji route examples from docs where the hex digits were uppercased; editors auto-capitalizing folder names; manually typing hex codes in uppercase.

Related errors

• Invalid character escape sequence in ${id}
• Hexadecimal escape sequence in ${id} must be two characters
• The "${lookup.get(key)}" and "${route.id}" routes conflict w
• Cannot build with ${JSON.stringify(file)} because Bun treats
• Cannot build with ${JSON.stringify(file)} because Bun treats

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/bd0711ecf69a604f. Report an issue: GitHub.