What causes "Missing parameter '${name}' in route ${id}" in sveltejs/kit?

Provide a value for every required parameter when resolving the route

sveltejs/kit · error · Error

Missing parameter '${name}' in route ${id}

Error message

Missing parameter '${name}' in route ${id}

What it means

`resolve_route` builds a concrete path/route id by substituting parameter values into a parsed route pattern. When a required (non-optional, non-rest) parameter is `undefined` (or an empty string) in the params object, the resulting route would be malformed, so this error names the missing parameter and the route id being resolved.

Source

Thrown at packages/kit/src/utils/routing.js:310

 * @returns {string}
 */
export function resolve_route(id, params) {
	const segments = get_route_segments(id);
	const has_id_trailing_slash = id != '/' && id.endsWith('/');

	return (
		'/' +
		segments
			.map((segment) =>
				segment.replace(segment_pattern, (_, escape_type, escape_code, optional, rest, name) => {
					if (escape_type) return encode_pathname_chars(decode_escape_sequence(escape_code));

					const value = params[name];

					if (value === undefined || value === '') {
						if (optional) return '';
						if (rest && value !== undefined) return '';
						throw new Error(`Missing parameter '${name}' in route ${id}`);
					}

					if (typeof value === 'string') {
						if (value.startsWith('/') || value.endsWith('/')) {
							throw new Error(
								`Parameter '${name}' in route ${id} cannot start or end with a slash -- this would cause an invalid route like foo//bar`
							);
						}

						return value;
					}

					if (
						typeof value === 'number' ||
						typeof value === 'boolean' ||
						typeof value === 'bigint'
					) {
						return String(value);

View on GitHub (pinned to 03f1687fe6)

Solutions

Provide a value for every required parameter when resolving the route
Make the parameter optional in the route id (`[x?]`) if it can legitimately be absent
Handle empty-string inputs before resolution (treat them as absent and adjust the route or params)

Example fix

// before
resolve_route('/blog/[slug]/edit', pattern, {});
// after
resolve_route('/blog/[slug]/edit', pattern, { slug: 'hello-world' });

Defensive patterns

Strategy: validation

Validate before calling

function requireParams(routeId, params) {
	for (const m of routeId.matchAll(/\[(?!\.\.\.)([^=\]]+?)(?:\?)?\]/g)) {
		const name = m[1];
		if (params[name] === undefined || params[name] === '') {
			throw new Error(`Missing parameter '${name}' for route ${routeId}`);
		}
	}
}

Type guard

function hasAllParams(routeId, params) {
	return [...routeId.matchAll(/\[(?!\.\.\.)([^=\]]+?)\]/g)].every((m) => params[m[1]] !== undefined && params[m[1]] !== '');
}

Try / catch

try {
	const href = resolveRoute(id, pattern, params);
} catch (e) {
	if (e.message.startsWith('Missing parameter')) {
		const name = e.message.match(/'([^']+)'/)[1];
		// supply params[name] or fall back to another route
	}
	throw e;
}

Prevention

Build params objects from the route id's parsed segments, not by hand
Treat empty form/query strings as absent before resolving
Update all call sites when adding a parameter to a route id

When it happens

Trigger: Calling route resolution (e.g. `resolve_route(route.id, route.pattern, params)`) with a params object that omits a required parameter or supplies `''` for it, and the parameter is neither optional (`[x?]`) nor a valid rest segment (`[...x]`).

Common situations: Programmatically generating hrefs/route ids while forgetting one parameter; a rest param handler returning undefined unexpectedly; refactoring route ids to add a parameter without updating all `resolve_route`-style call sites; empty-string values from form/query handling.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/8365fa23a7a6fc3c. Report an issue: GitHub.