What causes "Invalid content-type value "${type}". ${error_suffix}" in sveltejs/kit?

Use the full media type: e.g. 'application/json', 'text/html; charset=utf-8'

sveltejs/kit · error · Error

Invalid content-type value "${type}". ${error_suffix}

Error message

Invalid content-type value "${type}". ${error_suffix}

What it means

The content-type value's media type (part before ';') must match a type/subtype pattern. Values like 'text' (missing subtype), 'invalid' bare tokens, or whitespace-mangled types throw with the parsed type and original value shown.

Source

Thrown at packages/kit/src/runtime/server/validate-headers.js:45

		const parts = value.split(',').map((part) => part.trim());
		if (parts.some((part) => !part)) {
			throw new Error(`\`cache-control\` header contains empty directives. ${error_suffix}`);
		}

		const directives = parts.map((part) => part.split('=')[0].toLowerCase());
		const invalid = directives.find((directive) => !VALID_CACHE_CONTROL_DIRECTIVES.has(directive));
		if (invalid) {
			throw new Error(
				`Invalid cache-control directive "${invalid}". Did you mean one of: ${[...VALID_CACHE_CONTROL_DIRECTIVES].join(', ')}? ${error_suffix}`
			);
		}
	},

	'content-type': (value) => {
		const type = value.split(';')[0].trim();
		const error_suffix = `(While parsing "${value}".)`;
		if (!CONTENT_TYPE_PATTERN.test(type)) {
			throw new Error(`Invalid content-type value "${type}". ${error_suffix}`);
		}
	}
};

/**
 * @param {Record<string, string>} headers
 */
export function validateHeaders(headers) {
	for (const [key, value] of Object.entries(headers)) {
		const validator = HEADER_VALIDATORS[key.toLowerCase()];
		try {
			validator?.(value);
		} catch (error) {
			if (error instanceof Error) {
				console.warn(`[SvelteKit] ${error.message}`);
			}
		}
	}

View on GitHub (pinned to 03f1687fe6)

Solutions

Use the full media type: e.g. 'application/json', 'text/html; charset=utf-8'
Prefer Response.json() or the json() helper so the content-type is set correctly for you
Trim whitespace and ensure a '/' separates type and subtype

Example fix

// before
event.setHeaders({ 'content-type': 'json' });
// after
event.setHeaders({ 'content-type': 'application/json' });

Defensive patterns

Strategy: validation

Validate before calling

const CT = /^[*\w.+/-]+\/[\w.+-]+$/;
function isValidContentType(v) { return CT.test(v.split(';')[0].trim()); }
// if (!isValidContentType('application/json')) throw ...

Try / catch

try {
  event.setHeaders({ 'content-type': type });
} catch (e) {
  if (e.message.startsWith('Invalid content-type')) console.error('Bad media type:', type);
}

Prevention

Use full MIME types like application/json, never bare suffixes
Prefer Response.json()/json() helpers over manual content-type
Always include type/subtype separated by '/'

When it happens

Trigger: setHeaders({ 'content-type': 'json' }) instead of 'application/json'; returning a Response whose headers include a malformed content-type like 'application/;' ; empty content-type string.

Common situations: Hand-rolled endpoint responses with abbreviated types; string concatenation that drops the subtype; copying only the MIME suffix ('xml', 'html') instead of the full type.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/07413057c231d1e3. Report an issue: GitHub.