Cannot `return error(...)` — use `error(...)` or `return fai

Error message

Cannot `return error(...)` — use `error(...)` or `return fail(...)` instead

What it means

`error(...)` creates an `HttpError` that must be thrown so SvelteKit can convert it into an error response. Returning the instance from an action is invalid; `validate_action_return` throws this Error telling you to either throw the error or, for form validation feedback, return `fail(status, data)`.

Source

Thrown at packages/kit/src/runtime/server/page/actions.js:283

				current.setAttributes({
					'sveltekit.form_action.result.type': 'failure',
					'sveltekit.form_action.result.status': result.status
				});
			}

			return result;
		}
	});
}

/** @param {any} data */
function validate_action_return(data) {
	if (data instanceof Redirect) {
		throw new Error('Cannot `return redirect(...)` — use `redirect(...)` instead');
	}

	if (data instanceof HttpError) {
		throw new Error('Cannot `return error(...)` — use `error(...)` or `return fail(...)` instead');
	}
}

/**
 * Try to `devalue.uneval` the data object, and if it fails, return a proper Error with context
 * @param {any} data
 * @param {string} route_id
 */
export function uneval_action_response(data, route_id) {
	return try_serialize(data, uneval, route_id);
}

/**
 * @param {any} data
 * @param {(data: any) => string} fn
 * @param {string} route_id
 */
function try_serialize(data, fn, route_id) {

View on GitHub (pinned to 03f1687fe6)

Solutions

Use `return fail(status, data)` for validation failures in form actions
Use bare `error(status, message)` (thrown) for fatal errors: `throw error(404, 'Not found')`

Example fix

// before
export const actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    return error(400, 'invalid');
  }
};
// after
import { fail } from '@sveltejs/kit';
export const actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    return fail(400, { message: 'invalid' });
  }
};

Defensive patterns

Strategy: validation

Prevention

Rule of thumb: user input problems -> `return fail(...)`; unexpected fatal problems -> `throw error(...)`
Never prefix error/redirect with `return` in actions or load functions
Lint for `return error(` and `return redirect(` in the codebase

When it happens

Trigger: Writing `return error(400, 'bad input')` inside a form action instead of `error(400, 'bad input')` thrown, or `return fail(400, {...})`.

Common situations: Confusing `fail()` (expected failure data for forms) with `error()` (fatal error response); copying pre-SvelteKit-2 patterns where `error` was used differently; mixing up return-based APIs from other frameworks (Next.js, Remix).

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/b0c0659601b45565. Report an issue: GitHub.