sveltejs/kit · error

Can only use the `multiple` attribute when `name` includes a

Error message

Can only use the `multiple` attribute when `name` includes a `[]` suffix — consider changing "${name}" to "${name}[]"

What it means

A multi-file input (<input type="file" multiple>) inside a remote form must have a name ending in [] so SvelteKit serializes it as an array. Dev mode throws with a concrete rename suggestion when multiple is set without the suffix.

Source

Thrown at packages/kit/src/runtime/client/remote-functions/form.svelte.js:543

									throw new Error(
										`Cannot mix and match file and non-file inputs under the same name ("${element.name}")`
									);
								}
							}
						}

						value = is_file
							? elements.map((input) => Array.from(input.files ?? [])).flat()
							: elements.map((element) => element.value);
						if (element.type === 'checkbox') {
							value = /** @type {string[]} */ (value.filter((_, i) => elements[i].checked));
						}
					}

					set_nested_value(input, field, is_file ? value : coerce_form_value(field.type, value));
				} else if (is_file) {
					if (DEV && element.multiple) {
						throw new Error(
							`Can only use the \`multiple\` attribute when \`name\` includes a \`[]\` suffix — consider changing "${name}" to "${name}[]"`
						);
					}

					const file = /** @type {HTMLInputElement & { files: FileList }} */ (element).files[0];

					if (file) {
						set_nested_value(input, field, file);
					} else {
						set_nested_value(input, field, DELETE_KEY);
					}
				} else {
					set_nested_value(
						input,
						field,
						coerce_form_value(
							field.type,
							element.type === 'checkbox' && !element.checked ? null : element.value

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Rename the input's name to include a [] suffix, e.g. name="files" becomes name="files[]"
2. Remove the multiple attribute if only one file is expected
3. Update the remote function's argument shape to expect an array for that field

Example fix

// before
<input type="file" name="uploads" multiple />
// after
<input type="file" name="uploads[]" multiple />

function assertMultipleNaming(form) {
  for (const el of form.querySelectorAll('input[type="file"][multiple]')) {
    if (!el.name.endsWith('[]')) throw new Error(`rename ${el.name} to ${el.name}[]`);
  }
}

When it happens

Trigger: Submitting an enhanced remote form containing <input type="file" multiple name="files"> where the name lacks the [] suffix, detected in handle_input when the first selected file is processed.

Common situations: Adding the multiple attribute to an existing single-file input without updating the name; following older multipart form examples that don't use [] naming.

Related errors

• Cannot mix and match file and non-file inputs under the same
• Your form contains <input type="file"> fields, but is missin
• `$` is used to collect all FormData validation issues and ca
• exports is not available in dev mode
• Your form contains <input type="file"> fields, but is missin

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/302224aec021f9bd. Report an issue: GitHub.