UncheckedPrerenderHandlerMissingError

What this error means and what can trigger it

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UncheckedPrerenderHandlerMissingError means the first argument to Prerender is "unchecked", while the second is not a function. This case differs slightly from the other unchecked helpers because Prerender options may also occupy that second position.

For that reason, the guard checks callability rather than mere truthiness. An omitted value, a falsy value, an options object, or any other non-function triggers the error when paired with the unchecked sentinel.

Exactly where it triggers

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if (is_unchecked(validate_or_handler)) {
	throw new UncheckedPrerenderHandlerMissingError();
}

This branch is reached after the factory has tested the second argument with typeof value === "function". It throws synchronously during helper creation and is rethrown unchanged after normalization.

For reference, the class itself is declared at modules/svelte-effect-runtime/src/errors.ts L481 @ C14.

Why this is the case

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The sentinel disables input validation during build-time resolution. A callable handler is still required to compute the value that SvelteKit will prerender.

Options can configure prerender behavior, but they do not replace the computation. The callable check prevents an options object from being mistaken for executable work.