UncheckedFormHandlerMissingError

What this error means and what can trigger it

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UncheckedFormHandlerMissingError means Form("unchecked", handler) reached its server factory without a truthy handler. The sentinel suppresses schema validation for the submitted input, but it cannot process that input or produce a form result.

The declared TypeScript overload normally prevents this. Untyped code can still enter the branch with an omitted, undefined, null, or otherwise falsy second argument.

Exactly where it triggers

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

SER throws the error synchronously while SER builds the remote Form helper. Helper normalization preserves existing Error instances, so the dedicated class is rethrown unchanged.

For reference, the class is declared at modules/svelte-effect-runtime/src/errors.ts L461 @ C14.

Why this is the case

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Form has a one-argument path for an inputless effect-like handler. Without the sentinel guard, "unchecked" could be treated as that program and fail later with a less precise invocation error.

Validation policy and submission behavior are separate inputs to the factory. Disabling the former never supplies the latter.