AwaitInEffectWorkError

What this error means and what can trigger it

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AwaitInEffectWorkError means one script statement belongs to both JavaScript's Promise continuation model and SER's effect generator model. The statement contains a top-level yield*, SER successfully lowered at least one effect block from it, and the same statement also contains a top-level await.

Here, top-level is relative to the statement SER is inspecting; the detectors do not descend through nested function boundaries. An ordinary statement such as const response = await fetch(url) is left alone because it contains no SER work, whereas record(await transform(yield* Load())) mixes both execution models in one expression and is rejected.

Exactly where it triggers

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const has_top_level_yield_star = contains_top_level_yield_star(stmt);

if (!has_top_level_yield_star) {
	continue;
}

has_effect = true;
const lowered = lower_statement(stmt, content, context);

if (lowered.effect_blocks.length > 0 && contains_top_level_await(stmt)) {
	const text = slice(content, stmt);
	throw new AwaitInEffectWorkError(filename, text);
}

All three predicates are relevant: a top-level yield* must exist, lowering must have produced an effect block, and a top-level await must remain in that statement. The exception is synchronous and stops preprocessing.

For reference, the class itself is declared at modules/svelte-effect-runtime/src/errors.ts L126 @ C14.

Why this is the case

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The lowered statement runs inside generator-based effect sequencing, where yield* controls suspension, interruption, scope, and the typed error channel. Native await suspends an async JavaScript function around a Promise. SER would have to invent how Promise cancellation, rejection, and continuation order interact with the dispatcher-owned effect fiber.

By rejecting the mixed statement, SER keeps the two execution models explicit. Ordinary Svelte top-level await remains untouched in statements that SER does not lower.