What causes "Cannot reorder effect: source index not-found" in remotion-dev/remotion?

Clamp/recompute fromIndex against the current array length before calling

Cannot reorder effect: source index not-found

Error message

Cannot reorder effect: source index not-found

What it means

reorderEffect validates fromIndex against the enumerated effects array (enumerateEffectArrayElements) before moving. An index below zero or at/above the number of elements means the source effect to move does not exist, so it throws. A companion check validates toIndex ('target index not-found').

Solutions

Clamp/recompute fromIndex against the current array length before calling
Re-enumerate effects on the current source and refresh both indices
Guard: if (fromIndex >= 0 && fromIndex < elements.length) before invoking

Example fix

// before
reorderEffect({fromIndex: 5, toIndex: 0, ...}); // 2 effects exist
// after
const i = Math.min(fromIndex, elements.length - 1); reorderEffect({fromIndex: i, toIndex: 0, ...});

Defensive patterns

Strategy: validation

Validate before calling

const elements = enumerateEffectArrayElements(getEffectsArray(findEffectsAttr(jsx.openingElement.attributes)!, 'check'));
if (fromIndex < 0 || fromIndex >= elements.length) throw new Error('fromIndex out of range');
if (toIndex < 0 || toIndex >= elements.length) throw new Error('toIndex out of range');

Type guard

const inBounds = (i: number, len: number): i is number => Number.isInteger(i) && i >= 0 && i < len;

Try / catch

try { await reorderEffect({fromIndex, toIndex, input, sequenceNodePath}); } catch (e) { if ((e as Error).message.includes('source index not-found')) { /* refresh indices from current source and retry once */ } else throw e; }

Prevention

Recompute indices immediately before reordering — never cache across edits
Clamp UI drag targets to the current array length
Delete and reorder flows should share one fresh enumeration source

When it happens

Trigger: reorderEffect with fromIndex < 0 or fromIndex >= effects.length — typically an effect index captured before effects were deleted, or an index counted on a different element.

Common situations: Drag-and-drop reorder in Studio after another effect was deleted making stored indices stale; hard-coded index in an automation script; array shrank because non-call entries were cleaned up.

Related errors

AI-assisted analysis of remotion-dev/remotion@46a3a6bf13 (2026-09-18). Data as JSON: /api/errors/c07644c0ce56f2da. Report an issue: GitHub.

Appendix: source

Thrown at packages/studio-codemods/src/effect-operations.ts:596

} & EffectSourceStyleOptions): Promise<{
	output: string;
	formatted: boolean;
	effectLabel: string;
	logLine: number;
}> =>
	Promise.resolve().then(() => {
		const ast = parseAst(input);
		const snapshots = captureEffectSourceSnapshots(ast);
		const jsx = getJsx({action: 'reorder', ast, sequenceNodePath});
		const attr = findEffectsAttr(jsx.attributes);
		if (!attr) {
			throw new Error('Could not find effects on the target JSX element');
		}

		const array = getEffectsArray(attr, 'reorder');
		const elements = enumerateEffectArrayElements(array);
		if (fromIndex < 0 || fromIndex >= elements.length) {
			throw new Error('Cannot reorder effect: source index not-found');
		}

		if (toIndex < 0 || toIndex >= elements.length) {
			throw new Error('Cannot reorder effect: target index not-found');
		}

		const target = elements[fromIndex];
		if (target.kind !== 'call') {
			throw new Error(
				'Cannot reorder effect: source effect is not-call-expression',
			);
		}

		trackOpeningElement({jsx, snapshots});
		if (fromIndex !== toIndex) {
			const [moved] = array.elements.splice(fromIndex, 1);
			array.elements.splice(toIndex, 0, moved as never);
		}

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