n8n-io/n8n · error · GraphValidationError

Edge ${edge.from} → ${edge.to} has slot index ${index}; slot

Error message

Edge ${edge.from} → ${edge.to} has slot index ${index}; slot indices above ${MAX_SLOT_INDEX} are not supported yet

What it means

GraphValidationError (code: invalid_graph) thrown by validateExecutableGraph when an edge's outputIndex or inputIndex is a valid non-negative integer but exceeds MAX_SLOT_INDEX. Slot counts above the current cap are not yet supported (see CAT-3042 for raising the bound); the message names the edge, the value, and the current MAX_SLOT_INDEX.

Source

Thrown at packages/@n8n/engine/src/graph/validate-executable-graph.ts:47

	}

	// TODO(CAT-2875): loop iteration needs re-runnable steps; until that lands,
	// graphs with back-edges are rejected outright rather than deadlocking.
	if (graph.edges.some((edge) => edge.isBackEdge)) {
		throw new UnimplementedError('Graphs with back-edges (loops) are not supported yet');
	}

	// Slot indices are structural, so they're enforced here rather than left to
	// the transport boundary. TODO(CAT-3042): enforce an upper bound too.
	for (const edge of graph.edges) {
		for (const index of [edge.outputIndex, edge.inputIndex]) {
			if (!Number.isInteger(index) || index < 0) {
				throw new GraphValidationError(
					`Edge ${edge.from} → ${edge.to} has slot index ${index}; slot indices are non-negative integers`,
				);
			}
			if (index > MAX_SLOT_INDEX) {
				throw new GraphValidationError(
					`Edge ${edge.from} → ${edge.to} has slot index ${index}; slot indices above ${MAX_SLOT_INDEX} are not supported yet`,
				);
			}
		}
	}

	// TODO(CAT-2874): multi-slot outputs arrive with branching; until then only
	// output slot 0 fires, and the runtime can assume single-slot outputs.
	for (const edge of graph.edges) {
		if (edge.outputIndex !== 0) {
			throw new UnimplementedError(
				`Edge ${edge.from} → ${edge.to} leaves output slot ${edge.outputIndex}; only output slot 0 is supported yet`,
			);
		}
	}

	// TODO(CAT-3982): same-slot convergence gets a defined meaning (concatenation);
	// until then it is rejected rather than given accidental semantics.

View on GitHub (pinned to 5ac6606e81)

Solutions

  1. Clamp indices to <= MAX_SLOT_INDEX when constructing graphs; check the current value of MAX_SLOT_INDEX for your build.
  2. Redesign high-fan-out nodes to use a different primitive until branching/multi-slot lands (CAT-2874).
  3. If the graph originated from a newer n8n, downgrade the graph structure or upgrade the engine to a build with a higher MAX_SLOT_INDEX.
  4. Validate client-side: `if (index > MAX_SLOT_INDEX) throw ...` with a clearer producer-side message.

Example fix

// before
edges: [{ from: 'A', to: 'B', outputIndex: 99, inputIndex: 0, isBackEdge: false }]

// after
edges: [{ from: 'A', to: 'B', outputIndex: 0, inputIndex: 0, isBackEdge: false }]
Defensive patterns

Strategy: validation

Validate before calling

import { MAX_SLOT_INDEX } from '@n8n/engine/...';
function withinSlotCap(n: number): boolean {
  return Number.isInteger(n) && n >= 0 && n <= MAX_SLOT_INDEX;
}
for (const e of graph.edges) {
  if (!withinSlotCap(e.outputIndex) || !withinSlotCap(e.inputIndex)) {
    // refuse to submit; reduce slot fan-out
  }
}

Type guard

function isValidSlotIndex(n: unknown): n is number {
  return typeof n === 'number' && Number.isInteger(n) && n >= 0 && n <= MAX_SLOT_INDEX;
}

Try / catch

try {
  validateExecutableGraph(graph);
} catch (err) {
  if (err instanceof GraphValidationError && /slot indices above .* are not supported yet/.test(err.message)) {
    // clamp the named edge's index to <= MAX_SLOT_INDEX and retry
  } else throw err;
}

Prevention

When it happens

Trigger: Submitting a graph where a node claims to connect to input/output slot N and N > MAX_SLOT_INDEX. Both indices are checked per edge after the non-negative-integer check.

Common situations: A workflow exported from a future version that supports more slots; programmatically generated slot indices that overshoot; a node type designed for branching with many outputs before branching support landed (CAT-2874); off-by-one producing an index one past the cap.

Related errors

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/3e6f760477b018f5. Report an issue: GitHub.