n8n-io/n8n · error · NodeOperationError

Please connect a model to the Fallback Model input or disabl

Error message

Please connect a model to the Fallback Model input or disable the fallback option

What it means

Thrown by Tools Agent V3's buildExecutionContext when needsFallback=true but getChatModel(ctx, 1) returned a falsy value. This is the V3 equivalent of error 651: the fallback input is enabled but nothing is connected to it.

Source

Thrown at packages/@n8n/nodes-langchain/nodes/agents/Agent/agents/ToolsAgent/V3/helpers/buildExecutionContext.ts:49

): Promise<ToolsAgentExecutionContext> {
	const items = ctx.getInputData();
	const batchSize = ctx.getNodeParameter('options.batching.batchSize', 0, 1) as number;
	const delayBetweenBatches = ctx.getNodeParameter(
		'options.batching.delayBetweenBatches',
		0,
		0,
	) as number;
	const needsFallback = ctx.getNodeParameter('needsFallback', 0, false) as boolean;

	const memory = await getOptionalMemory(ctx);
	const model = await getChatModel(ctx, 0);
	assert(model, 'Please connect a model to the Chat Model input');

	let fallbackModel: BaseChatModel | null = null;
	if (needsFallback) {
		const maybeFallbackModel = await getChatModel(ctx, 1);
		if (!maybeFallbackModel) {
			throw new NodeOperationError(
				ctx.getNode(),
				'Please connect a model to the Fallback Model input or disable the fallback option',
			);
		}
		fallbackModel = maybeFallbackModel;
	}

	return {
		items,
		batchSize,
		delayBetweenBatches,
		needsFallback,
		model,
		fallbackModel,
		memory,
	};
}

View on GitHub (pinned to 5ac6606e81)

Solutions

1. Connect a chat model to the Fallback Model input.
2. Disable the needsFallback option if fallback is not required.
3. Confirm the fallback model node loads correctly on its own.

const needsFallback = ctx.getNodeParameter('needsFallback', 0, false) as boolean;
if (needsFallback) {
  const fb = await getChatModel(ctx, 1);
  if (!fb) {
    throw new NodeOperationError(ctx.getNode(), 'Fallback enabled but no model connected to the Fallback Model input.');
  }
}

function isChatModel(model: unknown): model is BaseChatModel {
  return !!model && typeof (model as any).invoke === 'function';
}

When it happens

Trigger: Tools Agent V3 with the fallback option enabled and no model node connected to the Fallback Model input.

Common situations: Option toggled without wiring the input; fallback model node deleted; fallback model node errored during getInputConnectionData.

Related errors

• Please connect a model to the Fallback Model input or disabl
• This fallback model is not supported in ${version} version o
• Max iterations (${maxIterations}) reached. The agent could n
• The "text" parameter is empty.
• Human-in-the-Loop nodes cannot be used inside a sub-agent. M

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/c919c3026d5bdf74. Report an issue: GitHub.