feat: Add fallback mechanism for agent and basic chain llm (#16617)

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/methods/processItem.ts

@@ -1,5 +1,6 @@
 import type { BaseLanguageModel } from '@langchain/core/language_models/base';
 import { type IExecuteFunctions, NodeConnectionTypes, NodeOperationError } from 'n8n-workflow';
+import assert from 'node:assert';
 
 import { getPromptInputByType } from '@utils/helpers';
 import { getOptionalOutputParser } from '@utils/output_parsers/N8nOutputParser';
@@ -7,11 +8,40 @@ import { getOptionalOutputParser } from '@utils/output_parsers/N8nOutputParser';
 import { executeChain } from './chainExecutor';
 import { type MessageTemplate } from './types';
 
+async function getChatModel(
+	ctx: IExecuteFunctions,
+	index: number = 0,
+): Promise<BaseLanguageModel | undefined> {
+	const connectedModels = await ctx.getInputConnectionData(NodeConnectionTypes.AiLanguageModel, 0);
+
+	let model;
+
+	if (Array.isArray(connectedModels) && index !== undefined) {
+		if (connectedModels.length <= index) {
+			return undefined;
+		}
+		// We get the models in reversed order from the workflow so we need to reverse them again to match the right index
+		const reversedModels = [...connectedModels].reverse();
+		model = reversedModels[index] as BaseLanguageModel;
+	} else {
+		model = connectedModels as BaseLanguageModel;
+	}
+
+	return model;
+}
+
 export const processItem = async (ctx: IExecuteFunctions, itemIndex: number) => {
-	const llm = (await ctx.getInputConnectionData(
-		NodeConnectionTypes.AiLanguageModel,
-		0,
-	)) as BaseLanguageModel;
+	const needsFallback = ctx.getNodeParameter('needsFallback', 0, false) as boolean;
+	const llm = await getChatModel(ctx, 0);
+	assert(llm, 'Please connect a model to the Chat Model input');
+
+	const fallbackLlm = needsFallback ? await getChatModel(ctx, 1) : null;
+	if (needsFallback && !fallbackLlm) {
+		throw new NodeOperationError(
+			ctx.getNode(),
+			'Please connect a model to the Fallback Model input or disable the fallback option',
+		);
+	}
 
 	// Get output parser if configured
 	const outputParser = await getOptionalOutputParser(ctx, itemIndex);
@@ -50,5 +80,6 @@ export const processItem = async (ctx: IExecuteFunctions, itemIndex: number) =>
 		llm,
 		outputParser,
 		messages,
+		fallbackLlm,
 	});
 };