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

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

@@ -88,15 +88,24 @@ async function executeSimpleChain({
 	llm,
 	query,
 	prompt,
+	fallbackLlm,
 }: {
 	context: IExecuteFunctions;
 	llm: BaseLanguageModel;
 	query: string;
 	prompt: ChatPromptTemplate | PromptTemplate;
+	fallbackLlm?: BaseLanguageModel | null;
 }) {
 	const outputParser = getOutputParserForLLM(llm);
+	let model;
 
-	const chain = prompt.pipe(llm).pipe(outputParser).withConfig(getTracingConfig(context));
+	if (fallbackLlm) {
+		model = llm.withFallbacks([fallbackLlm]);
+	} else {
+		model = llm;
+	}
+
+	const chain = prompt.pipe(model).pipe(outputParser).withConfig(getTracingConfig(context));
 
 	// Execute the chain
 	const response = await chain.invoke({
@@ -118,6 +127,7 @@ export async function executeChain({
 	llm,
 	outputParser,
 	messages,
+	fallbackLlm,
 }: ChainExecutionParams): Promise<unknown[]> {
 	// If no output parsers provided, use a simple chain with basic prompt template
 	if (!outputParser) {
@@ -134,6 +144,7 @@ export async function executeChain({
 			llm,
 			query,
 			prompt: promptTemplate,
+			fallbackLlm,
 		});
 	}
 

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

@@ -23,6 +23,17 @@ export function getInputs(parameters: IDataObject) {
 		},
 	];
 
+	const needsFallback = parameters?.needsFallback;
+
+	if (needsFallback === undefined || needsFallback === true) {
+		inputs.push({
+			displayName: 'Fallback Model',
+			maxConnections: 1,
+			type: 'ai_languageModel',
+			required: true,
+		});
+	}
+
 	// If `hasOutputParser` is undefined it must be version 1.3 or earlier so we
 	// always add the output parser input
 	const hasOutputParser = parameters?.hasOutputParser;
@@ -119,6 +130,18 @@ export const nodeProperties: INodeProperties[] = [
 			},
 		},
 	},
+	{
+		displayName: 'Enable Fallback Model',
+		name: 'needsFallback',
+		type: 'boolean',
+		default: false,
+		noDataExpression: true,
+		displayOptions: {
+			hide: {
+				'@version': [1, 1.1, 1.3],
+			},
+		},
+	},
 	{
 		displayName: 'Chat Messages (if Using a Chat Model)',
 		name: 'messages',
@@ -275,4 +298,16 @@ export const nodeProperties: INodeProperties[] = [
 			},
 		},
 	},
+	{
+		displayName:
+			'Connect an additional language model on the canvas to use it as a fallback if the main model fails',
+		name: 'fallbackNotice',
+		type: 'notice',
+		default: '',
+		displayOptions: {
+			show: {
+				needsFallback: [true],
+			},
+		},
+	},
 ];

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,
 	});
 };

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

@@ -38,4 +38,5 @@ export interface ChainExecutionParams {
 	llm: BaseLanguageModel;
 	outputParser?: N8nOutputParser;
 	messages?: MessageTemplate[];
+	fallbackLlm?: BaseLanguageModel | null;
 }