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

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],
+			},
+		},
+	},
 ];