fix(Basic LLM Chain Node): Prevent stringifying of structured output on previous versions (#14200)

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

@@ -9,6 +9,21 @@ import { getTracingConfig } from '@utils/tracing';
 import { createPromptTemplate } from './promptUtils';
 import type { ChainExecutionParams } from './types';
 
+export class NaiveJsonOutputParser<
+	T extends Record<string, any> = Record<string, any>,
+> extends JsonOutputParser<T> {
+	async parse(text: string): Promise<T> {
+		// First try direct JSON parsing
+		try {
+			const directParsed = JSON.parse(text);
+			return directParsed as T;
+		} catch (e) {
+			// If fails, fall back to JsonOutputParser parser
+			return await super.parse(text);
+		}
+	}
+}
+
 /**
  * Type guard to check if the LLM has a modelKwargs property(OpenAI)
  */
@@ -39,11 +54,11 @@ export function getOutputParserForLLM(
 	llm: BaseLanguageModel,
 ): BaseLLMOutputParser<string | Record<string, unknown>> {
 	if (isModelWithResponseFormat(llm) && llm.modelKwargs?.response_format?.type === 'json_object') {
-		return new JsonOutputParser();
+		return new NaiveJsonOutputParser();
 	}
 
 	if (isModelWithFormat(llm) && llm.format === 'json') {
-		return new JsonOutputParser();
+		return new NaiveJsonOutputParser();
 	}
 
 	return new StringOutputParser();

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

@@ -3,7 +3,7 @@ import type { IDataObject } from 'n8n-workflow';
 /**
  * Formats the response from the LLM chain into a consistent structure
  */
-export function formatResponse(response: unknown, version: number): IDataObject {
+export function formatResponse(response: unknown, returnUnwrappedObject: boolean): IDataObject {
 	if (typeof response === 'string') {
 		return {
 			text: response.trim(),
@@ -17,10 +17,12 @@ export function formatResponse(response: unknown, version: number): IDataObject
 	}
 
 	if (response instanceof Object) {
-		if (version >= 1.6) {
+		if (returnUnwrappedObject) {
 			return response as IDataObject;
 		}
 
+		// If the response is an object and we are not unwrapping it, we need to stringify it
+		// to be backwards compatible with older versions of the chain(< 1.6)
 		return {
 			text: JSON.stringify(response),
 		};