fix(core): Escape curly braces in LangChain prompt templates to prevent parsing errors (#25821)

Co-authored-by: aalises <aalises@users.noreply.github.com>

packages/@n8n/nodes-langchain/nodes/chains/InformationExtractor/processItem.ts

@@ -26,8 +26,13 @@ export async function processItem(
 		systemPromptTemplate?: string;
 	};
 
+	const escapedTemplate = (options.systemPromptTemplate ?? SYSTEM_PROMPT_TEMPLATE).replace(
+		/[{}]/g,
+		(match) => match + match,
+	);
+
 	const systemPromptTemplate = SystemMessagePromptTemplate.fromTemplate(
-		`${options.systemPromptTemplate ?? SYSTEM_PROMPT_TEMPLATE}
+		`${escapedTemplate}
 {format_instructions}`,
 	);
 

packages/@n8n/nodes-langchain/nodes/chains/InformationExtractor/test/processItem.test.ts

@@ -116,6 +116,28 @@ describe('processItem', () => {
 		expect(result).toEqual({ name: 'John', age: 30 });
 	});
 
+	it('should handle curly braces in custom system prompt template', async () => {
+		const customTemplate = 'Extract JSON like this: {"name": "value"} from the text.';
+		const mockExecuteFunctions = {
+			getNodeParameter: (param: string) => {
+				if (param === 'text') return 'John is 30 years old';
+				if (param === 'options') return { systemPromptTemplate: customTemplate };
+				return undefined;
+			},
+			getNode: () => ({ typeVersion: 1.1 }),
+		};
+
+		const llm = new FakeLLM({ response: formatFakeLlmResponse({ name: 'John', age: 30 }) });
+		const parser = OutputFixingParser.fromLLM(
+			llm,
+			StructuredOutputParser.fromZodSchema(makeZodSchemaFromAttributes(mockPersonAttributes)),
+		);
+
+		const result = await processItem(mockExecuteFunctions as any, 0, llm, parser);
+
+		expect(result).toEqual({ name: 'John', age: 30 });
+	});
+
 	it('should handle retries when LLM returns invalid data', async () => {
 		const mockExecuteFunctions = {
 			getNodeParameter: (param: string) => {

packages/@n8n/nodes-langchain/nodes/chains/SentimentAnalysis/SentimentAnalysis.node.ts

@@ -217,8 +217,12 @@ export class SentimentAnalysis implements INodeType {
 						? OutputFixingParser.fromLLM(llm, structuredParser)
 						: structuredParser;
 
+					const escapedTemplate = (options.systemPromptTemplate ?? DEFAULT_SYSTEM_PROMPT_TEMPLATE)
+						.replace(/[{}]/g, (match) => match + match)
+						.replaceAll('{{categories}}', '{categories}');
+
 					const systemPromptTemplate = SystemMessagePromptTemplate.fromTemplate(
-						`${options.systemPromptTemplate ?? DEFAULT_SYSTEM_PROMPT_TEMPLATE}
+						`${escapedTemplate}
 				{format_instructions}`,
 					);
 
@@ -357,8 +361,12 @@ export class SentimentAnalysis implements INodeType {
 						? OutputFixingParser.fromLLM(llm, structuredParser)
 						: structuredParser;
 
+					const escapedTemplate = (options.systemPromptTemplate ?? DEFAULT_SYSTEM_PROMPT_TEMPLATE)
+						.replace(/[{}]/g, (match) => match + match)
+						.replaceAll('{{categories}}', '{categories}');
+
 					const systemPromptTemplate = SystemMessagePromptTemplate.fromTemplate(
-						`${options.systemPromptTemplate ?? DEFAULT_SYSTEM_PROMPT_TEMPLATE}
+						`${escapedTemplate}
 			{format_instructions}`,
 					);
 

packages/@n8n/nodes-langchain/nodes/chains/TextClassifier/processItem.ts

@@ -36,8 +36,12 @@ export async function processItem(
 		itemIndex,
 		SYSTEM_PROMPT_TEMPLATE,
 	) as string;
+	const escapedTemplate = (systemPromptTemplateOpt ?? SYSTEM_PROMPT_TEMPLATE)
+		.replace(/[{}]/g, (match) => match + match)
+		.replaceAll('{{categories}}', '{categories}');
+
 	const systemPromptTemplate = SystemMessagePromptTemplate.fromTemplate(
-		`${systemPromptTemplateOpt ?? SYSTEM_PROMPT_TEMPLATE}
+		`${escapedTemplate}
 	{format_instructions}
 	${multiClassPrompt}
 	${fallbackPrompt}`,

packages/@n8n/nodes-langchain/nodes/chains/TextClassifier/test/processItem.test.ts

@@ -1,4 +1,4 @@
-import { ChatPromptTemplate } from '@langchain/core/prompts';
+import { ChatPromptTemplate, SystemMessagePromptTemplate } from '@langchain/core/prompts';
 import { FakeChatModel } from '@langchain/core/utils/testing';
 import { mock } from 'jest-mock-extended';
 import type { IExecuteFunctions } from 'n8n-workflow';
@@ -102,6 +102,50 @@ describe('processItem', () => {
 		expect(tracing.getTracingConfig).toHaveBeenCalledWith(mockContext);
 	});
 
+	it('should escape curly braces in custom system prompt template', async () => {
+		const templateWithBraces = 'Classify using format {"category": "value"}: {categories}';
+
+		mockContext.getNodeParameter.mockImplementation((param, _itemIndex, defaultValue) => {
+			if (param === 'inputText') return 'Test input';
+			if (param === 'options.systemPromptTemplate') return templateWithBraces;
+			return defaultValue;
+		});
+
+		const mockParser = {
+			getFormatInstructions: () => '[format instructions]',
+		};
+
+		const mockChain = {
+			invoke: jest.fn().mockResolvedValue({ category: 'test' }),
+		};
+
+		const mockPipe = jest.fn().mockReturnValue({
+			pipe: jest.fn().mockReturnValue({
+				withConfig: jest.fn().mockReturnValue(mockChain),
+			}),
+		});
+
+		jest.mocked(ChatPromptTemplate.fromMessages).mockReturnValue({ pipe: mockPipe } as any);
+
+		await processItem(
+			mockContext,
+			0,
+			{ json: {} },
+			fakeLLM,
+			mockParser as any,
+			[{ category: 'test', description: 'test category' }],
+			'multi class prompt',
+			undefined,
+		);
+
+		expect(SystemMessagePromptTemplate.fromTemplate).toHaveBeenCalledWith(
+			expect.stringContaining('{{"category": "value"}}'),
+		);
+		expect(SystemMessagePromptTemplate.fromTemplate).toHaveBeenCalledWith(
+			expect.stringContaining('{categories}'),
+		);
+	});
+
 	it('should handle fallback prompt', async () => {
 		const mockParser = {
 			getFormatInstructions: () => '[format instructions]',