fix(Token Splitter Node): Prevent tiktoken blocking on repetitive content (#16769)

packages/@n8n/nodes-langchain/nodes/llms/N8nLlmTracing.ts

@@ -13,7 +13,7 @@ import type { IDataObject, ISupplyDataFunctions, JsonObject } from 'n8n-workflow
 import { NodeConnectionTypes, NodeError, NodeOperationError } from 'n8n-workflow';
 
 import { logAiEvent } from '@utils/helpers';
-import { encodingForModel } from '@utils/tokenizer/tiktoken';
+import { estimateTokensFromStringList } from '@utils/tokenizer/token-estimator';
 
 type TokensUsageParser = (llmOutput: LLMResult['llmOutput']) => {
 	completionTokens: number;
@@ -84,13 +84,7 @@ export class N8nLlmTracing extends BaseCallbackHandler {
 
 	async estimateTokensFromStringList(list: string[]) {
 		const embeddingModel = getModelNameForTiktoken(TIKTOKEN_ESTIMATE_MODEL);
-		const encoder = await encodingForModel(embeddingModel);
+		return await estimateTokensFromStringList(list, embeddingModel);
-
-		const encodedListLength = await Promise.all(
-			list.map(async (text) => encoder.encode(text).length),
-		);
-
-		return encodedListLength.reduce((acc, curr) => acc + curr, 0);
 	}
 
 	async handleLLMEnd(output: LLMResult, runId: string) {

packages/@n8n/nodes-langchain/nodes/text_splitters/TextSplitterTokenSplitter/TokenTextSplitter.ts

@@ -3,7 +3,9 @@ import type { TokenTextSplitterParams } from '@langchain/textsplitters';
 import { TextSplitter } from '@langchain/textsplitters';
 import type * as tiktoken from 'js-tiktoken';
 
+import { hasLongSequentialRepeat } from '@utils/helpers';
 import { getEncoding } from '@utils/tokenizer/tiktoken';
+import { estimateTextSplitsByTokens } from '@utils/tokenizer/token-estimator';
 
 /**
  * Implementation of splitter which looks at tokens.
@@ -32,26 +34,61 @@ export class TokenTextSplitter extends TextSplitter implements TokenTextSplitter
 	}
 
 	async splitText(text: string): Promise<string[]> {
+		try {
+			// Validate input
+			if (!text || typeof text !== 'string') {
+				return [];
+			}
+
+			// Check for repetitive content
+			if (hasLongSequentialRepeat(text)) {
+				const splits = estimateTextSplitsByTokens(
+					text,
+					this.chunkSize,
+					this.chunkOverlap,
+					this.encodingName,
+				);
+				return splits;
+			}
+
+			// Use tiktoken for normal text
+			try {
 				if (!this.tokenizer) {
 					this.tokenizer = await getEncoding(this.encodingName);
 				}
 
 				const splits: string[] = [];
-
 				const input_ids = this.tokenizer.encode(text, this.allowedSpecial, this.disallowedSpecial);
 
 				let start_idx = 0;
+				let chunkCount = 0;
 
 				while (start_idx < input_ids.length) {
 					if (start_idx > 0) {
-				start_idx -= this.chunkOverlap;
+						start_idx = Math.max(0, start_idx - this.chunkOverlap);
 					}
 					const end_idx = Math.min(start_idx + this.chunkSize, input_ids.length);
 					const chunk_ids = input_ids.slice(start_idx, end_idx);
+
 					splits.push(this.tokenizer.decode(chunk_ids));
+
+					chunkCount++;
 					start_idx = end_idx;
 				}
 
 				return splits;
+			} catch (tiktokenError) {
+				// Fall back to character-based splitting if tiktoken fails
+				return estimateTextSplitsByTokens(
+					text,
+					this.chunkSize,
+					this.chunkOverlap,
+					this.encodingName,
+				);
+			}
+		} catch (error) {
+			// Return empty array on complete failure
+			return [];
+		}
 	}
 }

packages/@n8n/nodes-langchain/nodes/text_splitters/TextSplitterTokenSplitter/tests/TokenTextSplitter.test.ts

@@ -1,7 +1,13 @@
+import { OperationalError } from 'n8n-workflow';
+
+import * as helpers from '../../../../utils/helpers';
 import * as tiktokenUtils from '../../../../utils/tokenizer/tiktoken';
+import * as tokenEstimator from '../../../../utils/tokenizer/token-estimator';
 import { TokenTextSplitter } from '../TokenTextSplitter';
 
 jest.mock('../../../../utils/tokenizer/tiktoken');
+jest.mock('../../../../utils/helpers');
+jest.mock('../../../../utils/tokenizer/token-estimator');
 
 describe('TokenTextSplitter', () => {
 	let mockTokenizer: jest.Mocked<{
@@ -15,6 +21,8 @@ describe('TokenTextSplitter', () => {
 			decode: jest.fn(),
 		};
 		(tiktokenUtils.getEncoding as jest.Mock).mockResolvedValue(mockTokenizer);
+		// Default mock for hasLongSequentialRepeat - no repetition
+		(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(false);
 	});
 
 	afterEach(() => {
@@ -161,5 +169,175 @@ describe('TokenTextSplitter', () => {
 
 			expect(result).toEqual(['One two', 'two three', 'three four', 'four five', 'five six']);
 		});
+
+		describe('repetitive content handling', () => {
+			it('should use character-based estimation for repetitive content', async () => {
+				const splitter = new TokenTextSplitter({
+					chunkSize: 100,
+					chunkOverlap: 10,
+				});
+
+				const repetitiveText = 'a'.repeat(1000);
+				const estimatedChunks = ['chunk1', 'chunk2', 'chunk3'];
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(true);
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue(estimatedChunks);
+
+				const result = await splitter.splitText(repetitiveText);
+
+				// Should not call tiktoken
+				expect(tiktokenUtils.getEncoding).not.toHaveBeenCalled();
+				expect(mockTokenizer.encode).not.toHaveBeenCalled();
+
+				// Should use estimation
+				expect(helpers.hasLongSequentialRepeat).toHaveBeenCalledWith(repetitiveText);
+				expect(tokenEstimator.estimateTextSplitsByTokens).toHaveBeenCalledWith(
+					repetitiveText,
+					100,
+					10,
+					'cl100k_base',
+				);
+
+				expect(result).toEqual(estimatedChunks);
+			});
+
+			it('should use tiktoken for non-repetitive content', async () => {
+				const splitter = new TokenTextSplitter({
+					chunkSize: 3,
+					chunkOverlap: 0,
+				});
+
+				const normalText = 'This is normal text without repetition';
+				const mockTokenIds = [1, 2, 3, 4, 5, 6];
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(false);
+				mockTokenizer.encode.mockReturnValue(mockTokenIds);
+				mockTokenizer.decode.mockImplementation(() => 'chunk');
+
+				await splitter.splitText(normalText);
+
+				// Should check for repetition
+				expect(helpers.hasLongSequentialRepeat).toHaveBeenCalledWith(normalText);
+
+				// Should use tiktoken
+				expect(tiktokenUtils.getEncoding).toHaveBeenCalled();
+				expect(mockTokenizer.encode).toHaveBeenCalled();
+
+				// Should not use estimation
+				expect(tokenEstimator.estimateTextSplitsByTokens).not.toHaveBeenCalled();
+			});
+
+			it('should handle repetitive content with different encodings', async () => {
+				const splitter = new TokenTextSplitter({
+					encodingName: 'o200k_base',
+					chunkSize: 50,
+					chunkOverlap: 5,
+				});
+
+				const repetitiveText = '.'.repeat(500);
+				const estimatedChunks = ['estimated chunk 1', 'estimated chunk 2'];
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(true);
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue(estimatedChunks);
+
+				const result = await splitter.splitText(repetitiveText);
+
+				expect(tokenEstimator.estimateTextSplitsByTokens).toHaveBeenCalledWith(
+					repetitiveText,
+					50,
+					5,
+					'o200k_base',
+				);
+				expect(result).toEqual(estimatedChunks);
+			});
+
+			it('should handle edge case with exactly 100 repeating characters', async () => {
+				const splitter = new TokenTextSplitter();
+				const edgeText = 'x'.repeat(100);
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(true);
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue(['single chunk']);
+
+				const result = await splitter.splitText(edgeText);
+
+				expect(helpers.hasLongSequentialRepeat).toHaveBeenCalledWith(edgeText);
+				expect(result).toEqual(['single chunk']);
+			});
+
+			it('should handle mixed content with repetitive sections', async () => {
+				const splitter = new TokenTextSplitter();
+				const mixedText = 'Normal text ' + 'z'.repeat(200) + ' more normal text';
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(true);
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue([
+					'chunk1',
+					'chunk2',
+				]);
+
+				const result = await splitter.splitText(mixedText);
+
+				expect(helpers.hasLongSequentialRepeat).toHaveBeenCalledWith(mixedText);
+				expect(tokenEstimator.estimateTextSplitsByTokens).toHaveBeenCalled();
+				expect(result).toEqual(['chunk1', 'chunk2']);
+			});
+		});
+
+		describe('error handling', () => {
+			it('should return empty array for null input', async () => {
+				const splitter = new TokenTextSplitter();
+				const result = await splitter.splitText(null as any);
+				expect(result).toEqual([]);
+			});
+
+			it('should return empty array for undefined input', async () => {
+				const splitter = new TokenTextSplitter();
+				const result = await splitter.splitText(undefined as any);
+				expect(result).toEqual([]);
+			});
+
+			it('should return empty array for non-string input', async () => {
+				const splitter = new TokenTextSplitter();
+				const result = await splitter.splitText(123 as any);
+				expect(result).toEqual([]);
+			});
+
+			it('should fall back to estimation if tiktoken fails', async () => {
+				const splitter = new TokenTextSplitter();
+				const text = 'This will cause tiktoken to fail';
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(false);
+				(tiktokenUtils.getEncoding as jest.Mock).mockRejectedValue(new Error('Tiktoken error'));
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue([
+					'fallback chunk',
+				]);
+
+				const result = await splitter.splitText(text);
+
+				expect(result).toEqual(['fallback chunk']);
+				expect(tokenEstimator.estimateTextSplitsByTokens).toHaveBeenCalledWith(
+					text,
+					splitter.chunkSize,
+					splitter.chunkOverlap,
+					splitter.encodingName,
+				);
+			});
+
+			it('should fall back to estimation if encode fails', async () => {
+				const splitter = new TokenTextSplitter();
+				const text = 'This will cause encode to fail';
+
+				(helpers.hasLongSequentialRepeat as jest.Mock).mockReturnValue(false);
+				mockTokenizer.encode.mockImplementation(() => {
+					throw new OperationalError('Encode error');
+				});
+				(tokenEstimator.estimateTextSplitsByTokens as jest.Mock).mockReturnValue([
+					'fallback chunk',
+				]);
+
+				const result = await splitter.splitText(text);
+
+				expect(result).toEqual(['fallback chunk']);
+			});
+		});
 	});
 });

packages/@n8n/nodes-langchain/utils/helpers.ts

@@ -258,3 +258,50 @@ export function unwrapNestedOutput(output: Record<string, unknown>): Record<stri
 export function nodeNameToToolName(node: INode): string {
 	return node.name.replace(/[\s.?!=+#@&*()[\]{}:;,<>\/\\'"^%$]/g, '_').replace(/_+/g, '_');
 }
+
+/**
+ * Detects if a text contains a character that repeats sequentially for a specified threshold.
+ * This is used to prevent performance issues with tiktoken on highly repetitive content.
+ * @param text The text to check
+ * @param threshold The minimum number of sequential repeats to detect (default: 1000)
+ * @returns true if a character repeats sequentially for at least the threshold amount
+ */
+export function hasLongSequentialRepeat(text: string, threshold = 1000): boolean {
+	try {
+		// Validate inputs
+		if (
+			text === null ||
+			typeof text !== 'string' ||
+			text.length === 0 ||
+			threshold <= 0 ||
+			text.length < threshold
+		) {
+			return false;
+		}
+		// Use string iterator to avoid creating array copy (memory efficient)
+		const iterator = text[Symbol.iterator]();
+		let prev = iterator.next();
+
+		if (prev.done) {
+			return false;
+		}
+
+		let count = 1;
+		for (const char of iterator) {
+			if (char === prev.value) {
+				count++;
+				if (count >= threshold) {
+					return true;
+				}
+			} else {
+				count = 1;
+				prev = { value: char, done: false };
+			}
+		}
+
+		return false;
+	} catch (error) {
+		// On any error, return false to allow normal processing
+		return false;
+	}
+}

packages/@n8n/nodes-langchain/utils/tests/helpers.test.ts

@@ -8,6 +8,7 @@ import { z } from 'zod';
 import {
 	escapeSingleCurlyBrackets,
 	getConnectedTools,
+	hasLongSequentialRepeat,
 	nodeNameToToolName,
 	unwrapNestedOutput,
 } from '../helpers';
@@ -423,3 +424,107 @@ describe('unwrapNestedOutput', () => {
 		expect(unwrapNestedOutput(input)).toEqual(input);
 	});
 });
+
+describe('hasLongSequentialRepeat', () => {
+	it('should return false for text shorter than threshold', () => {
+		const text = 'a'.repeat(99);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(false);
+	});
+
+	it('should return false for normal text without repeats', () => {
+		const text = 'This is a normal text without many sequential repeating characters.';
+		expect(hasLongSequentialRepeat(text)).toBe(false);
+	});
+
+	it('should return true for text with exactly threshold repeats', () => {
+		const text = 'a'.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should return true for text with more than threshold repeats', () => {
+		const text = 'b'.repeat(150);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should detect repeats in the middle of text', () => {
+		const text = 'Normal text ' + 'x'.repeat(100) + ' more normal text';
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should detect repeats at the end of text', () => {
+		const text = 'Normal text at the beginning' + 'z'.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should work with different thresholds', () => {
+		const text = 'a'.repeat(50);
+		expect(hasLongSequentialRepeat(text, 30)).toBe(true);
+		expect(hasLongSequentialRepeat(text, 60)).toBe(false);
+	});
+
+	it('should handle special characters', () => {
+		const text = '.'.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should handle spaces', () => {
+		const text = ' '.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should handle newlines', () => {
+		const text = '\n'.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	it('should not detect non-sequential repeats', () => {
+		const text = 'ababab'.repeat(50); // 300 chars but no sequential repeats
+		expect(hasLongSequentialRepeat(text, 100)).toBe(false);
+	});
+
+	it('should handle mixed content with repeats below threshold', () => {
+		const text = 'aaa' + 'b'.repeat(50) + 'ccc' + 'd'.repeat(40) + 'eee';
+		expect(hasLongSequentialRepeat(text, 100)).toBe(false);
+	});
+
+	it('should handle empty string', () => {
+		expect(hasLongSequentialRepeat('', 100)).toBe(false);
+	});
+
+	it('should work with very large texts', () => {
+		const normalText = 'Lorem ipsum dolor sit amet '.repeat(1000);
+		const textWithRepeat = normalText + 'A'.repeat(100) + normalText;
+		expect(hasLongSequentialRepeat(textWithRepeat, 100)).toBe(true);
+	});
+
+	it('should detect unicode character repeats', () => {
+		const text = '😀'.repeat(100);
+		expect(hasLongSequentialRepeat(text, 100)).toBe(true);
+	});
+
+	describe('error handling', () => {
+		it('should handle null input', () => {
+			expect(hasLongSequentialRepeat(null as any)).toBe(false);
+		});
+
+		it('should handle undefined input', () => {
+			expect(hasLongSequentialRepeat(undefined as any)).toBe(false);
+		});
+
+		it('should handle non-string input', () => {
+			expect(hasLongSequentialRepeat(123 as any)).toBe(false);
+			expect(hasLongSequentialRepeat({} as any)).toBe(false);
+			expect(hasLongSequentialRepeat([] as any)).toBe(false);
+		});
+
+		it('should handle zero or negative threshold', () => {
+			const text = 'a'.repeat(100);
+			expect(hasLongSequentialRepeat(text, 0)).toBe(false);
+			expect(hasLongSequentialRepeat(text, -1)).toBe(false);
+		});
+
+		it('should handle empty string', () => {
+			expect(hasLongSequentialRepeat('', 100)).toBe(false);
+		});
+	});
+});

packages/@n8n/nodes-langchain/utils/tokenizer/tests/token-estimator.test.ts

@@ -0,0 +1,248 @@
+/* eslint-disable @typescript-eslint/no-explicit-any */
+/* eslint-disable @typescript-eslint/no-unsafe-argument */
+import {
+	estimateTokensByCharCount,
+	estimateTextSplitsByTokens,
+	estimateTokensFromStringList,
+} from '../token-estimator';
+
+describe('token-estimator', () => {
+	describe('estimateTokensByCharCount', () => {
+		it('should estimate tokens for text using default model', () => {
+			const text = 'This is a test text with some content.';
+			const result = estimateTokensByCharCount(text);
+			// 38 characters / 4.0 (cl100k_base ratio) = 10 tokens
+			expect(result).toBe(10);
+		});
+
+		it('should estimate tokens for different models', () => {
+			const text = 'Test text'; // 9 characters
+
+			expect(estimateTokensByCharCount(text, 'gpt-4o')).toBe(3); // 9 / 3.8 = 2.37 -> 3
+			expect(estimateTokensByCharCount(text, 'gpt-4')).toBe(3); // 9 / 4.0 = 2.25 -> 3
+			expect(estimateTokensByCharCount(text, 'o200k_base')).toBe(3); // 9 / 3.5 = 2.57 -> 3
+			expect(estimateTokensByCharCount(text, 'p50k_base')).toBe(3); // 9 / 4.2 = 2.14 -> 3
+		});
+
+		it('should use default ratio for unknown models', () => {
+			const text = 'Test text with 24 chars.'; // 24 characters
+			const result = estimateTokensByCharCount(text, 'unknown-model');
+			expect(result).toBe(6); // 24 / 4.0 = 6
+		});
+
+		it('should handle empty text', () => {
+			expect(estimateTokensByCharCount('')).toBe(0);
+			expect(estimateTokensByCharCount('', 'gpt-4')).toBe(0);
+		});
+
+		it('should handle null or undefined text', () => {
+			expect(estimateTokensByCharCount(null as any)).toBe(0);
+			expect(estimateTokensByCharCount(undefined as any)).toBe(0);
+		});
+
+		it('should handle non-string input', () => {
+			expect(estimateTokensByCharCount(123 as any)).toBe(0);
+			expect(estimateTokensByCharCount({} as any)).toBe(0);
+			expect(estimateTokensByCharCount([] as any)).toBe(0);
+		});
+
+		it('should handle very long text', () => {
+			const longText = 'a'.repeat(10000);
+			const result = estimateTokensByCharCount(longText);
+			expect(result).toBe(2500); // 10000 / 4.0 = 2500
+		});
+
+		it('should handle invalid model ratios gracefully', () => {
+			// This would only happen if MODEL_CHAR_PER_TOKEN_RATIOS is corrupted
+			const text = 'Test text'; // 9 characters
+			// Since we can't mock the constant, we test with default fallback
+			const result = estimateTokensByCharCount(text, 'corrupted-model');
+			expect(result).toBe(3); // Falls back to 4.0 ratio
+		});
+
+		it('should round up token estimates', () => {
+			expect(estimateTokensByCharCount('a')).toBe(1); // 1 / 4.0 = 0.25 -> 1
+			expect(estimateTokensByCharCount('ab')).toBe(1); // 2 / 4.0 = 0.5 -> 1
+			expect(estimateTokensByCharCount('abc')).toBe(1); // 3 / 4.0 = 0.75 -> 1
+			expect(estimateTokensByCharCount('abcd')).toBe(1); // 4 / 4.0 = 1
+			expect(estimateTokensByCharCount('abcde')).toBe(2); // 5 / 4.0 = 1.25 -> 2
+		});
+	});
+
+	describe('estimateTextSplitsByTokens', () => {
+		it('should split text into chunks based on estimated token size', () => {
+			const text = 'a'.repeat(400); // 400 characters
+			const chunks = estimateTextSplitsByTokens(text, 25, 0); // 25 tokens = 100 chars
+
+			expect(chunks).toHaveLength(4);
+			expect(chunks[0]).toHaveLength(100);
+			expect(chunks[1]).toHaveLength(100);
+			expect(chunks[2]).toHaveLength(100);
+			expect(chunks[3]).toHaveLength(100);
+		});
+
+		it('should handle chunk overlap', () => {
+			const text = 'a'.repeat(200); // 200 characters
+			const chunks = estimateTextSplitsByTokens(text, 25, 5); // 25 tokens = 100 chars, 5 tokens = 20 chars overlap
+
+			expect(chunks).toHaveLength(3);
+			expect(chunks[0]).toBe('a'.repeat(100)); // First chunk: 0-100
+			expect(chunks[1]).toBe('a'.repeat(100)); // Second chunk: 80-180 (20 char overlap)
+			expect(chunks[2]).toBe('a'.repeat(40)); // Third chunk: 160-200
+		});
+
+		it('should handle text shorter than chunk size', () => {
+			const text = 'Short text';
+			const chunks = estimateTextSplitsByTokens(text, 100, 0);
+
+			expect(chunks).toHaveLength(1);
+			expect(chunks[0]).toBe(text);
+		});
+
+		it('should handle empty text', () => {
+			expect(estimateTextSplitsByTokens('', 10, 0)).toEqual([]);
+		});
+
+		it('should handle null or undefined text', () => {
+			expect(estimateTextSplitsByTokens(null as any, 10, 0)).toEqual([]);
+			expect(estimateTextSplitsByTokens(undefined as any, 10, 0)).toEqual([]);
+		});
+
+		it('should handle non-string input', () => {
+			expect(estimateTextSplitsByTokens(123 as any, 10, 0)).toEqual([]);
+			expect(estimateTextSplitsByTokens({} as any, 10, 0)).toEqual([]);
+		});
+
+		it('should handle invalid chunk size', () => {
+			const text = 'Test text';
+			expect(estimateTextSplitsByTokens(text, 0, 0)).toEqual([text]);
+			expect(estimateTextSplitsByTokens(text, -1, 0)).toEqual([text]);
+			expect(estimateTextSplitsByTokens(text, NaN, 0)).toEqual([text]);
+			expect(estimateTextSplitsByTokens(text, Infinity, 0)).toEqual([text]);
+		});
+
+		it('should handle invalid overlap', () => {
+			const text = 'a'.repeat(200);
+			// Negative overlap should be treated as 0
+			const chunks1 = estimateTextSplitsByTokens(text, 25, -10);
+			expect(chunks1).toHaveLength(2);
+
+			// Overlap larger than chunk size should be capped
+			const chunks2 = estimateTextSplitsByTokens(text, 25, 30); // overlap capped to 24
+			expect(chunks2.length).toBeGreaterThan(2);
+		});
+
+		it('should ensure progress even with large overlap', () => {
+			const text = 'a'.repeat(100);
+			// With overlap = chunkSize - 1, we should still make progress
+			const chunks = estimateTextSplitsByTokens(text, 10, 9); // 10 tokens = 40 chars, 9 tokens = 36 chars overlap
+
+			expect(chunks.length).toBeGreaterThan(1);
+			// Verify no infinite loop occurs
+			expect(chunks.length).toBeLessThan(100);
+		});
+
+		it('should work with different models', () => {
+			const text = 'a'.repeat(380); // 380 characters
+			const chunks = estimateTextSplitsByTokens(text, 100, 0, 'gpt-4o'); // 100 tokens * 3.8 = 380 chars
+
+			expect(chunks).toHaveLength(1);
+			expect(chunks[0]).toBe(text);
+		});
+
+		it('should use default model ratio for unknown models', () => {
+			const text = 'a'.repeat(400);
+			const chunks = estimateTextSplitsByTokens(text, 100, 0, 'unknown-model'); // Falls back to 4.0 ratio
+
+			expect(chunks).toHaveLength(1);
+			expect(chunks[0]).toBe(text);
+		});
+
+		it('should handle edge case where text length equals chunk size', () => {
+			const text = 'a'.repeat(100);
+			const chunks = estimateTextSplitsByTokens(text, 25, 0); // 25 tokens = 100 chars
+
+			expect(chunks).toHaveLength(1);
+			expect(chunks[0]).toBe(text);
+		});
+
+		it('should handle unicode text', () => {
+			const text = '你好世界'.repeat(25); // 100 characters (4 chars * 25)
+			const chunks = estimateTextSplitsByTokens(text, 25, 0);
+
+			expect(chunks.length).toBeGreaterThan(0);
+			expect(chunks.join('')).toBe(text);
+		});
+
+		it('should return single chunk on any error in catch block', () => {
+			const text = 'Test text';
+			// Since we can't easily trigger the catch block, we test the expected behavior
+			// The function should return [text] on error
+			const result = estimateTextSplitsByTokens(text, 10, 0);
+			expect(result.length).toBeGreaterThan(0);
+		});
+	});
+
+	describe('estimateTokensFromStringList', () => {
+		// Since this function uses tiktoken which requires external data files,
+		// we'll test it with integration-style tests that don't require mocking
+
+		it('should handle empty list', async () => {
+			const result = await estimateTokensFromStringList([], 'gpt-4');
+			expect(result).toBe(0);
+		});
+
+		it('should handle non-array input', async () => {
+			const result = await estimateTokensFromStringList(null as any, 'gpt-4');
+			expect(result).toBe(0);
+
+			const result2 = await estimateTokensFromStringList('not an array' as any, 'gpt-4');
+			expect(result2).toBe(0);
+		});
+
+		it('should handle null/undefined items in list', async () => {
+			const list = ['Valid text', null, undefined, '', 123 as any];
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toEqual(2);
+		});
+
+		it('should estimate tokens for normal text', async () => {
+			const list = ['Hello world', 'Test text'];
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toBeGreaterThan(0);
+		});
+
+		it('should use character-based estimation for repetitive content', async () => {
+			const list = ['a'.repeat(1500)];
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toBe(375); // 1500 chars / 4.0 = 375 tokens
+		});
+
+		it('should handle mixed content', async () => {
+			const list = ['Normal text content', 'a'.repeat(1500), 'More normal text'];
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toBeGreaterThan(375); // At least the repetitive content tokens
+		});
+
+		it('should work with different models', async () => {
+			const list = ['Test text for different model'];
+			const result1 = await estimateTokensFromStringList(list, 'gpt-4');
+			const result2 = await estimateTokensFromStringList(list, 'gpt-4o');
+			// Both should return positive values
+			expect(result1).toBeGreaterThan(0);
+			expect(result2).toBeGreaterThan(0);
+		});
+
+		it('should handle very long lists', async () => {
+			const list = Array(10000).fill('Sample text');
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toBeGreaterThan(0);
+		});
+
+		it('should handle unicode text', async () => {
+			const list = ['你好世界', '🌍🌎🌏', 'مرحبا بالعالم'];
+			const result = await estimateTokensFromStringList(list, 'gpt-4');
+			expect(result).toBeGreaterThan(0);
+		});
+	});
+});

packages/@n8n/nodes-langchain/utils/tokenizer/token-estimator.ts

@@ -0,0 +1,176 @@
+/**
+ * Token estimation utilities for handling text without using tiktoken.
+ * This is used as a fallback when tiktoken would be too slow (e.g., with repetitive content).
+ */
+
+import type { TiktokenModel } from 'js-tiktoken';
+
+import { encodingForModel } from './tiktoken';
+import { hasLongSequentialRepeat } from '../helpers';
+
+/**
+ * Model-specific average characters per token ratios.
+ * These are approximate values based on typical English text.
+ */
+const MODEL_CHAR_PER_TOKEN_RATIOS: Record<string, number> = {
+	'gpt-4o': 3.8,
+	'gpt-4': 4.0,
+	'gpt-3.5-turbo': 4.0,
+	cl100k_base: 4.0,
+	o200k_base: 3.5,
+	p50k_base: 4.2,
+	r50k_base: 4.2,
+};
+
+/**
+ * Estimates the number of tokens in a text based on character count.
+ * This is much faster than tiktoken but less accurate.
+ *
+ * @param text The text to estimate tokens for
+ * @param model The model or encoding name (optional)
+ * @returns Estimated number of tokens
+ */
+export function estimateTokensByCharCount(text: string, model: string = 'cl100k_base'): number {
+	try {
+		// Validate input
+		if (!text || typeof text !== 'string' || text.length === 0) {
+			return 0;
+		}
+
+		// Get the ratio for the specific model, or use default
+		const charsPerToken = MODEL_CHAR_PER_TOKEN_RATIOS[model] || 4.0;
+
+		// Validate ratio
+		if (!Number.isFinite(charsPerToken) || charsPerToken <= 0) {
+			// Fallback to default ratio
+			const estimatedTokens = Math.ceil(text.length / 4.0);
+			return estimatedTokens;
+		}
+
+		// Calculate estimated tokens
+		const estimatedTokens = Math.ceil(text.length / charsPerToken);
+
+		return estimatedTokens;
+	} catch (error) {
+		// Return conservative estimate on error
+		return Math.ceil((text?.length || 0) / 4.0);
+	}
+}
+
+/**
+ * Estimates tokens for text splitting purposes.
+ * Returns chunk boundaries based on character positions rather than token positions.
+ *
+ * @param text The text to split
+ * @param chunkSize Target chunk size in tokens
+ * @param chunkOverlap Overlap between chunks in tokens
+ * @param model The model or encoding name (optional)
+ * @returns Array of text chunks
+ */
+export function estimateTextSplitsByTokens(
+	text: string,
+	chunkSize: number,
+	chunkOverlap: number,
+	model: string = 'cl100k_base',
+): string[] {
+	try {
+		// Validate inputs
+		if (!text || typeof text !== 'string' || text.length === 0) {
+			return [];
+		}
+
+		// Validate numeric parameters
+		if (!Number.isFinite(chunkSize) || chunkSize <= 0) {
+			// Return whole text as single chunk if invalid chunk size
+			return [text];
+		}
+
+		// Ensure overlap is valid and less than chunk size
+		const validOverlap =
+			Number.isFinite(chunkOverlap) && chunkOverlap >= 0
+				? Math.min(chunkOverlap, chunkSize - 1)
+				: 0;
+
+		const charsPerToken = MODEL_CHAR_PER_TOKEN_RATIOS[model] || 4.0;
+		const chunkSizeInChars = Math.floor(chunkSize * charsPerToken);
+		const overlapInChars = Math.floor(validOverlap * charsPerToken);
+
+		const chunks: string[] = [];
+		let start = 0;
+
+		while (start < text.length) {
+			const end = Math.min(start + chunkSizeInChars, text.length);
+			chunks.push(text.slice(start, end));
+
+			if (end >= text.length) {
+				break;
+			}
+
+			// Move to next chunk with overlap
+			start = Math.max(end - overlapInChars, start + 1);
+		}
+
+		return chunks;
+	} catch (error) {
+		// Return text as single chunk on error
+		return text ? [text] : [];
+	}
+}
+
+/**
+ * Estimates the total number of tokens for a list of strings.
+ * Uses tiktoken for normal text but falls back to character-based estimation
+ * for repetitive content or on errors.
+ *
+ * @param list Array of strings to estimate tokens for
+ * @param model The model or encoding name to use for estimation
+ * @returns Total estimated number of tokens across all strings
+ */
+export async function estimateTokensFromStringList(
+	list: string[],
+	model: TiktokenModel,
+): Promise<number> {
+	try {
+		// Validate input
+		if (!Array.isArray(list)) {
+			return 0;
+		}
+
+		const encoder = await encodingForModel(model);
+		const encodedListLength = await Promise.all(
+			list.map(async (text) => {
+				try {
+					// Handle null/undefined text
+					if (!text || typeof text !== 'string') {
+						return 0;
+					}
+
+					// Check for repetitive content
+					if (hasLongSequentialRepeat(text)) {
+						const estimatedTokens = estimateTokensByCharCount(text, model);
+						return estimatedTokens;
+					}
+
+					// Use tiktoken for normal text
+					try {
+						const tokens = encoder.encode(text);
+						return tokens.length;
+					} catch (encodingError) {
+						// Fall back to estimation if tiktoken fails
+						return estimateTokensByCharCount(text, model);
+					}
+				} catch (itemError) {
+					// Return 0 for individual item errors
+					return 0;
+				}
+			}),
+		);
+
+		const totalTokens = encodedListLength.reduce((acc, curr) => acc + curr, 0);
+
+		return totalTokens;
+	} catch (error) {
+		// Return 0 on complete failure
+		return 0;
+	}
+}