How to Prevent Duplicate LLM Calls in the Same n8n Workflow

1// Merge node configuration to prevent re-execution:
2// Mode: Combine
3// Join Mode: Wait for Both
4// This ensures both inputs arrive before the Merge fires,
5// preventing re-execution of upstream nodes.
6
7// Alternative: use 'Append' mode if you need all items from both branches
8// without any re-triggering.

1// Code node — JavaScript
2// Check cache before LLM call
3
4const staticData = $getWorkflowStaticData('global');
5if (!staticData.cache) {
6  staticData.cache = {};
7}
8
9const prompt = $input.first().json.prompt || $input.first().json.message || '';
10const cacheKey = prompt.trim().toLowerCase().substring(0, 200); // Normalize
11const CACHE_TTL = 3600000; // 1 hour in milliseconds
12const now = Date.now();
13
14// Clean expired entries
15for (const [key, entry] of Object.entries(staticData.cache)) {
16  if (now - entry.timestamp > CACHE_TTL) {
17    delete staticData.cache[key];
18  }
19}
20
21// Check cache
22if (staticData.cache[cacheKey] && (now - staticData.cache[cacheKey].timestamp < CACHE_TTL)) {
23  return [{
24    json: {
25      response: staticData.cache[cacheKey].response,
26      cached: true,
27      cacheAge: Math.round((now - staticData.cache[cacheKey].timestamp) / 1000)
28    }
29  }];
30}
31
32// Cache miss — continue to LLM
33return [{
34  json: {
35    prompt,
36    cached: false,
37    cacheKey
38  }
39}];

1// IF node configuration:
2// Condition: {{ $json.cached }} equals true
3// True output → response formatting (skip LLM)
4// False output → LLM node
5
6// Code node after LLM (cache store):
7const staticData = $getWorkflowStaticData('global');
8if (!staticData.cache) staticData.cache = {};
9
10const cacheKey = $('Cache Check').first().json.cacheKey;
11const response = $input.first().json.choices?.[0]?.message?.content
12  || $input.first().json.content?.[0]?.text
13  || $input.first().json.output
14  || '';
15
16staticData.cache[cacheKey] = {
17  response,
18  timestamp: Date.now()
19};
20
21return [{ json: { response, cached: false } }];

1// Code node — JavaScript
2// Deduplicate items before LLM call
3
4const items = $input.all();
5const seen = new Map();
6const uniqueItems = [];
7const duplicateMap = []; // Maps original index to unique index
8
9for (let i = 0; i < items.length; i++) {
10  const prompt = (items[i].json.prompt || items[i].json.message || '').trim();
11  
12  if (seen.has(prompt)) {
13    duplicateMap.push({ originalIndex: i, uniqueIndex: seen.get(prompt) });
14  } else {
15    seen.set(prompt, uniqueItems.length);
16    duplicateMap.push({ originalIndex: i, uniqueIndex: uniqueItems.length });
17    uniqueItems.push(items[i]);
18  }
19}
20
21// Store the map for re-expansion after LLM call
22const staticData = $getWorkflowStaticData('global');
23staticData.duplicateMap = duplicateMap;
24staticData.originalCount = items.length;
25
26// Only unique items go to the LLM
27return uniqueItems;