How to Get Consistent JSON Outputs from a Language Model in n8n

1// Example JSON Schema for the Structured Output Parser
2// Set this in the 'JSON Schema' field of the Structured Output Parser node
3{
4  "type": "object",
5  "properties": {
6    "summary": {
7      "type": "string",
8      "description": "A 2-3 sentence summary of the content"
9    },
10    "sentiment": {
11      "type": "string",
12      "enum": ["positive", "negative", "neutral"],
13      "description": "Overall sentiment of the content"
14    },
15    "key_topics": {
16      "type": "array",
17      "items": { "type": "string" },
18      "description": "List of main topics discussed"
19    },
20    "confidence": {
21      "type": "number",
22      "minimum": 0,
23      "maximum": 1,
24      "description": "Confidence score between 0 and 1"
25    }
26  },
27  "required": ["summary", "sentiment", "key_topics", "confidence"]
28}

1// When using the HTTP Request node to call OpenAI directly,
2// add response_format to the request body:
3{
4  "model": "gpt-4o",
5  "messages": [
6    {
7      "role": "system",
8      "content": "You are a data extraction assistant. Return your analysis as JSON."
9    },
10    {
11      "role": "user",
12      "content": "Analyze this text: {{ $json.input_text }}"
13    }
14  ],
15  "response_format": { "type": "json_object" },
16  "temperature": 0
17}

1// System prompt template for consistent JSON output
2const systemPrompt = `You are a structured data extraction assistant.
3
4OUTPUT FORMAT:
5- Return ONLY a JSON object matching the schema below
6- Do NOT wrap the JSON in markdown code fences
7- Do NOT add any text before or after the JSON
8- Do NOT include comments in the JSON
9- Use double quotes for all strings
10- Ensure all required fields are present
11
12SCHEMA:
13{
14  "summary": "string (2-3 sentences)",
15  "sentiment": "positive | negative | neutral",
16  "key_topics": ["string", ...],
17  "confidence": number (0 to 1)
18}
19
20EXAMPLE OUTPUT:
21{"summary": "The article discusses...", "sentiment": "positive", "key_topics": ["AI", "automation"], "confidence": 0.92}
22
23Respond with ONLY the JSON object. No other text.`;

1const rawOutput = $json.output || $json.text || '';
2
3function extractAndValidate(text, requiredFields) {
4  let cleaned = text.trim();
5
6  // Strip markdown fences
7  const fenceMatch = cleaned.match(/```(?:json)?\s*\n?([\s\S]*?)\n?\s*```/);
8  if (fenceMatch) cleaned = fenceMatch[1].trim();
9
10  // Find JSON boundaries
11  const start = cleaned.indexOf('{');
12  const end = cleaned.lastIndexOf('}');
13  if (start === -1 || end === -1) return null;
14
15  cleaned = cleaned.substring(start, end + 1);
16
17  try {
18    const parsed = JSON.parse(cleaned);
19    // Validate required fields
20    const missing = requiredFields.filter(f => !(f in parsed));
21    if (missing.length > 0) {
22      return { data: parsed, valid: false, missing_fields: missing };
23    }
24    return { data: parsed, valid: true };
25  } catch (e) {
26    return null;
27  }
28}
29
30const required = ['summary', 'sentiment', 'key_topics', 'confidence'];
31const result = extractAndValidate(rawOutput, required);
32
33if (result && result.valid) {
34  return [{ json: result.data }];
35} else if (result && !result.valid) {
36  return [{ json: { ...result.data, _validation_warning: 'Missing fields: ' + result.missing_fields.join(', ') } }];
37} else {
38  return [{ json: { _parse_error: true, raw_text: rawOutput } }];
39}