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In the middle of the desert you can say anything you want

04 Feb 2026

Simple test to see if VLLM supports structured output

curl 127.0.0.1:8000/v1/chat/completions -s \
  -H "Content-Type: application/json" \
  -d '{
    "model": "/data_pvc2/cache/models--CohereLabs--c4ai-command-r-08-2024/snapshots/96b61ca90ba9a25548d3d4bf68e1938b13506852/",
    "messages": [
      {
        "role": "user",
        "content": "Generate a short analysis in the given JSON schema."
      }
    ],
    "response_format": {
      "type": "json_schema",
      "json_schema": {
        "name": "analysis_schema",
        "schema": {
          "type": "object",
          "properties": {
            "summary": { "type": "string" },
            "importance_score": { "type": "number" },
            "key_points": {
              "type": "array",
              "items": { "type": "string" }
            }
          },
          "required": ["summary", "importance_score", "key_points"],
          "additionalProperties": false
        },
        "strict": true
      }
    }
  }' | jq . > /tmp/out

Sample otuput:

{
  "id": "chatcmpl-9c3932339c7d4e1bace32c24ef6dbada",
  "object": "chat.completion",
  "created": 1770201387,
  "model": "/data_pvc2/cache/models--CohereLabs--c4ai-command-r-08-2024/snapshots/96b61ca90ba9a25548d3d4bf68e1938b13506852/",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "reasoning_content": null,
        "content": "{\n    \"summary\": \"This analysis focuses on the impact of marketing strategies on customer engagement and sales performance for a hypothetical e-commerce company.\",\n    \"importance_score\": 0.85,\n    \"key_points\": [\n        \"Implemented an email marketing campaign with personalized recommendations based on past purchases: Increased open rates by 15% and contributed to a 12% rise in average order value.\",\n        \"Launched an influencer marketing program on social media: Achieved a 20% growth in brand awareness and drove a significant rise in website traffic from social platforms.\",\n        \"Optimised product listings through enhanced SEO techniques: Boosted organic visibility, resulting in a 25% increase in search-driven purchases.\",\n        \"Implemented an automated email nurturing funnel for abandoned carts: Improved cart recovery rate by 10% and increased overall conversions.\",\n        \"Analyzed customer reviews for product feedback and sentiment: Used insights to refine product strategies and enhance customer satisfaction.\"\n    ]\n}",
        "tool_calls": []
      },
      "logprobs": null,
      "finish_reason": "stop",
      "stop_reason": null
    }
  ],
  "usage": {
    "prompt_tokens": 16,
    "total_tokens": 219,
    "completion_tokens": 203,
    "prompt_tokens_details": null
  },
  "prompt_logprobs": null
}

To pretty-print the content it returned:

jq -r .choices[0].message.content /tmp/out | jq .

Simpler OpenAI-compatible JSON mode test

curl 127.0.0.1:8000/v1/chat/completions -s \
  -H "Content-Type: application/json" \
  -d '{
    "model": "/data_pvc2/cache/models--CohereLabs--c4ai-command-r-08-2024/snapshots/96b61ca90ba9a25548d3d4bf68e1938b13506852/",
    "messages": [
      {
        "role": "user",
        "content": "Return a JSON object with keys summary, importance_score (number), key_points (string array)."
      }
    ],
    "response_format": { "type": "json_object" }
  }'

Bonus: literature

Nel mezzo del deserto posso dire tutto quello che voglio.
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