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When you include response_format in your body, the model guarantees the response matches the JSON schema you sent. Useful for deterministic parsing without retries. If the selected model doesn’t support it, Geek Hub rejects the request before leaving the gateway and returns the list of compatible models.

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Example: invoice extraction

curl -X POST https://api.geekhub.mx/v1/chat/completions \
  -H "Authorization: Bearer ghub_sk_live_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-4.1-mini",
    "messages": [
      {"role":"system","content":"You extract data from invoices."},
      {"role":"user","content":"Invoice #A123 from Geek Vibes for $12,345.67 dated June 12, 2026."}
    ],
    "response_format": {
      "type": "json_schema",
      "json_schema": {
        "name": "invoice",
        "strict": true,
        "schema": {
          "type": "object",
          "properties": {
            "number": { "type": "string" },
            "issuer": { "type": "string" },
            "total_usd": { "type": "number" },
            "date_iso": { "type": "string", "format": "date" }
          },
          "required": ["number", "issuer", "total_usd", "date_iso"],
          "additionalProperties": false
        }
      }
    }
  }'
content arrives as a parseable JSON string. JSON.parse(content) gives you the typed object directly.

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Simple mode

"response_format": { "type": "json_object" }
OpenAI requires the prompt to contain the word “json” when using json_object. Not a Geek Hub requirement — provider-specific.

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Compatible models

12 of 17 chat models support structured outputs.
FamilyModels
Anthropicclaude-opus-4-8, claude-sonnet-4-6, claude-haiku-4-5
Googlegemini-2.5-pro, gemini-2.5-flash
OpenAIgpt-5, gpt-4.1, gpt-4.1-mini, o4-mini
xAIgrok-4, grok-3, grok-3-mini

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Rejection

HTTP/1.1 422 Unprocessable Entity

{
  "error": {
    "type": "structured_outputs_not_supported",
    "message": "Model \"deepseek/deepseek-chat\" does not support structured outputs.",
    "model": "deepseek/deepseek-chat",
    "compatible_models": [
      "anthropic/claude-opus-4-8",
      "openai/gpt-4.1-mini",
      "..."
    ]
  }
}
Combine with Model Fallbacks: pass a list of candidates and unsupported ones get skipped instead of failing.