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Structured output depends on the model honoring response_format. The most common surprise is that a model silently ignores it. See Structured Output for the full guide.

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I get plain text instead of JSON (no error)

Not every model enforces response_format. On a model that doesn’t support it, the request still succeeds and returns ordinary text — it does not error.
  • Switch to a model with first-class support, e.g. OpenAI models or google/gemini-2.5-flash.
  • Always parse defensively and handle the case where the content isn’t valid JSON.

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422 — malformed response_format

The response_format object is invalid. Check:
  • type is one of "text", "json_object", "json_schema".
  • For json_schema, the json_schema object has both a name and a schema.
  • The schema is valid JSON Schema (no trailing commas, correct types).

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Valid JSON but the wrong keys

json_object only guarantees syntactically valid JSON — not which keys appear. The structure follows your prompt, not a contract.
  • Describe the exact keys you expect in the prompt, or
  • Use json_schema for a guaranteed shape. Set additionalProperties: false to forbid extra fields, and list every required field in required.

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JSON is truncated / won’t parse

If the output is cut off mid-object, max_tokens was too low — the model ran out of room before closing the JSON.
  • Raise max_tokens.
  • Check finish_reason: "length" means truncated, "stop" means complete.

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content is a string, not an object

The JSON is returned as a string in choices[0].message.content. You must parse it yourself:

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Output isn’t deterministic across runs

Sampling introduces variation even with a schema.
  • Set temperature: 0 for the most deterministic, schema-faithful output.
  • Note that for Gemini models, response_format is translated server-side to Gemini’s native responseMimeType / responseSchema, so enforcement happens on the provider — behavior can differ subtly from OpenAI.

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Still stuck?

See the Mesh API error reference or email contact@meshapi.ai.