Why does OpenAI JSON mode require the word "JSON" in the prompt?

When you set response_format to json_object, the API refuses the request unless the word "JSON" appears somewhere in the messages. This is a guardrail that prevents the model from being asked for guaranteed JSON without any instruction to actually produce it. This tool flags that requirement before you hit the error in production.

Is JSON mode the same as structured outputs?

No. JSON mode (json_object) only guarantees the response is syntactically valid JSON — it does not enforce a schema. Structured Outputs (json_schema with strict mode) additionally guarantees the response matches a JSON Schema you supply. This tool helps with both: it readies your prompt for JSON mode and validates the shape against your schema for the structured case.

How does the schema validation work?

The schema is a JSON sketch. Objects and arrays are matched structurally and recursively; leaf values are JSON type names ("string", "number", "boolean", "null"). The tool reports every missing required key and every type mismatch with its path, so you can see exactly where the response diverges.

Does my data get uploaded anywhere?

No. The prompt check, JSON parsing, and schema validation all run locally in your browser using JavaScript. Nothing is sent to a server, which makes it safe to paste real responses.

My valid JSON is being rejected — why?

Common causes are trailing commas, single quotes instead of double quotes, comments, or markdown code fences wrapping the JSON. Strict JSON does not allow any of these. Paste only the raw JSON object, and the parser will tell you the exact position of the syntax error.

LLM JSON Mode Converter

Getting an LLM to return reliable JSON is two problems, not one: your prompt has to satisfy the provider’s JSON-mode requirements, and the output has to match the shape your code expects. This tool checks both — without sending anything anywhere.

How it works

OpenAI’s response_format: { type: "json_object" } mode has a quiet requirement: the word JSON must appear somewhere in the conversation, or the API returns an error. The first check scans your system prompt for an explicit JSON instruction and, if it is missing, gives you a ready-to-paste rewrite that appends a clear “respond with a single valid JSON object only” line.

The second check parses your sample response and walks it against your schema sketch. The schema is just JSON where the structure is matched recursively and each leaf is a type name — "string", "number", "boolean", "null". Missing required keys and type mismatches are reported with their full path (for example address.zip: expected string, got number) so you know exactly what to fix.

When to use which mode

Plain JSON mode only guarantees syntactic validity — you still have to validate the shape yourself, which is what the schema check here mirrors. If your provider supports Structured Outputs (json_schema with strict: true), the shape is guaranteed by the API and you can trust the keys and types without runtime validation. Use this tool to design and sanity-check the schema before you wire it into either path.

Tips

Keep the schema minimal — only the keys your code actually reads. Extra keys in the response are fine and are ignored by the validator.
For arrays, supply a single example element; every item in the response is validated against it.
If you are on a model without native structured outputs, keep the validation in your own code too — JSON mode can still return an empty object or omit optional keys.