# Structured Output and Validating Replies — Claude API / OpenAI API Basics

Source: https://www.geekswithgeeks.com/en/llm-apis/m-structured

> Get machine-readable output and check it in code.

## Ask, then verify

When code consumes the reply, ask for JSON with exact keys and allowed values, give an example, and **validate** what comes back. Both providers offer features that make valid output much more likely: **tool/function calling** with a schema, and **structured output / JSON-schema modes** on supported models. Even then check types, ranges and allowed values, and handle failures: models sometimes wrap JSON in prose or code fences, get truncated at `max_tokens`, or choose a value outside your list. Extract the JSON object, parse it with `json.loads`, validate against your rules, and **retry with the error message** or fall back safely. Never pass model output to `eval`, a shell or SQL without strict checks.

## Extracting and validating JSON, run

I ran this plain-Python (standard library only) example. Clean JSON passes; JSON inside prose and a code fence is extracted and passes; a category outside the allowed set is rejected; a reply with no JSON is reported. These strings stand in for model replies.

```python
import json

def parse_json_reply(text):
    """Models sometimes wrap JSON in prose or code fences; extract and validate it."""
    start, end = text.find("{"), text.rfind("}")
    if start == -1 or end == -1:
        return None, "no JSON object found"
    try:
        data = json.loads(text[start:end + 1])
    except json.JSONDecodeError as e:
        return None, f"invalid JSON: {e.msg}"
    if data.get("category") not in {"billing", "technical", "other"}:
        return None, "category not allowed"
    return data, None

for reply in ['{"category": "billing"}',
              'Sure! Here you go:\n```json\n{"category": "technical"}\n```',
              '{"category": "refunds"}',
              'I cannot help with that.']:
    print(parse_json_reply(reply))

```

Output:

```
({'category': 'billing'}, None)
({'category': 'technical'}, None)
(None, 'category not allowed')
(None, 'no JSON object found')
```

## Show one example of the exact JSON

A single example output in the prompt usually fixes key names and formatting more reliably than a long description.

**Quiz:** Why validate JSON even when using a structured-output feature?

- [ ] It is forbidden to trust models
- [ ] JSON cannot be parsed
- [x] Values can still be wrong or outside your business rules, and failures happen
- [ ] It makes replies shorter

*Answer:* Values can still be wrong or outside your business rules, and failures happen. Schema validity is not the same as correctness for your rules.