llm-output-guard 0.1.0

Validate LLM outputs against schemas with automatic retry and JSON extraction.

llm-output-guard

A production-ready Python package that validates LLM outputs against schemas, with automatic JSON extraction and retry logic.

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Features

• Schema-agnostic — works with Pydantic v2/v1, JSON Schema, and plain Python dicts
• Automatic JSON extraction — strips markdown fences and prose wrappers from LLM responses
• Configurable retry — fixed, exponential, or linear back-off with jitter
• Zero hard dependencies — the core works with nothing installed; extras add integrations
• Integrations — LangChain, FastAPI, OpenAI SDK
• CLI — validate JSON files against schemas from the terminal
• Fully typed — py.typed marker, complete mypy --strict coverage

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The Problem It Solves

Getting structured data out of an LLM reliably requires a surprising amount of boilerplate — JSON extraction, schema validation, retry logic with error feedback, and prompt engineering. Here's what that looks like today versus with this package.

Before — ~100 lines of manual plumbing

import json
import re
import time
from typing import Any, Dict, Optional


class LLMJobValidator:
    def __init__(self, model="gpt-4"):
        self.model = model
        self.max_retries = 3

    def generate_job(self, description: str) -> Optional[Dict]:
        prompt = self._build_prompt(description)

        for attempt in range(self.max_retries):
            response = self._call_llm(prompt)

            data = self._extract_json(response)
            if not data:
                continue

            errors = self._validate_job_data(data)
            if not errors:
                return data

            prompt = self._build_retry_prompt(prompt, errors, response)
            time.sleep(1 * (attempt + 1))

        return None

    def _build_prompt(self, description):
        return f"""
        Create a job posting for: {description}

        Output must be JSON with:
        - title (string): Job title
        - salary (integer): Annual salary in USD
        - skills (array of strings): Required skills
        - is_remote (boolean): Whether remote work is possible

        Example:
        {{
            "title": "Software Engineer",
            "salary": 100000,
            "skills": ["Python", "SQL"],
            "is_remote": true
        }}
        """

    def _extract_json(self, text):
        try:
            return json.loads(text)
        except Exception:
            pass

        match = re.search(r'```(?:json)?\s*([\s\S]*?)\s*```', text)
        if match:
            try:
                return json.loads(match.group(1))
            except Exception:
                pass

        match = re.search(r'\{[^{}]*(?:\{[^{}]*\}[^{}]*)*\}', text)
        if match:
            try:
                return json.loads(match.group(0))
            except Exception:
                pass

        return None

    def _validate_job_data(self, data):
        errors = {}

        if 'title' not in data:
            errors['title'] = 'Missing title'
        elif not isinstance(data['title'], str):
            errors['title'] = 'Title must be string'

        if 'salary' not in data:
            errors['salary'] = 'Missing salary'
        elif not isinstance(data['salary'], (int, float)):
            errors['salary'] = 'Salary must be number'
        elif data['salary'] <= 0:
            errors['salary'] = 'Salary must be positive'

        if 'skills' not in data:
            errors['skills'] = 'Missing skills'
        elif not isinstance(data['skills'], list):
            errors['skills'] = 'Skills must be array'

        if 'is_remote' not in data:
            errors['is_remote'] = 'Missing is_remote'
        elif not isinstance(data['is_remote'], bool):
            errors['is_remote'] = 'is_remote must be boolean'

        return errors

    def _build_retry_prompt(self, original_prompt, errors, bad_response):
        error_text = "\n".join([f"- {k}: {v}" for k, v in errors.items()])
        return f"""
        {original_prompt}

        Your previous response had these errors:
        {error_text}

        Bad response: {bad_response}

        Please fix these errors and provide valid JSON.
        """

    def _call_llm(self, prompt):
        # your real API call here
        ...

After — just the schema:

from pydantic import BaseModel, Field
from llm_output_guard.integrations.openai import GuardedOpenAI


class JobPost(BaseModel):
    title: str = Field(description="Job title")
    salary: int = Field(description="Annual salary in USD", gt=0)
    skills: list[str] = Field(description="Required skills")
    is_remote: bool = Field(description="Whether remote work is possible")


# That's ALL you write. Everything else is handled automatically:
# JSON extraction · schema validation · retry with error feedback · prompt engineering
guard = GuardedOpenAI(schema=JobPost, model="gpt-4o-mini", max_retries=3)

result = guard.guard("Create a software engineer job posting.")
if result.success:
    job: JobPost = result.data          # fully typed Pydantic instance
    print(f"{job.title} — ${job.salary:,} — remote: {job.is_remote}")

---

Installation

# Core (no dependencies)
pip install llm-output-guard

# With Pydantic v2
pip install "llm-output-guard[pydantic]"

# With JSON Schema validation
pip install "llm-output-guard[jsonschema]"

# With OpenAI integration
pip install "llm-output-guard[openai]"

# With LangChain integration
pip install "llm-output-guard[langchain]"

# Everything
pip install "llm-output-guard[all]"

---

Quick Start

JSON Schema

from llm_output_guard import Validator

schema = {
    "type": "object",
    "properties": {
        "name": {"type": "string"},
        "age":  {"type": "integer"},
    },
    "required": ["name", "age"],
}

def my_llm(prompt: str) -> str:
    return '{"name": "Alice", "age": 30}'

validator = Validator(schema=schema, llm_callable=my_llm, max_retries=2)
result = validator.guard("Who is Alice?")

print(result.success)   # True
print(result.data)      # {'name': 'Alice', 'age': 30}
print(result.attempts)  # 1

Pydantic Model

from pydantic import BaseModel
from llm_output_guard import Validator

class Person(BaseModel):
    name: str
    age: int

validator = Validator(schema=Person, llm_callable=my_llm)
result = validator.guard("Describe Alice.")

person: Person = result.data   # fully typed Pydantic instance
print(person.name)             # Alice

Validate an Existing String

result = Validator(schema=Person).validate_output('{"name": "Bob", "age": 25}')
print(result.success)  # True

Raise on Failure

from llm_output_guard.core.exceptions import MaxRetriesExceededError

validator = Validator(schema=schema, llm_callable=my_llm, raise_on_failure=True)
try:
    result = validator.guard("…")
except MaxRetriesExceededError as e:
    print(f"Failed after {e.attempts} attempts.")

---

Integrations

OpenAI

from llm_output_guard.integrations.openai import GuardedOpenAI

guard = GuardedOpenAI(schema=Person, model="gpt-4o-mini", max_retries=3)
result = guard.guard("Tell me about Alice.")

LangChain

from langchain_openai import ChatOpenAI
from llm_output_guard.integrations.langchain import GuardedLLM

guarded = GuardedLLM(llm=ChatOpenAI(), schema=Person, max_retries=3)
result = guarded.invoke("Tell me about Alice.")

FastAPI

from fastapi import FastAPI
from llm_output_guard import Validator

app = FastAPI()
validator = Validator(schema=Person, llm_callable=my_llm)

@app.post("/person")
async def get_person(prompt: str):
    result = validator.guard(prompt)
    result.raise_for_status()
    return result.data

---

CLI

# Validate a JSON file against a JSON Schema file
llm-guard validate output.json schema.json

# Print the JSON Schema of a Pydantic model
llm-guard schema mypackage.models.Person

---

GuardResult

Every .guard() or .validate_output() call returns a GuardResult:

Attribute	Type	Description
success	bool	True when validation passed
data	Any	Validated output (Pydantic instance or dict)
raw_output	str	Raw string from the LLM
errors	list[dict]	Validation error dicts (empty on success)
attempts	int	Number of LLM calls made
schema_type	str	"pydantic" / "json_schema" / "dict"

---

Retry Strategies

Strategy	Description
fixed	Constant delay between retries
exponential	Exponential back-off with optional jitter (default)
linear	Linearly increasing delay

Validator(schema=schema, llm_callable=llm, retry_strategy="exponential", retry_delay=1.0)

---

Development

git clone https://github.com/Ujjwal-Bajpayee/llm-output-guard
cd llm-output-guard
pip install -e ".[dev]"
pytest --cov=llm_output_guard

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Contributing

See CONTRIBUTING.md.

License

MIT