Outlines Has a Free API: Guarantee Valid JSON Output From Any LLM

#ai #programming #llm #python

LLMs sometimes return invalid JSON. Outlines makes it physically impossible — by constraining token generation to match your schema.

What Is Outlines?

Outlines is a Python library that constrains LLM text generation using formal grammars. Unlike Instructor (which validates after generation), Outlines constrains DURING generation — the LLM can only produce valid tokens.

import outlines

model = outlines.models.transformers("mistralai/Mistral-7B-v0.3")

# JSON schema constraint
schema = '''{
  "type": "object",
  "properties": {
    "name": {"type": "string"},
    "age": {"type": "integer"},
    "city": {"type": "string"}
  },
  "required": ["name", "age", "city"]
}'''

generator = outlines.generate.json(model, schema)
result = generator("Extract info: John is 30 and lives in NYC")
# GUARANTEED valid JSON matching schema
# {"name": "John", "age": 30, "city": "NYC"}

Regex Constraints

# Phone number — only valid phone patterns
phone_gen = outlines.generate.regex(model, r"\+1-\d{3}-\d{3}-\d{4}")
phone = phone_gen("Give me a US phone number")
# "+1-555-123-4567" — ALWAYS matches regex

# Email
email_gen = outlines.generate.regex(model, r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}")
email = email_gen("Generate an email for John Doe")
# "john.doe@example.com"

# Date
date_gen = outlines.generate.regex(model, r"\d{4}-\d{2}-\d{2}")

Enum/Choice

from enum import Enum

class Sentiment(str, Enum):
    POSITIVE = "positive"
    NEGATIVE = "negative"
    NEUTRAL = "neutral"

sentiment_gen = outlines.generate.choice(model, list(Sentiment))
result = sentiment_gen("The movie was absolutely terrible")
# "negative" — can ONLY be one of the three options

How It Works

Outlines builds a finite-state machine from your schema/regex. At each generation step, it masks all tokens that would lead to invalid output. The LLM can only choose valid next tokens.

This means: 100% valid output. Not 99%. Not "retry until valid." One hundred percent. Every time.

Outlines vs Instructor

Feature	Instructor	Outlines
Method	Validate after generation	Constrain during generation
Guarantee	Retry on failure	100% valid by construction
Speed	May need retries	One pass
Models	Cloud APIs	Local models (transformers)