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Guarantee valid JSON/XML/code structure during generation, use Pydantic models for type-safe outputs, support local models (Transformers, vLLM), and maximize inference speed with Outlines - dottxt.ai's structured generation library
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Guarantee valid JSON/XML/code structure during generation, use Pydantic models for type-safe outputs, support local models (Transformers, vLLM), and maximize inference speed with Outlines - dottxt.ai's structured generation library 📦 Ships with claude-code-templates
</> SKILL.md
prompt-engineering-outlines.SKILL.md
---
name : outlines
description : Guarantee valid JSON/XML/code structure during generation, use Pydantic models for type-safe outputs, support local models (Transformers, vLLM), and maximize inference speed with Outlines - dottxt.ai's structured generation library
version : 1.0.0
author : Orchestra Research
license : MIT
tags : [Prompt Engineering, Outlines, Structured Generation, JSON Schema, Pydantic, Local Models, Grammar-Based Generation, vLLM, Transformers, Type Safety]
dependencies : [outlines, transformers, vllm, pydantic]
---
# Outlines: Structured Text Generation
## When to Use This Skill
Use Outlines when you need to:
- **Guarantee valid JSON/XML/code** structure during generation
- **Use Pydantic models** for type-safe outputs
- **Support local models** (Transformers, llama.cpp, vLLM)
- **Maximize inference speed** with zero-overhead structured generation
- **Generate against JSON schemas** automatically
- **Control token sampling** at the grammar level
**GitHub Stars**: 8,000+ | **From**: dottxt.ai (formerly .txt)
## Installation
```bash
# Base installation
pip install outlines
Flowy AI Flows that just work. Hand-picked Claude Code plugins that fire the right skill as you prompt.
# With specific backends
pip install outlines transformers # Hugging Face models
pip install outlines llama-cpp-python # llama.cpp
pip install outlines vllm # vLLM for high-throughput
```
## Quick Start
### Basic Example: Classification
```python
import outlines
from typing import Literal
# Load model
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
# Generate with type constraint
prompt = "Sentiment of 'This product is amazing!': "
generator = outlines.generate.choice(model, ["positive", "negative", "neutral"])
sentiment = generator(prompt)
print(sentiment) # "positive" (guaranteed one of these)
```
### With Pydantic Models
```python
from pydantic import BaseModel
import outlines
class User(BaseModel):
name: str
age: int
email: str
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
# Generate structured output
prompt = "Extract user: John Doe, 30 years old, john@example.com"
generator = outlines.generate.json(model, User)
user = generator(prompt)
print(user.name) # "John Doe"
print(user.age) # 30
print(user.email) # "john@example.com"
```
## Core Concepts
### 1. Constrained Token Sampling
Outlines uses Finite State Machines (FSM) to constrain token generation at the logit level.
**How it works:**
1. Convert schema (JSON/Pydantic/regex) to context-free grammar (CFG)
2. Transform CFG into Finite State Machine (FSM)
3. Filter invalid tokens at each step during generation
4. Fast-forward when only one valid token exists
**Benefits:**
- **Zero overhead**: Filtering happens at token level
- **Speed improvement**: Fast-forward through deterministic paths
- **Guaranteed validity**: Invalid outputs impossible
```python
import outlines
# Pydantic model -> JSON schema -> CFG -> FSM
class Person(BaseModel):
name: str
age: int
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
# Behind the scenes:
# 1. Person -> JSON schema
# 2. JSON schema -> CFG
# 3. CFG -> FSM
# 4. FSM filters tokens during generation
generator = outlines.generate.json(model, Person)
result = generator("Generate person: Alice, 25")
```
### 2. Structured Generators
Outlines provides specialized generators for different output types.
#### Choice Generator
```python
# Multiple choice selection
generator = outlines.generate.choice(
model,
["positive", "negative", "neutral"]
)
sentiment = generator("Review: This is great!")
# Result: One of the three choices
```
#### JSON Generator
```python
from pydantic import BaseModel
class Product(BaseModel):
name: str
price: float
in_stock: bool
# Generate valid JSON matching schema
generator = outlines.generate.json(model, Product)
product = generator("Extract: iPhone 15, $999, available")
# Guaranteed valid Product instance
print(type(product)) # <class '__main__.Product'>
```
#### Regex Generator
```python
# Generate text matching regex
generator = outlines.generate.regex(
model,
r"[0-9]{3}-[0-9]{3}-[0-9]{4}" # Phone number pattern
)
phone = generator("Generate phone number:")
# Result: "555-123-4567" (guaranteed to match pattern)
```
#### Integer/Float Generators
```python
# Generate specific numeric types
int_generator = outlines.generate.integer(model)
age = int_generator("Person's age:") # Guaranteed integer
float_generator = outlines.generate.float(model)
price = float_generator("Product price:") # Guaranteed float
```
### 3. Model Backends
Outlines supports multiple local and API-based backends.
#### Transformers (Hugging Face)
```python
import outlines
# Load from Hugging Face
model = outlines.models.transformers(
"microsoft/Phi-3-mini-4k-instruct",
device="cuda" # Or "cpu"
)
# Use with any generator
generator = outlines.generate.json(model, YourModel)
```
#### llama.cpp
```python
# Load GGUF model
model = outlines.models.llamacpp(
"./models/llama-3.1-8b-instruct.Q4_K_M.gguf",
n_gpu_layers=35
)
generator = outlines.generate.json(model, YourModel)
```
#### vLLM (High Throughput)
```python
# For production deployments
model = outlines.models.vllm(
"meta-llama/Llama-3.1-8B-Instruct",
tensor_parallel_size=2 # Multi-GPU
)
generator = outlines.generate.json(model, YourModel)
```
#### OpenAI (Limited Support)
```python
# Basic OpenAI support
model = outlines.models.openai(
"gpt-4o-mini",
api_key="your-api-key"
)
# Note: Some features limited with API models
generator = outlines.generate.json(model, YourModel)
```
### 4. Pydantic Integration
Outlines has first-class Pydantic support with automatic schema translation.
#### Basic Models
```python
from pydantic import BaseModel, Field
class Article(BaseModel):
title: str = Field(description="Article title")
author: str = Field(description="Author name")
word_count: int = Field(description="Number of words", gt=0)
tags: list[str] = Field(description="List of tags")
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, Article)
article = generator("Generate article about AI")
print(article.title)
print(article.word_count) # Guaranteed > 0
```
#### Nested Models
```python
class Address(BaseModel):
street: str
city: str
country: str
class Person(BaseModel):
name: str
age: int
address: Address # Nested model
generator = outlines.generate.json(model, Person)
person = generator("Generate person in New York")
print(person.address.city) # "New York"
```
#### Enums and Literals
```python
from enum import Enum
from typing import Literal
class Status(str, Enum):
PENDING = "pending"
APPROVED = "approved"
REJECTED = "rejected"
class Application(BaseModel):
applicant: str
status: Status # Must be one of enum values
priority: Literal["low", "medium", "high"] # Must be one of literals
generator = outlines.generate.json(model, Application)
app = generator("Generate application")
print(app.status) # Status.PENDING (or APPROVED/REJECTED)
```
## Common Patterns
### Pattern 1: Data Extraction
```python
from pydantic import BaseModel
import outlines
class CompanyInfo(BaseModel):
name: str
founded_year: int
industry: str
employees: int
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, CompanyInfo)
text = """
Apple Inc. was founded in 1976 in the technology industry.
The company employs approximately 164,000 people worldwide.
"""
prompt = f"Extract company information:\n{text}\n\nCompany:"
company = generator(prompt)
print(f"Name: {company.name}")
print(f"Founded: {company.founded_year}")
print(f"Industry: {company.industry}")
print(f"Employees: {company.employees}")
```
### Pattern 2: Classification
```python
from typing import Literal
import outlines
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
# Binary classification
generator = outlines.generate.choice(model, ["spam", "not_spam"])
result = generator("Email: Buy now! 50% off!")
# Multi-class classification
categories = ["technology", "business", "sports", "entertainment"]
category_gen = outlines.generate.choice(model, categories)
category = category_gen("Article: Apple announces new iPhone...")
# With confidence
class Classification(BaseModel):
label: Literal["positive", "negative", "neutral"]
confidence: float
classifier = outlines.generate.json(model, Classification)
result = classifier("Review: This product is okay, nothing special")
```
### Pattern 3: Structured Forms
```python
class UserProfile(BaseModel):
full_name: str
age: int
email: str
phone: str
country: str
interests: list[str]
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, UserProfile)
prompt = """
Extract user profile from:
Name: Alice Johnson
Age: 28
Email: alice@example.com
Phone: 555-0123
Country: USA
Interests: hiking, photography, cooking
"""
profile = generator(prompt)
print(profile.full_name)
print(profile.interests) # ["hiking", "photography", "cooking"]
```
### Pattern 4: Multi-Entity Extraction
```python
class Entity(BaseModel):
name: str
type: Literal["PERSON", "ORGANIZATION", "LOCATION"]
class DocumentEntities(BaseModel):
entities: list[Entity]
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, DocumentEntities)
text = "Tim Cook met with Satya Nadella at Microsoft headquarters in Redmond."
prompt = f"Extract entities from: {text}"
result = generator(prompt)
for entity in result.entities:
print(f"{entity.name} ({entity.type})")
```
### Pattern 5: Code Generation
```python
class PythonFunction(BaseModel):
function_name: str
parameters: list[str]
docstring: str
body: str
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, PythonFunction)
prompt = "Generate a Python function to calculate factorial"
func = generator(prompt)
print(f"def {func.function_name}({', '.join(func.parameters)}):")
print(f' """{func.docstring}"""')
print(f" {func.body}")
```
### Pattern 6: Batch Processing
```python
def batch_extract(texts: list[str], schema: type[BaseModel]):
"""Extract structured data from multiple texts."""
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
generator = outlines.generate.json(model, schema)
results = []
for text in texts:
result = generator(f"Extract from: {text}")
results.append(result)
return results
class Person(BaseModel):
name: str
age: int
texts = [
"John is 30 years old",
"Alice is 25 years old",
"Bob is 40 years old"
]
people = batch_extract(texts, Person)
for person in people:
print(f"{person.name}: {person.age}")
```
## Backend Configuration
### Transformers
```python
import outlines
# Basic usage
model = outlines.models.transformers("microsoft/Phi-3-mini-4k-instruct")
# GPU configuration
model = outlines.models.transformers(
"microsoft/Phi-3-mini-4k-instruct",
device="cuda",
model_kwargs={"torch_dtype": "float16"}
)
# Popular models
model = outlines.models.transformers("meta-llama/Llama-3.1-8B-Instruct")
model = outlines.models.transformers("mistralai/Mistral-7B-Instruct-v0.3")
model = outlines.models.transformers("Qwen/Qwen2.5-7B-Instruct")
```
### llama.cpp
```python
# Load GGUF model
model = outlines.models.llamacpp(
"./models/llama-3.1-8b.Q4_K_M.gguf",
n_ctx=4096, # Context window
n_gpu_layers=35, # GPU layers
n_threads=8 # CPU threads
)
# Full GPU offload
model = outlines.models.llamacpp(
"./models/model.gguf",
n_gpu_layers=-1 # All layers on GPU
)
```
### vLLM (Production)
```python
# Single GPU
model = outlines.models.vllm("meta-llama/Llama-3.1-8B-Instruct")
# Multi-GPU
model = outlines.models.vllm(
"meta-llama/Llama-3.1-70B-Instruct",
tensor_parallel_size=4 # 4 GPUs
)
# With quantization
model = outlines.models.vllm(
"meta-llama/Llama-3.1-8B-Instruct",
quantization="awq" # Or "gptq"
)
```
## Best Practices
### 1. Use Specific Types
```python
# ✅ Good: Specific types
class Product(BaseModel):
name: str
price: float # Not str
quantity: int # Not str
in_stock: bool # Not str
# ❌ Bad: Everything as string
class Product(BaseModel):
name: str
price: str # Should be float
quantity: str # Should be int
```
### 2. Add Constraints
```python
from pydantic import Field
# ✅ Good: With constraints
class User(BaseModel):
name: str = Field(min_length=1, max_length=100)
age: int = Field(ge=0, le=120)
email: str = Field(pattern=r"^[\w\.-]+@[\w\.-]+\.\w+$")
# ❌ Bad: No constraints
class User(BaseModel):
name: str
age: int
email: str
```
### 3. Use Enums for Categories
```python
# ✅ Good: Enum for fixed set
class Priority(str, Enum):
LOW = "low"
MEDIUM = "medium"
HIGH = "high"
class Task(BaseModel):
title: str
priority: Priority
# ❌ Bad: Free-form string
class Task(BaseModel):
title: str
priority: str # Can be anything
```
### 4. Provide Context in Prompts
```python
# ✅ Good: Clear context
prompt = """
Extract product information from the following text.
Text: iPhone 15 Pro costs $999 and is currently in stock.
Product:
"""
# ❌ Bad: Minimal context
prompt = "iPhone 15 Pro costs $999 and is currently in stock."
```
### 5. Handle Optional Fields
```python
from typing import Optional
# ✅ Good: Optional fields for incomplete data
class Article(BaseModel):
title: str # Required
author: Optional[str] = None # Optional
date: Optional[str] = None # Optional
tags: list[str] = [] # Default empty list
# Can succeed even if author/date missing
```
## Comparison to Alternatives
| Feature | Outlines | Instructor | Guidance | LMQL |
|---------|----------|------------|----------|------|
| Pydantic Support | ✅ Native | ✅ Native | ❌ No | ❌ No |
| JSON Schema | ✅ Yes | ✅ Yes | ⚠️ Limited | ✅ Yes |
| Regex Constraints | ✅ Yes | ❌ No | ✅ Yes | ✅ Yes |
| Local Models | ✅ Full | ⚠️ Limited | ✅ Full | ✅ Full |
| API Models | ⚠️ Limited | ✅ Full | ✅ Full | ✅ Full |
| Zero Overhead | ✅ Yes | ❌ No | ⚠️ Partial | ✅ Yes |
| Automatic Retrying | ❌ No | ✅ Yes | ❌ No | ❌ No |
| Learning Curve | Low | Low | Low | High |
**When to choose Outlines:**
- Using local models (Transformers, llama.cpp, vLLM)
- Need maximum inference speed
- Want Pydantic model support
- Require zero-overhead structured generation
- Control token sampling process
**When to choose alternatives:**
- Instructor: Need API models with automatic retrying
- Guidance: Need token healing and complex workflows
- LMQL: Prefer declarative query syntax
## Performance Characteristics
**Speed:**
- **Zero overhead**: Structured generation as fast as unconstrained
- **Fast-forward optimization**: Skips deterministic tokens
- **1.2-2x faster** than post-generation validation approaches
**Memory:**
- FSM compiled once per schema (cached)
- Minimal runtime overhead
- Efficient with vLLM for high throughput
**Accuracy:**
- **100% valid outputs** (guaranteed by FSM)
- No retry loops needed
- Deterministic token filtering
## Resources
- **Documentation**: https://outlines-dev.github.io/outlines
- **GitHub**: https://github.com/outlines-dev/outlines (8k+ stars)
- **Discord**: https://discord.gg/R9DSu34mGd
- **Blog**: https://blog.dottxt.co
## See Also
- `references/json_generation.md` - Comprehensive JSON and Pydantic patterns
- `references/backends.md` - Backend-specific configuration
- `references/examples.md` - Production-ready examples
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