---
name: outlines
description: Generates guaranteed-valid structured output from LLMs with Outlines (dottxt.ai), constraining token sampling via finite state machines for JSON schemas, Pydantic models, regex, choice lists, and integer/float types, on Transformers, llama.cpp, vLLM, and limited OpenAI backends. Use when you need output that always parses as valid JSON or matches a regex. Use when extracting typed data into Pydantic models from a local model. Use when classifying text into a fixed set of categories. Use when running batch structured generation on vLLM or Transformers. Use when controlling token sampling at the grammar level. Not for API models needing automatic retries (use Instructor).
license: MIT
metadata:
  version: 1.0.0
  category: llm-applications
  maintainer: Kalaris Labs
  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


## Installation

```bash
# Base installation
pip install outlines

# 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

Details, code examples and parameter tables: [references/common-patterns.md](references/common-patterns.md). Read it when this step applies.

## Backend Configuration

Details, code examples and parameter tables: [references/backend-configuration.md](references/backend-configuration.md). Read it when this step applies.

## 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
- **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

## Agent operating procedure

1. **Check the environment.** Confirm the framework version, model provider, API keys and rate limits.
2. **Pin down the inputs.** Confirm formats, identifiers and parameters from the data or the user. Ask rather than guess any value that changes the result.
3. **Run a small version first.** Test a single call or chain with a known input and inspect raw outputs.
4. **Execute the full task** using the instructions and references above.
5. **Validate the result.** Evaluate on a small labeled set; check structured outputs against their schema; log prompts and responses.
6. **Report.** State what was run (versions, commands, parameters), what was checked, and what is still uncertain.

| If this happens | Do this |
|---|---|
| Outputs do not match the expected schema | Add validation and retries, tighten the schema, or simplify the prompt. |
| A function, flag or endpoint in these instructions is missing in the installed version | Check the installed version's own documentation (`help()`, `--help`, official docs), adapt, and tell the user. Never invent an API. |
| A required input, identifier or parameter is ambiguous | Ask the user, or state the assumption explicitly before running. |

**Integrity rules**

- Never fabricate results, parameters, identifiers, citations or statistics. If something cannot be run or verified, say so plainly.
- Never send private or sensitive data to external APIs without the user's consent.
- Treat version-specific details here as possibly outdated: confirm them against the official documentation for the installed version.
- Ask before actions that cost money, consume shared GPUs or cloud quota, touch personal or patient data, or cannot be undone.