Guidance: Constrained LLM Generation

When to Use This Skill

Use Guidance when you need to:

• Control LLM output syntax with regex or grammars
• Guarantee valid JSON/XML/code generation
• Reduce latency vs traditional prompting approaches
• Enforce structured formats (dates, emails, IDs, etc.)
• Build multi-step workflows with Pythonic control flow
• Prevent invalid outputs through grammatical constraints

GitHub Stars: 18,000+ | From: Microsoft Research

Installation

# Base installation
pip install guidance

# With specific backends
pip install guidance[transformers]  # Hugging Face models
pip install guidance[llama_cpp]     # llama.cpp models

Quick Start

Basic Example: Structured Generation

from guidance import models, gen

# Load model (supports OpenAI, Transformers, llama.cpp)
lm = models.OpenAI("gpt-4")

# Generate with constraints
result = lm + "The capital of France is " + gen("capital", max_tokens=5)

print(result["capital"])  # "Paris"

Chat format with a local model

Constraint support requires local logit access. Regex, select(), and grammar-based constrained generation only work with local backends (Transformers, LlamaCpp). Remote API backends (OpenAI, and Azure variants) support unconstrained gen() / chat only — they cannot enforce token-level constraints. guidance 0.3.x has no models.Anthropic class.

from guidance import models, gen, system, user, assistant

# Local model (supports constrained generation)
lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# Use context managers for chat format
with system():
    lm += "You are a helpful assistant."

with user():
    lm += "What is the capital of France?"

with assistant():
    lm += gen(max_tokens=20)

Core Concepts

1. Context Managers

Guidance uses Pythonic context managers for chat-style interactions.

from guidance import system, user, assistant, gen

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# System message
with system():
    lm += "You are a JSON generation expert."

# User message
with user():
    lm += "Generate a person object with name and age."

# Assistant response
with assistant():
    lm += gen("response", max_tokens=100)

print(lm["response"])

Benefits:

• Natural chat flow
• Clear role separation
• Easy to read and maintain

2. Constrained Generation

Guidance ensures outputs match specified patterns using regex or grammars.

Regex Constraints

from guidance import models, gen

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# Constrain to valid email format
lm += "Email: " + gen("email", regex=r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}")

# Constrain to date format (YYYY-MM-DD)
lm += "Date: " + gen("date", regex=r"\d{4}-\d{2}-\d{2}")

# Constrain to phone number
lm += "Phone: " + gen("phone", regex=r"\d{3}-\d{3}-\d{4}")

print(lm["email"])  # Guaranteed valid email
print(lm["date"])   # Guaranteed YYYY-MM-DD format

How it works:

• Regex converted to grammar at token level
• Invalid tokens filtered during generation
• Model can only produce matching outputs

Selection Constraints

from guidance import models, gen, select

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# Constrain to specific choices
lm += "Sentiment: " + select(["positive", "negative", "neutral"], name="sentiment")

# Multiple-choice selection
lm += "Best answer: " + select(
    ["A) Paris", "B) London", "C) Berlin", "D) Madrid"],
    name="answer"
)

print(lm["sentiment"])  # One of: positive, negative, neutral
print(lm["answer"])     # One of: A, B, C, or D

3. Token Healing

Guidance automatically "heals" token boundaries between prompt and generation.

Problem: Tokenization creates unnatural boundaries.

# Without token healing
prompt = "The capital of France is "
# Last token: " is "
# First generated token might be " Par" (with leading space)
# Result: "The capital of France is  Paris" (double space!)

Solution: Guidance backs up one token and regenerates.

from guidance import models, gen

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# Token healing enabled by default
lm += "The capital of France is " + gen("capital", max_tokens=5)
# Result: "The capital of France is Paris" (correct spacing)

Benefits:

• Natural text boundaries
• No awkward spacing issues
• Better model performance (sees natural token sequences)

4. Grammar-Based Generation

Define complex structures by composing grammar functions. The template-string grammar= form is not part of current guidance — build grammars from composable functions, or use guidance.json() for JSON.

from guidance import models, gen
from guidance import json as gen_json
from pydantic import BaseModel, Field

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

# JSON via a Pydantic schema (guidance.json compiles the schema to a grammar)
class Person(BaseModel):
    name: str = Field(pattern=r"[A-Za-z ]+")
    age: int
    email: str = Field(pattern=r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}")

lm += gen_json(name="person", schema=Person)

print(lm["person"])  # Guaranteed valid JSON matching the schema

# Or compose grammar functions directly:
grammar = "name=" + gen("name", regex=r"[A-Za-z ]+") + " age=" + gen("age", regex=r"[0-9]+")
lm += grammar

Use cases:

• Complex structured outputs
• Nested data structures
• Programming language syntax
• Domain-specific languages

5. Guidance Functions

Create reusable generation patterns with the @guidance decorator.

from guidance import guidance, gen, models

@guidance
def generate_person(lm):
    """Generate a person with name and age."""
    lm += "Name: " + gen("name", max_tokens=20, stop="\n")
    lm += "\nAge: " + gen("age", regex=r"[0-9]+", max_tokens=3)
    return lm

# Use the function
lm = models.Transformers("microsoft/Phi-4-mini-instruct")
lm = generate_person(lm)

print(lm["name"])
print(lm["age"])

Stateful Functions:

@guidance(stateless=False)
def react_agent(lm, question, tools, max_rounds=5):
    """ReAct agent with tool use."""
    lm += f"Question: {question}\n\n"

    for i in range(max_rounds):
        # Thought
        lm += f"Thought {i+1}: " + gen("thought", stop="\n")

        # Action
        lm += "\nAction: " + select(list(tools.keys()), name="action")

        # Execute tool
        tool_result = tools[lm["action"]]()
        lm += f"\nObservation: {tool_result}\n\n"

        # Check if done
        lm += "Done? " + select(["Yes", "No"], name="done")
        if lm["done"] == "Yes":
            break

    # Final answer
    lm += "\nFinal Answer: " + gen("answer", max_tokens=100)
    return lm

Backend Configuration

OpenAI (remote — unconstrained only)

Remote API backends cannot do constrained generation (regex/select/grammar); use them only for plain chat/gen(). For constraints, use a local backend.

from guidance import models

lm = models.OpenAI(
    model="gpt-4o-mini",
    api_key="your-api-key"  # Or set OPENAI_API_KEY env var
)

Local Models (Transformers)

from guidance.models import Transformers

lm = Transformers(
    "microsoft/Phi-4-mini-instruct",
    device="cuda"  # Or "cpu"
)

Local Models (llama.cpp)

from guidance.models import LlamaCpp

lm = LlamaCpp(
    model_path="/path/to/model.gguf",
    n_ctx=4096,
    n_gpu_layers=35
)

Common Patterns

Pattern 1: JSON Generation

from guidance import models, gen, system, user, assistant

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

with system():
    lm += "You generate valid JSON."

with user():
    lm += "Generate a user profile with name, age, and email."

with assistant():
    lm += """{
    "name": """ + gen("name", regex=r'"[A-Za-z ]+"', max_tokens=30) + """,
    "age": """ + gen("age", regex=r"[0-9]+", max_tokens=3) + """,
    "email": """ + gen("email", regex=r'"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}"', max_tokens=50) + """
}"""

print(lm)  # Valid JSON guaranteed

Pattern 2: Classification

from guidance import models, gen, select

lm = models.Transformers("microsoft/Phi-4-mini-instruct")

text = "This product is amazing! I love it."

lm += f"Text: {text}\n"
lm += "Sentiment: " + select(["positive", "negative", "neutral"], name="sentiment")
lm += "\nConfidence: " + gen("confidence", regex=r"[0-9]+", max_tokens=3) + "%"

print(f"Sentiment: {lm['sentiment']}")
print(f"Confidence: {lm['confidence']}%")

Pattern 3: Multi-Step Reasoning

from guidance import models, gen, guidance

@guidance
def chain_of_thought(lm, question):
    """Generate answer with step-by-step reasoning."""
    lm += f"Question: {question}\n\n"

    # Generate multiple reasoning steps
    for i in range(3):
        lm += f"Step {i+1}: " + gen(f"step_{i+1}", stop="\n", max_tokens=100) + "\n"

    # Final answer
    lm += "\nTherefore, the answer is: " + gen("answer", max_tokens=50)

    return lm

lm = models.Transformers("microsoft/Phi-4-mini-instruct")
lm = chain_of_thought(lm, "What is 15% of 200?")

print(lm["answer"])

Pattern 4: ReAct Agent

from guidance import models, gen, select, guidance

@guidance(stateless=False)
def react_agent(lm, question):
    """ReAct agent with tool use."""
    tools = {
        "calculator": lambda expr: eval(expr),
        "search": lambda query: f"Search results for: {query}",
    }

    lm += f"Question: {question}\n\n"

    for round in range(5):
        # Thought
        lm += f"Thought: " + gen("thought", stop="\n") + "\n"

        # Action selection
        lm += "Action: " + select(["calculator", "search", "answer"], name="action")

        if lm["action"] == "answer":
            lm += "\nFinal Answer: " + gen("answer", max_tokens=100)
            break

        # Action input
        lm += "\nAction Input: " + gen("action_input", stop="\n") + "\n"

        # Execute tool
        if lm["action"] in tools:
            result = tools[lm["action"]](lm["action_input"])
            lm += f"Observation: {result}\n\n"

    return lm

lm = models.Transformers("microsoft/Phi-4-mini-instruct")
lm = react_agent(lm, "What is 25 * 4 + 10?")
print(lm["answer"])

Pattern 5: Data Extraction

from guidance import models, gen, guidance

@guidance
def extract_entities(lm, text):
    """Extract structured entities from text."""
    lm += f"Text: {text}\n\n"

    # Extract person
    lm += "Person: " + gen("person", stop="\n", max_tokens=30) + "\n"

    # Extract organization
    lm += "Organization: " + gen("organization", stop="\n", max_tokens=30) + "\n"

    # Extract date
    lm += "Date: " + gen("date", regex=r"\d{4}-\d{2}-\d{2}", max_tokens=10) + "\n"

    # Extract location
    lm += "Location: " + gen("location", stop="\n", max_tokens=30) + "\n"

    return lm

text = "Tim Cook announced at Apple Park on 2024-09-15 in Cupertino."

lm = models.Transformers("microsoft/Phi-4-mini-instruct")
lm = extract_entities(lm, text)

print(f"Person: {lm['person']}")
print(f"Organization: {lm['organization']}")
print(f"Date: {lm['date']}")
print(f"Location: {lm['location']}")

Best Practices

1. Use Regex for Format Validation

# ✅ Good: Regex ensures valid format
lm += "Email: " + gen("email", regex=r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}")

# ❌ Bad: Free generation may produce invalid emails
lm += "Email: " + gen("email", max_tokens=50)

2. Use select() for Fixed Categories

# ✅ Good: Guaranteed valid category
lm += "Status: " + select(["pending", "approved", "rejected"], name="status")

# ❌ Bad: May generate typos or invalid values
lm += "Status: " + gen("status", max_tokens=20)

3. Leverage Token Healing

# Token healing is enabled by default
# No special action needed - just concatenate naturally
lm += "The capital is " + gen("capital")  # Automatic healing

4. Use stop Sequences

# ✅ Good: Stop at newline for single-line outputs
lm += "Name: " + gen("name", stop="\n")

# ❌ Bad: May generate multiple lines
lm += "Name: " + gen("name", max_tokens=50)

5. Create Reusable Functions

# ✅ Good: Reusable pattern
@guidance
def generate_person(lm):
    lm += "Name: " + gen("name", stop="\n")
    lm += "\nAge: " + gen("age", regex=r"[0-9]+")
    return lm

# Use multiple times
lm = generate_person(lm)
lm += "\n\n"
lm = generate_person(lm)

6. Balance Constraints

# ✅ Good: Reasonable constraints
lm += gen("name", regex=r"[A-Za-z ]+", max_tokens=30)

# ❌ Too strict: May fail or be very slow
lm += gen("name", regex=r"^(John|Jane)$", max_tokens=10)

Comparison to Alternatives

When to choose Guidance:

• Need regex/grammar constraints
• Want token healing
• Building complex workflows with control flow
• Using local models (Transformers, llama.cpp)
• Prefer Pythonic syntax

When to choose alternatives:

• Instructor: Need Pydantic validation with automatic retrying
• Outlines: Need JSON schema validation
• LMQL: Prefer declarative query syntax

Performance Characteristics

Latency Reduction:

• 30-50% faster than traditional prompting for constrained outputs
• Token healing reduces unnecessary regeneration
• Grammar constraints prevent invalid token generation

Memory Usage:

• Minimal overhead vs unconstrained generation
• Grammar compilation cached after first use
• Efficient token filtering at inference time

Token Efficiency:

• Prevents wasted tokens on invalid outputs
• No need for retry loops
• Direct path to valid outputs

Resources

• Documentation: https://guidance.readthedocs.io
• GitHub: https://github.com/guidance-ai/guidance (18k+ stars)
• Notebooks: https://github.com/guidance-ai/guidance/tree/main/notebooks
• Discord: Community support available

See Also

• references/constraints.md - Comprehensive regex and grammar patterns
• references/backends.md - Backend-specific configuration
• references/examples.md - Production-ready examples