---
name: guidance
description: Constrains LLM output during generation with Guidance (Microsoft Research), using regex, select() choices, context-free grammars, token healing, and @guidance functions, with Anthropic, OpenAI, Transformers, and llama.cpp backends. Use when you need generated text to match a regex or fixed format such as dates, emails, or IDs. Use when you need guaranteed valid JSON, XML, or code from a model. Use when building multi-step generation workflows or ReAct-style agents with Python control flow. Use when classifying text into fixed categories with select(). Use when running local models and wanting grammar constraints. Not for Pydantic validation with automatic retries; use Instructor for that.
license: MIT
metadata:
  version: 1.0.0
  category: llm-applications
  maintainer: Kalaris Labs
  tags: Prompt Engineering, Guidance, Constrained Generation, Structured Output, JSON Validation, Grammar, Microsoft Research, Format Enforcement, Multi-Step Workflows
  dependencies: guidance, transformers
---

# 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


## Installation

```bash
# 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

```python
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"
```

### With Anthropic Claude

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

# Configure Claude
lm = models.Anthropic("claude-sonnet-4-5-20250929")

# 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.

```python
from guidance import system, user, assistant, gen

lm = models.Anthropic("claude-sonnet-4-5-20250929")

# 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

```python
from guidance import models, gen

lm = models.Anthropic("claude-sonnet-4-5-20250929")

# 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

```python
from guidance import models, gen, select

lm = models.Anthropic("claude-sonnet-4-5-20250929")

# 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.

```python
# 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.

```python
from guidance import models, gen

lm = models.Anthropic("claude-sonnet-4-5-20250929")

# 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 using context-free grammars.

```python
from guidance import models, gen

lm = models.Anthropic("claude-sonnet-4-5-20250929")

# JSON grammar (simplified)
json_grammar = """
{
    "name": <gen name regex="[A-Za-z ]+" max_tokens=20>,
    "age": <gen age regex="[0-9]+" max_tokens=3>,
    "email": <gen email regex="[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}" max_tokens=50>
}
"""

# Generate valid JSON
lm += gen("person", grammar=json_grammar)

print(lm["person"])  # Guaranteed valid JSON structure
```

**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.

```python
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.Anthropic("claude-sonnet-4-5-20250929")
lm = generate_person(lm)

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

**Stateful Functions:**

```python
@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

### Anthropic Claude

```python
from guidance import models

lm = models.Anthropic(
    model="claude-sonnet-4-5-20250929",
    api_key="your-api-key"  # Or set ANTHROPIC_API_KEY env var
)
```

### OpenAI

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

### Local Models (Transformers)

```python
from guidance.models import Transformers

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

### Local Models (llama.cpp)

```python
from guidance.models import LlamaCpp

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

## Common Patterns

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

## Best Practices

### 1. Use Regex for Format Validation

```python
# ✅ 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

```python
# ✅ 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

```python
# 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

```python
# ✅ 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

```python
# ✅ 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

```python
# ✅ 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

| Feature | Guidance | Instructor | Outlines | LMQL |
|---------|----------|------------|----------|------|
| Regex Constraints | ✅ Yes | ❌ No | ✅ Yes | ✅ Yes |
| Grammar Support | ✅ CFG | ❌ No | ✅ CFG | ✅ CFG |
| Pydantic Validation | ❌ No | ✅ Yes | ✅ Yes | ❌ No |
| Token Healing | ✅ Yes | ❌ No | ✅ Yes | ❌ No |
| Local Models | ✅ Yes | ⚠️ Limited | ✅ Yes | ✅ Yes |
| API Models | ✅ Yes | ✅ Yes | ⚠️ Limited | ✅ Yes |
| Pythonic Syntax | ✅ Yes | ✅ Yes | ✅ Yes | ❌ SQL-like |
| Learning Curve | Low | Low | Medium | High |

**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
- **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

## 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.