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Control LLM output with regex and grammars, guarantee valid JSON/XML/code generation, enforce structured formats, and build multi-step workflows with Guidance - Microsoft Research's constrained generation framework
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Control LLM output with regex and grammars, guarantee valid JSON/XML/code generation, enforce structured formats, and build multi-step workflows with Guidance - Microsoft Research's constrained generation framework 📦 Ships with claude-code-templates
</> SKILL.md
prompt-engineering-guidance.SKILL.md
---
name : guidance
description : Control LLM output with regex and grammars, guarantee valid JSON/XML/code generation, enforce structured formats, and build multi-step workflows with Guidance - Microsoft Research's constrained generation framework
version : 1.0.0
author : Orchestra Research
license : MIT
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
**GitHub Stars**: 18,000+ | **From**: Microsoft Research
## Installation
```bash
# Base installation
pip install guidance
Flowy AI Flows that just work. Hand-picked Claude Code plugins that fire the right skill as you prompt.
# 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
### Pattern 1: JSON Generation
```python
from guidance import models, gen, system, user, assistant
lm = models.Anthropic("claude-sonnet-4-5-20250929")
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
```python
from guidance import models, gen, select
lm = models.Anthropic("claude-sonnet-4-5-20250929")
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
```python
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.Anthropic("claude-sonnet-4-5-20250929")
lm = chain_of_thought(lm, "What is 15% of 200?")
print(lm["answer"])
```
### Pattern 4: ReAct Agent
```python
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.Anthropic("claude-sonnet-4-5-20250929")
lm = react_agent(lm, "What is 25 * 4 + 10?")
print(lm["answer"])
```
### Pattern 5: Data Extraction
```python
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.Anthropic("claude-sonnet-4-5-20250929")
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
```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 (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
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