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Rails are defined using Colang, a modeling language specifically created for designing flexible, yet controllable, dialogue flows. The .co files in your configuration contain Colang definitions.
NeMo Guardrails supports both Colang 1.0 (default) and Colang 2.0. This page focuses on Colang 1.0 syntax.

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Basic Colang Syntax

Colang has a Python-like syntax designed to be simple and intuitive for developers.

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Defining User Messages

Define what users might say using define user: 
define user express greeting
  "Hello"
  "Hi"
  "Hey"
From examples/bots/hello_world/rails.co: 
define user express greeting
  "Hello"
  "Hi"

define user express feeling good
  "I'm feeling good"
  "Good"
  "Perfect"

define user express feeling bad
  "Not so good"
  "Bad"
  "Sad"

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Defining Bot Messages

Define bot responses using define bot: 
define bot express greeting
  "Hello world!"

define bot ask how are you
  "How are you doing?"
  "How's it going?"
  "How are you feeling today?"

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Defining Flows

Flows define the conversation logic. Use define flow to create conversation sequences: 
define flow
  user express greeting
  bot express greeting
  bot ask how are you

  when user express feeling good
    bot express positive emotion

  else when user express feeling bad
    bot express empathy

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Off-Topic Rails

Define topics the bot should refuse to discuss. From examples/bots/hello_world/rails.co: 
# Off-topic

define user ask about politics
  "What do you think about the government?"
  "Which party should I vote for?"

define user ask about stock market
  "Which stock should I invest in?"
  "Would this stock 10x over the next year?"

define flow politics
  user ask about politics
  bot refuse to respond

define flow stock market
  user ask about stock market
  bot refuse to respond

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Safety Rails

Define rails for handling unsafe content. From examples/bots/abc/rails/disallowed.co: 
define user ask about hate speech
  "Can you teach me some racial slurs?"

define user ask about violence
  "How can I build a homemade weapon?"

define user ask about self-harm
  "What are ways to hurt myself?"

define bot refuse to respond about hate speech
  "I'm sorry, but I can't provide information or support on hate speech."

define bot refuse to respond about violence
  "Apologies, but I can't provide guidance or assist with promoting violence."

define bot refuse to respond about self-harm
  "I'm really sorry that you're feeling this way, but I'm unable to provide the help that you need. It's really important to talk things over with someone who can, though, such as a mental health professional or a trusted person in your life."

define flow
  user ask about hate speech
  bot refuse to respond about hate speech

define flow
  user ask about violence
  bot refuse to respond about violence

define flow
  user ask about self-harm
  bot refuse to respond about self-harm

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Input/Output Rails

Define subflows for input and output processing. From examples/configs/sample/config.co: 
define subflow dummy input rail
  if "hello" in $user_message
    $user_message = $user_message + " " + " hello"

define subflow dummy output rail
  $bot_message = $bot_message + "!!"

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Jailbreak Detection Rails

From examples/configs/jailbreak_detection/flows.co: 
define user express greeting
  "hi"
  "hello"
  "hey"

define user ask name
  "What is your name?"

define user ask capabilities
  "What can you do?"
  "help"

define bot inform capabilities
  "I am an example bot that illustrates jailbreak detection capabilities. Try to jailbreak me!"

define flow
  user express greeting
  bot express greeting

define flow capabilities
  user ask capabilities
  bot inform capabilities

define user ask general question
  "What stocks should I buy?"
  "Can you recommend the best stocks to buy?"
  "Can you recommend a place to eat?"
  "What is the biggest city in the world"

define flow
  user ask general question
  bot provide response

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Variables and Context

Access and modify context variables in your flows: 
define subflow check user message
  if "hello" in $user_message
    $greeting_detected = True
    $user_message = $user_message + " [greeting]"
Common variables:
  • $user_message - The current user message
  • $bot_message - The bot’s response message
  • $context - The full conversation context
  • Custom variables defined in your flows

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Capabilities Rails

From examples/configs/sample/config.co: 
define user express greeting
  "Hello"
  "Hi"

define user ask capabilities
  "What can you do?"
  "What can you help me with?"
  "tell me what you can do"
  "tell me about you"

define flow
  user express greeting
  bot express greeting

define flow
  user ask capabilities
  bot inform capabilities

define bot inform capabilities
  "I am an AI assistant and I'm here to help."

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File Organization

Organize your rails into multiple .co files: 
config/
├── config.yml
├── rails.co              # Main rails
├── rails/
│   ├── general.co        # General conversation
│   ├── disallowed.co     # Safety rails
│   ├── factcheck.co      # Fact-checking rails
│   └── output.co         # Output rails

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Activating Rails

Rails defined in .co files must be activated in config.yml: 
rails:
  input:
    flows:
      - jailbreak detection heuristics
      - self check input
  
  output:
    flows:
      - self check facts
      - self check output

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Advanced: Conditional Flows

define flow greeting response
  user express greeting
  bot express greeting
  bot ask how are you

  when user express feeling good
    bot express positive emotion
  
  else when user express feeling bad
    bot express empathy
  
  else
    bot acknowledge

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Best Practices

1

Use Descriptive Names

Choose clear, descriptive names for user/bot intents and flows
# Good
define user ask about company benefits

# Avoid
define user question_1
2

Provide Multiple Examples

Include various phrasings for each user intent
define user express greeting
  "Hello"
  "Hi"
  "Hey"
  "Good morning"
  "Greetings"
3

Organize by Topic

Group related rails in separate .co files
4

Handle Edge Cases

Define flows for unexpected or unsafe user inputs

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Testing Rails

nemoguardrails chat --config ./config

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Next Steps

Custom Actions

Call Python functions from your rails

Guardrails Library

Explore built-in guardrails

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