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Execution Rails

Execution rails control and validate tool calls, function execution, and action invocations. They ensure that external integrations, APIs, and custom actions are called safely with validated parameters and produce trusted results.

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When Execution Rails Execute

Execution rails run before and after action/tool execution: 
Tool Call Request → Input Validation → Execute Tool → Output Validation → Return Result
                    ↓                                  ↓
                Validate Params                    Verify Result
Execution rails are critical for agentic applications where LLMs make autonomous tool calls and API requests.

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Key Concepts

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Action Input Validation

Validate parameters before executing actions: 
define flow validate database query
  user request data
  
  $query = generate_sql_query
  
  # Validate before execution
  $is_safe = execute validate_sql($query)
  
  if $is_safe
    $result = execute run_query($query)
    bot present results
  else
    bot inform unsafe query

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Action Output Validation

Verify results after execution: 
define flow validated api call
  user request external data
  
  $response = execute call_external_api
  
  # Validate response
  $contains_sensitive = execute detect_sensitive_data(source="output", text=$response)
  
  if $contains_sensitive
    $response = execute mask_sensitive_data(source="output", text=$response)
  
  bot present data

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Authorization Control

Enforce access control for tool calls: 
define flow authorized action
  user request privileged action
  
  $is_authorized = execute check_user_permissions($action_name)
  
  if $is_authorized
    $result = execute perform_action
    bot confirm action
  else
    bot refuse unauthorized action

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Built-in Execution Rails

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Action Parameter Validation

Validate action inputs using custom validators: 
from nemoguardrails.actions import action

@action()
async def validate_email(email: str) -> bool:
    """Validate email format before sending."""
    import re
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return bool(re.match(pattern, email))

@action()
async def send_email(to: str, subject: str, body: str) -> dict:
    """Send email with validated recipient."""
    # Email sending logic
    return {"status": "sent", "to": to}
Usage in flows: 
define flow send validated email
  user request send email
  
  $email = extract_email($user_message)
  $is_valid = execute validate_email($email)
  
  if $is_valid
    execute send_email(to=$email, subject="...", body="...")
    bot confirm email sent
  else
    bot inform invalid email

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SQL Injection Prevention

Validate database queries: 
from nemoguardrails.actions import action
import sqlparse

@action()
async def validate_sql(query: str) -> bool:
    """Check if SQL query is safe to execute."""
    # Parse the query
    parsed = sqlparse.parse(query)
    
    if not parsed:
        return False
    
    # Block dangerous operations
    dangerous_keywords = ['DROP', 'DELETE', 'TRUNCATE', 'ALTER', 'GRANT']
    query_upper = query.upper()
    
    for keyword in dangerous_keywords:
        if keyword in query_upper:
            return False
    
    return True

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File System Access Control

Validate file operations: 
from nemoguardrails.actions import action
import os
from pathlib import Path

@action()
async def validate_file_path(file_path: str, allowed_dir: str = "/data") -> bool:
    """Ensure file access is within allowed directory."""
    try:
        # Resolve absolute paths
        abs_path = Path(file_path).resolve()
        allowed_path = Path(allowed_dir).resolve()
        
        # Check if file is within allowed directory
        return abs_path.is_relative_to(allowed_path)
    except Exception:
        return False

@action()
async def read_file(file_path: str) -> str:
    """Read file with path validation."""
    if not await validate_file_path(file_path):
        raise ValueError(f"Access to {file_path} is not allowed")
    
    with open(file_path, 'r') as f:
        return f.read()

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API Rate Limiting

Control API call frequency: 
from nemoguardrails.actions import action
import time
from collections import defaultdict

# Simple in-memory rate limiter
rate_limits = defaultdict(list)

@action()
async def check_rate_limit(
    action_name: str,
    max_calls: int = 10,
    window_seconds: int = 60
) -> bool:
    """Check if action can be called within rate limit."""
    now = time.time()
    
    # Remove old entries
    rate_limits[action_name] = [
        t for t in rate_limits[action_name]
        if now - t < window_seconds
    ]
    
    # Check limit
    if len(rate_limits[action_name]) >= max_calls:
        return False
    
    # Record this call
    rate_limits[action_name].append(now)
    return True
Usage: 
define flow rate limited api call
  user request api data
  
  $can_call = execute check_rate_limit(action_name="call_external_api", max_calls=10, window_seconds=60)
  
  if $can_call
    $data = execute call_external_api
    bot present data
  else
    bot inform rate limit exceeded

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Common Patterns

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Pre-Execution Validation

define flow execute with validation
  user request action
  
  # Extract parameters
  $params = extract_action_params
  
  # Validate all parameters
  $valid_email = execute validate_email($params.email)
  $valid_amount = execute validate_amount($params.amount)
  $authorized = execute check_permissions($action_name)
  
  if $valid_email and $valid_amount and $authorized
    $result = execute perform_action($params)
    bot confirm success
  else
    bot inform validation failed

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Post-Execution Sanitization

define flow sanitize action result
  user request external data
  
  $raw_result = execute fetch_external_data
  
  # Sanitize result
  $has_pii = execute detect_sensitive_data(source="output", text=$raw_result)
  
  if $has_pii
    $sanitized = execute mask_sensitive_data(source="output", text=$raw_result)
    bot present sanitized data
  else
    bot present raw data

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Retry with Validation

define flow retry validated action
  user request action
  
  $max_retries = 3
  $attempt = 0
  
  while $attempt < $max_retries
    $result = execute perform_action
    $is_valid = execute validate_result($result)
    
    if $is_valid
      bot confirm success
      stop
    
    $attempt = $attempt + 1
  
  bot inform action failed

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Sandboxed Execution

from nemoguardrails.actions import action
import subprocess
import tempfile
import os

@action()
async def execute_sandboxed_code(code: str, language: str = "python") -> dict:
    """Execute code in sandboxed environment."""
    # Create temporary file
    with tempfile.NamedTemporaryFile(mode='w', suffix=f'.{language}', delete=False) as f:
        f.write(code)
        temp_file = f.name
    
    try:
        # Run in container with resource limits
        result = subprocess.run(
            ['docker', 'run', '--rm',
             '--memory=128m',  # Limit memory
             '--cpus=0.5',     # Limit CPU
             '--network=none', # No network access
             '-v', f'{temp_file}:/code/{language}',
             f'{language}-sandbox',
             f'{language}', f'/code/{language}'
            ],
            capture_output=True,
            text=True,
            timeout=5  # 5 second timeout
        )
        
        return {
            "output": result.stdout,
            "error": result.stderr,
            "success": result.returncode == 0
        }
    finally:
        os.unlink(temp_file)

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Real-World Examples

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E-commerce Transaction Validation

define flow process payment
  user request purchase
  
  # Validate order
  $order = extract_order_details
  $valid_amount = execute validate_amount($order.total)
  $valid_items = execute validate_inventory($order.items)
  $authorized = execute check_user_credit_limit($order.total)
  
  if not ($valid_amount and $valid_items and $authorized)
    bot inform validation failed
    stop
  
  # Process payment
  $payment_result = execute process_payment($order)
  
  # Validate result
  if $payment_result.status == "success"
    execute update_inventory($order.items)
    execute send_confirmation_email($order.email)
    bot confirm order
  else
    bot inform payment failed

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Database Query Execution

define flow execute database query
  user request data query
  
  # Generate SQL
  $query = generate_sql_from_natural_language
  
  # Validate query safety
  $is_safe = execute validate_sql($query)
  $is_read_only = execute check_read_only($query)
  $within_limit = execute check_row_limit($query, max_rows=1000)
  
  if not ($is_safe and $is_read_only and $within_limit)
    bot inform unsafe query
    stop
  
  # Execute with timeout
  $result = execute run_query($query, timeout=10)
  
  # Validate result size
  if $result.row_count > 1000
    bot inform result too large
  else
    bot present query results

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External API Integration

define flow call external api
  user request external information
  
  # Check rate limits
  $can_call = execute check_rate_limit(action_name="external_api")
  
  if not $can_call
    bot inform rate limit
    stop
  
  # Validate request parameters
  $params = extract_api_params
  $valid_params = execute validate_api_params($params)
  
  if not $valid_params
    bot inform invalid parameters
    stop
  
  # Call API
  $response = execute call_api($params)
  
  # Validate and sanitize response
  $has_sensitive = execute detect_sensitive_data(source="output", text=$response)
  
  if $has_sensitive
    $response = execute mask_sensitive_data(source="output", text=$response)
  
  bot present api results

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Best Practices

  1. Validate all external inputs - Never trust user-provided parameters
  2. Implement defense in depth - Layer multiple validation checks
  3. Use least privilege - Only grant necessary permissions for actions
  4. Set timeouts - Prevent long-running or hanging operations
  5. Sanitize outputs - Remove sensitive data from action results
  6. Log all executions - Track action calls for security auditing
  7. Implement rate limiting - Prevent abuse of expensive operations
  8. Use sandboxing - Isolate untrusted code execution

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Configuration

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Register Custom Actions

# actions.py
from nemoguardrails.actions import action

@action()
async def validate_action_params(action_name: str, params: dict) -> bool:
    """Custom parameter validator."""
    # Validation logic
    return True

def register_actions(rails):
    rails.register_action(validate_action_params)

# config.yml
actions_server_url: null  # Use in-process actions

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Enable Action Logging

logging:
  level: INFO
  log_actions: true
  log_action_params: true  # Be careful with sensitive data

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Security Considerations

Critical Security Practices:
  1. Never execute arbitrary code from user input without sandboxing
  2. Always validate file paths to prevent directory traversal
  3. Use parameterized queries to prevent SQL injection
  4. Implement proper authentication for privileged actions
  5. Rate limit expensive operations to prevent DoS
  6. Audit all action executions for security monitoring

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Performance Impact

Validation TypeLatency ImpactSecurity Value
Parameter validationLow (~10-50ms)High
SQL safety checkLow (~10-50ms)Very High
File path validationLow (~5-10ms)Very High
Rate limitingLow (~5-10ms)Medium
Output sanitizationMedium (~50-100ms)High
Sandboxed executionHigh (~1-5s)Very High

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Troubleshooting

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Action Validation Failures

Enable detailed logging: 
logging:
  level: DEBUG
  show_internal_events: true

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Performance Issues

Cache validation results: 
from functools import lru_cache

@action()
@lru_cache(maxsize=1000)
async def validate_email(email: str) -> bool:
    # Validation logic
    return True

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See Also