Custom Code Guardrail

Write custom guardrail logic using Python-like code that runs in a sandboxed environment.

Quick Start

1. Define the guardrail in config

model_list:
    - model_name: gpt-4
        litellm_params:
        model: gpt-4
        api_key: os.environ/OPENAI_API_KEY

guardrails:
    - guardrail_name: block-ssn
        litellm_params:
        guardrail: custom_code
        mode: pre_call
        custom_code: |
            def apply_guardrail(inputs, request_data, input_type):
                for text in inputs["texts"]:
                    if regex_match(text, r"\d{3}-\d{2}-\d{4}"):
                        return block("SSN detected")
                return allow()

2. Start proxy

litellm --config config.yaml

3. Test

curl -X POST http://localhost:4000/chat/completions \
  -H "Authorization: Bearer sk-1234" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4",
    "messages": [{"role": "user", "content": "My SSN is 123-45-6789"}],
    "guardrails": ["block-ssn"]
  }'

Configuration

Parameter	Type	Required	Description
guardrail	string	✅	Must be custom_code
mode	string	✅	When to run: pre_call, post_call, during_call
custom_code	string	✅	Python-like code with apply_guardrail function
default_on	bool	❌	Run on all requests (default: false)

Writing Custom Code

Function Signature

Your code must define an apply_guardrail function:

def apply_guardrail(inputs, request_data, input_type):
    # inputs: see table below
    # request_data: {"model": "...", "user_id": "...", "team_id": "...", "metadata": {...}}
    # input_type: "request" or "response"
    
    return allow()  # or block() or modify()

inputs Parameter

Field	Type	Description
texts	List[str]	Extracted text from the request/response
images	List[str]	Extracted images (for image guardrails)
tools	List[dict]	Tools sent to the LLM
tool_calls	List[dict]	Tool calls returned from the LLM
structured_messages	List[dict]	Full messages with role info (system/user/assistant)
model	str	The model being used

request_data Parameter

Field	Type	Description
model	str	Model name
user_id	str	User ID from API key
team_id	str	Team ID from API key
end_user_id	str	End user ID
metadata	dict	Request metadata

Return Values

Function	Description
allow()	Let request/response through
block(reason)	Reject with message
modify(texts=[], images=[], tool_calls=[])	Transform content

Built-in Primitives

Regex

Function	Description
regex_match(text, pattern)	Returns True if pattern found
regex_replace(text, pattern, replacement)	Replace all matches
regex_find_all(text, pattern)	Return list of matches

JSON

Function	Description
json_parse(text)	Parse JSON string, returns None on error
json_stringify(obj)	Convert to JSON string
json_schema_valid(obj, schema)	Validate against JSON schema

URL

Function	Description
extract_urls(text)	Extract all URLs from text
is_valid_url(url)	Check if URL is valid
all_urls_valid(text)	Check all URLs in text are valid

Code Detection

Function	Description
detect_code(text)	Returns True if code detected
detect_code_languages(text)	Returns list of detected languages
contains_code_language(text, ["sql", "python"])	Check for specific languages

Text Utilities

Function	Description
contains(text, substring)	Check if substring exists
contains_any(text, [substr1, substr2])	Check if any substring exists
word_count(text)	Count words
char_count(text)	Count characters
lower(text) / upper(text) / trim(text)	String transforms

Examples

Block PII (SSN)

def apply_guardrail(inputs, request_data, input_type):
    for text in inputs["texts"]:
        if regex_match(text, r"\d{3}-\d{2}-\d{4}"):
            return block("SSN detected")
    return allow()

Redact Email Addresses

def apply_guardrail(inputs, request_data, input_type):
    pattern = r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}"
    modified = []
    for text in inputs["texts"]:
        modified.append(regex_replace(text, pattern, "[EMAIL REDACTED]"))
    return modify(texts=modified)

Block SQL Injection

def apply_guardrail(inputs, request_data, input_type):
    if input_type != "request":
        return allow()
    for text in inputs["texts"]:
        if contains_code_language(text, ["sql"]):
            return block("SQL code not allowed")
    return allow()

Validate JSON Response

def apply_guardrail(inputs, request_data, input_type):
    if input_type != "response":
        return allow()
    
    schema = {
        "type": "object",
        "required": ["name", "value"]
    }
    
    for text in inputs["texts"]:
        obj = json_parse(text)
        if obj is None:
            return block("Invalid JSON response")
        if not json_schema_valid(obj, schema):
            return block("Response missing required fields")
    return allow()

Check URLs in Response

def apply_guardrail(inputs, request_data, input_type):
    if input_type != "response":
        return allow()
    for text in inputs["texts"]:
        if not all_urls_valid(text):
            return block("Response contains invalid URLs")
    return allow()

Combine Multiple Checks

def apply_guardrail(inputs, request_data, input_type):
    modified = []
    
    for text in inputs["texts"]:
        # Redact SSN
        text = regex_replace(text, r"\d{3}-\d{2}-\d{4}", "[SSN]")
        # Redact credit cards
        text = regex_replace(text, r"\d{16}", "[CARD]")
        modified.append(text)
    
    # Block SQL in requests
    if input_type == "request":
        for text in inputs["texts"]:
            if contains_code_language(text, ["sql"]):
                return block("SQL injection blocked")
    
    return modify(texts=modified)

Sandbox Restrictions

Custom code runs in a restricted environment:

• ❌ No import statements
• ❌ No file I/O
• ❌ No network access
• ❌ No exec() or eval()
• ✅ Only LiteLLM-provided primitives available

Per-Request Usage

Enable guardrail per request:

curl -X POST http://localhost:4000/chat/completions \
  -H "Authorization: Bearer sk-1234" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4",
    "messages": [{"role": "user", "content": "Hello"}],
    "guardrails": ["block-ssn"]
  }'

Default On

Run guardrail on all requests:

litellm_settings:
  guardrails:
    - guardrail_name: block-ssn
      litellm_params:
        guardrail: custom_code
        mode: pre_call
        default_on: true
        custom_code: |
          def apply_guardrail(inputs, request_data, input_type):
              ...