Implementation:Guardrails ai Guardrails Guard Use

Knowledge Sources	Guardrails Guardrails Guard API
Domains	Validation, API
Last Updated	2026-02-14 00:00 GMT

Overview

Concrete method for attaching validators to a Guard instance provided by the guardrails package.

Description

The Guard.use method adds a single validator to the Guard's validation pipeline. It accepts an instantiated Validator (or deprecated: a Validator class with arguments), hydrates it, registers it in the Guard's internal validator map keyed by target path, and persists the Guard state if running in server mode.

Usage

Use this method when constructing a Guard pipeline by attaching validators one at a time. Chain multiple .use() calls for a fluent builder pattern.

Code Reference

Source Location

• Repository: guardrails
• File: guardrails/guard.py
• Lines: L907-940

Signature

def use(
    self,
    validator: UseValidatorSpec,
    *args,
    on: str = "output",
    **kwargs,
) -> "Guard":
    """Use a validator to validate either of the following:
    - The output of an LLM request
    - The message history

    Args:
        validator: The validator to use. Either the class or an instance.
        on: The part of the LLM request to validate. Defaults to "output".
    """

Import

from guardrails import Guard

I/O Contract

Inputs

Name	Type	Required	Description
validator	UseValidatorSpec	Yes	A Validator instance (or deprecated: Validator class with args)
on	str	No	Target for validation: "output" (default), "messages", or JSON path like "$.field"

Outputs

Name	Type	Description
self	Guard	Returns the Guard instance (fluent interface for chaining)

Usage Examples

Basic Validator Attachment

from guardrails import Guard
from guardrails.hub import RegexMatch, ToxicLanguage

# Chain validators using fluent API
guard = Guard().use(
    RegexMatch(regex=r"\d{3}-\d{2}-\d{4}", on_fail="exception")
).use(
    ToxicLanguage(on_fail="fix")
)

Field-Level Validation

from guardrails import Guard
from guardrails.hub import ValidLength

# Target a specific JSON path
guard = Guard().use(
    ValidLength(min=1, max=100, on_fail="fix"),
    on="$.name"
)

Related Pages

Implements Principle

• Principle:Guardrails_ai_Guardrails_Guard_Validator_Composition

Requires Environment

• Environment:Guardrails_ai_Guardrails_Python_3_10_Runtime