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Guardrails help you build safe, compliant AI applications by validating and filtering content at key points in your agent’s execution. They can detect sensitive information, enforce content policies, validate outputs, and prevent unsafe behaviors before they cause problems. Common use cases include:
  • Preventing PII leakage
  • Detecting and blocking prompt injection attacks
  • Blocking inappropriate or harmful content
  • Enforcing business rules and compliance requirements
  • Validating output quality and accuracy
You can implement guardrails using middleware to intercept execution at strategic points - before the agent starts, after it completes, or around model and tool calls.
Middleware flow diagram
Guardrails can be implemented using two complementary approaches:

Deterministic guardrails

Use rule-based logic like regex patterns, keyword matching, or explicit checks. Fast, predictable, and cost-effective, but may miss nuanced violations.

Model-based guardrails

Use LLMs or classifiers to evaluate content with semantic understanding. Catch subtle issues that rules miss, but are slower and more expensive.
LangChain provides both built-in guardrails (e.g., PII detection, human-in-the-loop) and a flexible middleware system for building custom guardrails using either approach.

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Built-in guardrails

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PII detection

LangChain provides built-in middleware for detecting and handling Personally Identifiable Information (PII) in conversations. This middleware can detect common PII types like emails, credit cards, IP addresses, and more. PII detection middleware is helpful for cases such as health care and financial applications with compliance requirements, customer service agents that need to sanitize logs, and generally any application handling sensitive user data. The PII middleware supports multiple strategies for handling detected PII:
StrategyDescriptionExample
redactReplace with [REDACTED_{PII_TYPE}][REDACTED_EMAIL]
maskPartially obscure (e.g., last 4 digits)****-****-****-1234
hashReplace with deterministic hasha8f5f167...
blockRaise exception when detectedError thrown
from langchain.agents import create_agent
from langchain.agents.middleware import PIIMiddleware


agent = create_agent(
    model="gpt-5.4",
    tools=[customer_service_tool, email_tool],
    middleware=[
        # Redact emails in user input before sending to model
        PIIMiddleware(
            "email",
            strategy="redact",
            apply_to_input=True,
        ),
        # Mask credit cards in user input
        PIIMiddleware(
            "credit_card",
            strategy="mask",
            apply_to_input=True,
        ),
        # Block API keys - raise error if detected
        PIIMiddleware(
            "api_key",
            detector=r"sk-[a-zA-Z0-9]{32}",
            strategy="block",
            apply_to_input=True,
        ),
    ],
)

# When user provides PII, it will be handled according to the strategy
result = agent.invoke({
    "messages": [{"role": "user", "content": "My email is john.doe@example.com and card is 5105-1051-0510-5100"}]
})
Built-in PII types:
  • email - Email addresses
  • credit_card - Credit card numbers (Luhn validated)
  • ip - IP addresses
  • mac_address - MAC addresses
  • url - URLs
Configuration options:
ParameterDescriptionDefault
pii_typeType of PII to detect (built-in or custom)Required
strategyHow to handle detected PII ("block", "redact", "mask", "hash")"redact"
detectorCustom detector function or regex patternNone (uses built-in)
apply_to_inputCheck user messages before model callTrue
apply_to_outputCheck AI messages after model callFalse
apply_to_tool_resultsCheck tool result messages after executionFalse
See the middleware documentation for complete details on PII detection capabilities.

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Human-in-the-loop

LangChain provides built-in middleware for requiring human approval before executing sensitive operations. This is one of the most effective guardrails for high-stakes decisions. Human-in-the-loop middleware is helpful for cases such as financial transactions and transfers, deleting or modifying production data, sending communications to external parties, and any operation with significant business impact. 
from langchain.agents import create_agent
from langchain.agents.middleware import HumanInTheLoopMiddleware
from langgraph.checkpoint.memory import InMemorySaver
from langgraph.types import Command


agent = create_agent(
    model="gpt-5.4",
    tools=[search_tool, send_email_tool, delete_database_tool],
    middleware=[
        HumanInTheLoopMiddleware(
            interrupt_on={
                # Require approval for sensitive operations
                "send_email": True,
                "delete_database": True,
                # Auto-approve safe operations
                "search": False,
            }
        ),
    ],
    # Persist the state across interrupts
    checkpointer=InMemorySaver(),
)

# Human-in-the-loop requires a thread ID for persistence
config = {"configurable": {"thread_id": "some_id"}}

# Agent will pause and wait for approval before executing sensitive tools
result = agent.invoke(
    {"messages": [{"role": "user", "content": "Send an email to the team"}]},
    config=config
)

result = agent.invoke(
    Command(resume={"decisions": [{"type": "approve"}]}),
    config=config  # Same thread ID to resume the paused conversation
)
See the human-in-the-loop documentation for complete details on implementing approval workflows.

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Custom guardrails

For more sophisticated guardrails, you can create custom middleware that runs before or after the agent executes. This gives you full control over validation logic, content filtering, and safety checks.

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Before agent guardrails

Use “before agent” hooks to validate requests once at the start of each invocation. This is useful for session-level checks like authentication, rate limiting, or blocking inappropriate requests before any processing begins. 
from typing import Any

from langchain.agents.middleware import AgentMiddleware, AgentState, hook_config
from langgraph.runtime import Runtime

class ContentFilterMiddleware(AgentMiddleware):
    """Deterministic guardrail: Block requests containing banned keywords."""

    def __init__(self, banned_keywords: list[str]):
        super().__init__()
        self.banned_keywords = [kw.lower() for kw in banned_keywords]

    @hook_config(can_jump_to=["end"])
    def before_agent(self, state: AgentState, runtime: Runtime) -> dict[str, Any] | None:
        # Get the first user message
        if not state["messages"]:
            return None

        first_message = state["messages"][0]
        if first_message.type != "human":
            return None

        content = first_message.content.lower()

        # Check for banned keywords
        for keyword in self.banned_keywords:
            if keyword in content:
                # Block execution before any processing
                return {
                    "messages": [{
                        "role": "assistant",
                        "content": "I cannot process requests containing inappropriate content. Please rephrase your request."
                    }],
                    "jump_to": "end"
                }

        return None

# Use the custom guardrail
from langchain.agents import create_agent

agent = create_agent(
    model="gpt-5.4",
    tools=[search_tool, calculator_tool],
    middleware=[
        ContentFilterMiddleware(
            banned_keywords=["hack", "exploit", "malware"]
        ),
    ],
)

# This request will be blocked before any processing
result = agent.invoke({
    "messages": [{"role": "user", "content": "How do I hack into a database?"}]
})

​
After agent guardrails

Use “after agent” hooks to validate final outputs once before returning to the user. This is useful for model-based safety checks, quality validation, or final compliance scans on the complete agent response. 
from langchain.agents.middleware import AgentMiddleware, AgentState, hook_config
from langgraph.runtime import Runtime
from langchain.messages import AIMessage
from langchain.chat_models import init_chat_model
from typing import Any

class SafetyGuardrailMiddleware(AgentMiddleware):
    """Model-based guardrail: Use an LLM to evaluate response safety."""

    def __init__(self):
        super().__init__()
        self.safety_model = init_chat_model("gpt-5.4-mini")

    @hook_config(can_jump_to=["end"])
    def after_agent(self, state: AgentState, runtime: Runtime) -> dict[str, Any] | None:
        # Get the final AI response
        if not state["messages"]:
            return None

        last_message = state["messages"][-1]
        if not isinstance(last_message, AIMessage):
            return None

        # Use a model to evaluate safety
        safety_prompt = f"""Evaluate if this response is safe and appropriate.
        Respond with only 'SAFE' or 'UNSAFE'.

        Response: {last_message.content}"""

        result = self.safety_model.invoke([{"role": "user", "content": safety_prompt}])

        if "UNSAFE" in result.content:
            last_message.content = "I cannot provide that response. Please rephrase your request."

        return None

# Use the safety guardrail
from langchain.agents import create_agent

agent = create_agent(
    model="gpt-5.4",
    tools=[search_tool, calculator_tool],
    middleware=[SafetyGuardrailMiddleware()],
)

result = agent.invoke({
    "messages": [{"role": "user", "content": "How do I make explosives?"}]
})

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Combine multiple guardrails

You can stack multiple guardrails by adding them to the middleware array. They execute in order, allowing you to build layered protection: 
from langchain.agents import create_agent
from langchain.agents.middleware import PIIMiddleware, HumanInTheLoopMiddleware

agent = create_agent(
    model="gpt-5.4",
    tools=[search_tool, send_email_tool],
    middleware=[
        # Layer 1: Deterministic input filter (before agent)
        ContentFilterMiddleware(banned_keywords=["hack", "exploit"]),

        # Layer 2: PII protection (before and after model)
        PIIMiddleware("email", strategy="redact", apply_to_input=True),
        PIIMiddleware("email", strategy="redact", apply_to_output=True),

        # Layer 3: Human approval for sensitive tools
        HumanInTheLoopMiddleware(interrupt_on={"send_email": True}),

        # Layer 4: Model-based safety check (after agent)
        SafetyGuardrailMiddleware(),
    ],
)

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