Pydantic AI Guardrails

Guardrails for Pydantic AI 2.x, built as native capabilities.

Each guardrail is a pydantic_ai.capabilities.AbstractCapability. You add one the same way you add Pydantic AI's own FileSystem or Shell: drop it into Agent(capabilities=[...]). There is no wrapper class and no separate run method to learn.

from pydantic_ai import Agent
from pydantic_ai_guardrails import DetectPII, DetectPromptInjection, LimitCost

agent = Agent(
    'openai:gpt-4o',
    capabilities=[
        DetectPromptInjection(sensitivity='high'),
        DetectPII(),
        LimitCost(max_total_tokens=100_000),
    ],
)

result = await agent.run('Plan a trip to Lisbon')

Installation

pip install pydantic-ai-guardrails

Requires pydantic-ai>=2.0.0. Some guardrails use optional extras:

pip install pydantic-ai-guardrails[pii-detection,toxicity-detection,evals]

How a guardrail fails

The failure mode follows Pydantic AI's own conventions, so a guardrail behaves like the rest of the framework.

• Input guardrails run before the model and raise InputBlocked to stop the run.
• Output guardrails run on the result and raise ModelRetry by default, so the model rewrites a failing answer. Set on_fail='raise' to stop with OutputBlocked, or on_fail='warn' to log and continue.
• Tool guardrails raise ModelRetry when a tool is denied, so the model picks another path.
• LimitCost raises BudgetExceeded once token usage crosses the budget.

from pydantic_ai_guardrails import DetectPII, InputBlocked

agent = Agent('openai:gpt-4o', capabilities=[DetectPII()])

try:
    await agent.run('my SSN is 123-45-6789')
except InputBlocked as e:
    print(e.guardrail, e.reason)  # DetectPII  detected PII: ssn

Built-in guardrails

Import every guardrail from the package root.

Input

Output

Tool and cost

Custom guardrails

Pass your own check to InputGuardrail or OutputGuardrail. A check returns True to pass, False or a reason string to fail, or (passed, reason). Sync and async both work, and a context_guard variant receives the RunContext.

from pydantic_ai_guardrails import InputGuardrail, OutputGuardrail

agent = Agent(
    'openai:gpt-4o',
    capabilities=[
        InputGuardrail(guard=lambda text: 'DROP TABLE' not in text),
        OutputGuardrail(guard=lambda out: len(out) >= 20),
    ],
)

To build a reusable guardrail with its own fields, subclass InputGuardrailBase or OutputGuardrailBase and implement check.

from dataclasses import dataclass
from pydantic_ai_guardrails import InputGuardrailBase

@dataclass
class BlockLanguage(InputGuardrailBase):
    code: str = 'en'
    on_fail: str = 'raise'

    async def check(self, ctx, text):
        if detect_language(text) != self.code:
            return f'expected {self.code}'
        return None

Tool argument validation

For validation declared on the tool itself, use the args_validator helpers. They raise ModelRetry on failure, so the model corrects its own arguments.

from pydantic import BaseModel, Field
from pydantic_ai_guardrails import args_schema_validator

class WeatherArgs(BaseModel):
    location: str = Field(max_length=50)
    units: str = Field(pattern='^(celsius|fahrenheit)$')

@agent.tool(args_validator=args_schema_validator(WeatherArgs))
def get_weather(ctx, location: str, units: str = 'celsius') -> str: ...

args_custom_validator and args_allowlist_validator cover ad-hoc checks and value allowlists.

Config files

Describe a guardrail set in JSON or YAML and build it at startup.

# guardrails.yaml
version: 1
guardrails:
  - type: DetectPII
    config: {types: [ssn, credit_card]}
  - type: LimitOutputLength
    config: {min_chars: 10}
  - type: LimitCost
    config: {max_total_tokens: 50000}

from pydantic_ai_guardrails import build_guardrails, load_config

agent = Agent('openai:gpt-4o', capabilities=build_guardrails(load_config('guardrails.yaml')))

pydantic-evals integration

Wrap any pydantic-evals evaluator as an output guardrail through pydantic_ai_guardrails.evals.

from pydantic_ai_guardrails.evals import output_contains

agent = Agent('openai:gpt-4o', capabilities=[output_contains('thank you', case_sensitive=False)])

Migrating from 1.x

The v2 API renames the built-ins to capability classes and changes the failure model. See CHANGELOG.md for the full list. The short version:

• pii_detector() becomes DetectPII(), secret_redaction() becomes RedactSecrets(), and the rest follow the same pattern.
• CostGuard becomes LimitCost; ToolGuard becomes RestrictTools.
• Import from the package root instead of pydantic_ai_guardrails.shields.*.

License

MIT