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This guide shows how to contribute a evaluator directly to the agent-control repo. If you want to publish a standalone evaluator as a separate wheel, see Custom Evaluators. For a working reference, see the Galileo Luna-2.

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Quick Start

Pick an evaluator name. Everything else derives from this:
Example: evaluator = toxicity
  • Package module: agent_control_evaluators.toxicity
  • Entry point: toxicity
  • Evaluator class: ToxicityEvaluator
From the repo root: 
mkdir -p evaluators/builtin/src/agent_control_evaluators/toxicity
mkdir -p evaluators/builtin/tests

touch evaluators/builtin/src/agent_control_evaluators/toxicity/__init__.py
touch evaluators/builtin/src/agent_control_evaluators/toxicity/config.py
touch evaluators/builtin/src/agent_control_evaluators/toxicity/evaluator.py
touch evaluators/builtin/tests/test_toxicity.py
You’ll end up with: 
builtin/
├── pyproject.toml
├── src/agent_control_evaluators/
│   ├── __init__.py
│   └── toxicity/
│       ├── __init__.py
│       ├── config.py
│       └── evaluator.py
└── tests/
    └── test_toxicity.py

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Writing the Evaluator

Config — extend EvaluatorConfig with your evaluator’s settings: 
# toxicity/config.py
from pydantic import Field
from agent_control_evaluators import EvaluatorConfig

class ToxicityConfig(EvaluatorConfig):
    threshold: float = Field(default=0.7, ge=0.0, le=1.0)
    categories: list[str] = Field(default_factory=lambda: ["hate", "violence"])
Evaluator — extend Evaluator and decorate with @register_evaluator: 
# toxicity/evaluator.py
from typing import Any

from agent_control_evaluators import Evaluator, EvaluatorMetadata, register_evaluator
from agent_control_models import EvaluatorResult

from agent_control_evaluators.toxicity.config import ToxicityConfig

@register_evaluator
class ToxicityEvaluator(Evaluator[ToxicityConfig]):
    metadata = EvaluatorMetadata(
        name="toxicity",            # Must match entry point key exactly
        version="1.0.0",
        description="Toxicity detection",
        requires_api_key=False,
        timeout_ms=5000,
    )
    config_model = ToxicityConfig

    async def evaluate(self, data: Any) -> EvaluatorResult:
        if data is None:
            return EvaluatorResult(matched=False, confidence=1.0, message="No data")

        try:
            score = await self._score(str(data))
            return EvaluatorResult(
                matched=score >= self.config.threshold,
                confidence=score,
                message=f"Toxicity: {score:.2f}",
            )
        except Exception as e:
            # Fail-open on infrastructure errors
            return EvaluatorResult(
                matched=False,
                confidence=0.0,
                message=f"Failed: {e}",
                error=str(e),
            )

    async def _score(self, text: str) -> float:
        # Your API call or local logic here
        ...

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Register the Entry Point

Add the entry point to evaluators/builtin/pyproject.toml: 
[project.entry-points."agent_control.evaluators"]
"toxicity" = "agent_control_evaluators.toxicity:ToxicityEvaluator"
The entry point key (toxicity) must exactly match metadata.name in the evaluator class. Exports in toxicity/__init__.py: 
from agent_control_evaluators.toxicity.config import ToxicityConfig
from agent_control_evaluators.toxicity.evaluator import ToxicityEvaluator

__all__ = ["ToxicityEvaluator", "ToxicityConfig"]

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Testing

Write tests using Given/When/Then style. Cover at least three cases:
  1. Null input — returns matched=False, no error
  2. Normal evaluation — returns correct matched based on threshold
  3. Infrastructure failure — returns matched=False with error set (fail-open)
# tests/test_toxicity.py
import pytest
from agent_control_evaluators.toxicity import ToxicityEvaluator, ToxicityConfig

@pytest.fixture
def evaluator() -> ToxicityEvaluator:
    return ToxicityEvaluator(ToxicityConfig(threshold=0.5))

@pytest.mark.asyncio
async def test_none_input(evaluator):
    result = await evaluator.evaluate(None)
    assert result.matched is False
    assert result.error is None

@pytest.mark.asyncio
async def test_score_above_threshold_matches(evaluator, monkeypatch):
    async def _high(self, text):
        return 0.8

    monkeypatch.setattr(ToxicityEvaluator, "_score", _high)
    result = await evaluator.evaluate("test")
    assert result.matched is True
    assert result.error is None

@pytest.mark.asyncio
async def test_api_failure_fails_open(evaluator, monkeypatch):
    async def _fail(self, text):
        raise ConnectionError("timeout")

    monkeypatch.setattr(ToxicityEvaluator, "_score", _fail)
    result = await evaluator.evaluate("test")
    assert result.matched is False
    assert result.error is not None

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Rules to Know

Error handling — The error field is only for infrastructure failures (network errors, API 500s, missing credentials). If your evaluator ran and produced a judgment, that’s matched=True or matched=False — not an error. When error is set, matched must be False (fail-open). Thread safety — Evaluator instances are cached and reused across concurrent requests. Never store request-scoped state on self. Use local variables in evaluate(). Performance — Pre-compile patterns in __init__(). Use asyncio.to_thread() for CPU-bound work. Respect timeout_ms for external calls.

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Before You Submit

From the repo root: 
PKG=evaluators/builtin

# Lint, typecheck, test
(cd "$PKG" && uv run --extra dev ruff check --config ../../pyproject.toml src/)
(cd "$PKG" && uv run --extra dev mypy --config-file ../../pyproject.toml src/)
(cd "$PKG" && uv run pytest)

# Verify discovery works
(cd "$PKG" && uv run python -c "
from agent_control_evaluators import discover_evaluators, get_evaluator
discover_evaluators()
ev = get_evaluator('toxicity')
assert ev is not None, 'Discovery failed - entry point key does not match metadata.name'
print(f'OK: {ev.metadata.name}')
")