Custom Evaluator

Overview

The Strands Evals SDK allows you to create custom evaluators by extending the base Evaluator class. This enables you to implement domain-specific evaluation logic tailored to your unique requirements. A complete example can be found here.

When to Create a Custom Evaluator

Create a custom evaluator when:

• Built-in evaluators don’t meet your specific needs
• You need specialized evaluation logic for your domain
• You want to integrate external evaluation services
• You need custom scoring algorithms
• You require specific data processing or analysis

Base Evaluator Class

All evaluators inherit from the base Evaluator class, which provides the structure for evaluation:

from strands_evals.evaluators import Evaluator
from strands_evals.types.evaluation import EvaluationData, EvaluationOutput
from typing_extensions import TypeVar

InputT = TypeVar("InputT")
OutputT = TypeVar("OutputT")

class CustomEvaluator(Evaluator[InputT, OutputT]):
    def __init__(self, custom_param: str):
        super().__init__()
        self.custom_param = custom_param

    def evaluate(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        """Synchronous evaluation implementation"""
        # Your evaluation logic here
        pass

    async def evaluate_async(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        """Asynchronous evaluation implementation"""
        # Your async evaluation logic here
        pass

Required Methods

evaluate(evaluation_case: EvaluationData) -> list[EvaluationOutput]

Synchronous evaluation method that must be implemented.

Parameters:

• evaluation_case: Contains input, output, expected values, and trajectory

Returns:

• List of EvaluationOutput objects with scores and reasoning

evaluate_async(evaluation_case: EvaluationData) -> list[EvaluationOutput]

Asynchronous evaluation method that must be implemented.

Parameters:

• Same as evaluate()

Returns:

• Same as evaluate()

EvaluationData Structure

The evaluation_case parameter provides:

• input: The input to the task
• actual_output: The actual output from the agent
• expected_output: The expected output (if provided)
• actual_trajectory: The execution trajectory (if captured)
• expected_trajectory: The expected trajectory (if provided)
• actual_interactions: Interactions between agents (if applicable)
• expected_interactions: Expected interactions (if provided)

EvaluationOutput Structure

Your evaluator should return EvaluationOutput objects with:

• score: Float between 0.0 and 1.0
• test_pass: Boolean indicating pass/fail
• reason: String explaining the evaluation
• label: Optional categorical label

Example: Simple Custom Evaluator

from strands_evals.evaluators import Evaluator
from strands_evals.types.evaluation import EvaluationData, EvaluationOutput
from typing_extensions import TypeVar

InputT = TypeVar("InputT")
OutputT = TypeVar("OutputT")

class LengthEvaluator(Evaluator[InputT, OutputT]):
    """Evaluates if output length is within acceptable range."""

    def __init__(self, min_length: int, max_length: int):
        super().__init__()
        self.min_length = min_length
        self.max_length = max_length

    def evaluate(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        output_text = str(evaluation_case.actual_output)
        length = len(output_text)

        if self.min_length <= length <= self.max_length:
            score = 1.0
            test_pass = True
            reason = f"Output length {length} is within acceptable range [{self.min_length}, {self.max_length}]"
        else:
            score = 0.0
            test_pass = False
            reason = f"Output length {length} is outside acceptable range [{self.min_length}, {self.max_length}]"

        return [EvaluationOutput(score=score, test_pass=test_pass, reason=reason)]

    async def evaluate_async(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        # For simple evaluators, async can just call sync version
        return self.evaluate(evaluation_case)

Example: LLM-Based Custom Evaluator

from strands import Agent
from strands_evals.evaluators import Evaluator
from strands_evals.types.evaluation import EvaluationData, EvaluationOutput
from typing_extensions import TypeVar

InputT = TypeVar("InputT")
OutputT = TypeVar("OutputT")

class ToneEvaluator(Evaluator[InputT, OutputT]):
    """Evaluates the tone of agent responses."""

    def __init__(self, expected_tone: str, model: str = None):
        super().__init__()
        self.expected_tone = expected_tone
        self.model = model

    def evaluate(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        judge = Agent(
            model=self.model,
            system_prompt=f"""
            Evaluate if the response has a {self.expected_tone} tone.
            Score 1.0 if tone matches perfectly.
            Score 0.5 if tone is partially appropriate.
            Score 0.0 if tone is inappropriate.
            """,
            callback_handler=None
        )

        prompt = f"""
        Input: {evaluation_case.input}
        Response: {evaluation_case.actual_output}

        Evaluate the tone of the response.
        """

        result = judge.structured_output(EvaluationOutput, prompt)
        return [result]

    async def evaluate_async(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        judge = Agent(
            model=self.model,
            system_prompt=f"""
            Evaluate if the response has a {self.expected_tone} tone.
            Score 1.0 if tone matches perfectly.
            Score 0.5 if tone is partially appropriate.
            Score 0.0 if tone is inappropriate.
            """,
            callback_handler=None
        )

        prompt = f"""
        Input: {evaluation_case.input}
        Response: {evaluation_case.actual_output}

        Evaluate the tone of the response.
        """

        result = await judge.structured_output_async(EvaluationOutput, prompt)
        return [result]

Example: Metric-Based Custom Evaluator

from strands_evals.evaluators import Evaluator
from strands_evals.types.evaluation import EvaluationData, EvaluationOutput
from typing_extensions import TypeVar
import re

InputT = TypeVar("InputT")
OutputT = TypeVar("OutputT")

class KeywordPresenceEvaluator(Evaluator[InputT, OutputT]):
    """Evaluates if required keywords are present in output."""

    def __init__(self, required_keywords: list[str], case_sensitive: bool = False):
        super().__init__()
        self.required_keywords = required_keywords
        self.case_sensitive = case_sensitive

    def evaluate(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        output_text = str(evaluation_case.actual_output)
        if not self.case_sensitive:
            output_text = output_text.lower()
            keywords = [k.lower() for k in self.required_keywords]
        else:
            keywords = self.required_keywords

        found_keywords = [kw for kw in keywords if kw in output_text]
        missing_keywords = [kw for kw in keywords if kw not in output_text]

        score = len(found_keywords) / len(keywords) if keywords else 1.0
        test_pass = score == 1.0

        if test_pass:
            reason = f"All required keywords found: {found_keywords}"
        else:
            reason = f"Missing keywords: {missing_keywords}. Found: {found_keywords}"

        return [EvaluationOutput(
            score=score,
            test_pass=test_pass,
            reason=reason,
            label=f"{len(found_keywords)}/{len(keywords)} keywords"
        )]

    async def evaluate_async(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        return self.evaluate(evaluation_case)

Using Custom Evaluators

from strands_evals import Case, Experiment

# Create test cases
test_cases = [
    Case[str, str](
        name="test-1",
        input="Write a professional email",
        metadata={"category": "email"}
    ),
]

# Use custom evaluator
evaluator = ToneEvaluator(expected_tone="professional")

# Run evaluation
experiment = Experiment[str, str](cases=test_cases, evaluators=[evaluator])
reports = experiment.run_evaluations(task_function)
reports[0].run_display()

Best Practices

1. Inherit from Base Evaluator: Always extend the Evaluator class
2. Implement Both Methods: Provide both sync and async implementations
3. Return List: Always return a list of EvaluationOutput objects
4. Provide Clear Reasoning: Include detailed explanations in the reason field
5. Use Appropriate Scores: Keep scores between 0.0 and 1.0
6. Handle Edge Cases: Account for missing or malformed data
7. Document Parameters: Clearly document what your evaluator expects
8. Test Thoroughly: Validate your evaluator with diverse test cases

Advanced: Multi-Level Evaluation

class MultiLevelEvaluator(Evaluator[InputT, OutputT]):
    """Evaluates at multiple levels (e.g., per tool call)."""

    def evaluate(self, evaluation_case: EvaluationData[InputT, OutputT]) -> list[EvaluationOutput]:
        results = []

        # Evaluate each tool call in trajectory
        if evaluation_case.actual_trajectory:
            for tool_call in evaluation_case.actual_trajectory:
                # Evaluate this tool call
                score = self._evaluate_tool_call(tool_call)
                results.append(EvaluationOutput(
                    score=score,
                    test_pass=score >= 0.5,
                    reason=f"Tool call evaluation: {tool_call}"
                ))

        return results

    def _evaluate_tool_call(self, tool_call):
        # Your tool call evaluation logic
        return 1.0

Related Documentation

• OutputEvaluator: LLM-based output evaluation with custom rubrics
• TrajectoryEvaluator: Sequence-based evaluation
• Evaluator Base Class: Core evaluator interface