How to define a custom evaluator

Key concepts

• Evaluators

Custom evaluators are just functions that take a dataset example and the resulting application output, and return one or more metrics. These functions can be passed directly into evaluate() / aevaluate().

Basic example

Python

Requires langsmith>=0.2.0

from langsmith import evaluate

def correct(outputs: dict, reference_outputs: dict) -> bool:
"""Check if the answer exactly matches the expected answer."""
    return outputs["answer"] == reference_outputs["answer"]

def dummy_app(inputs: dict) -> dict:
    return {"answer": "hmm i'm not sure", "reasoning": "i didn't understand the question"}

results = evaluate(
    dummy_app,
    data="dataset_name",
    evaluators=[correct]
)

TypeScript

Requires langsmith>=0.2.9

import type { EvaluationResult } from "langsmith/evaluation";

const correct = async ({ outputs, referenceOutputs }: {
  outputs: Record<string, any>;
  referenceOutputs?: Record<string, any>;
}): Promise<EvaluationResult> => {
  const score = outputs?.answer === referenceOutputs?.answer;
  return { key: "correct", score };
}

Evaluator args

Custom evaluator functions must have specific argument names. They can take any subset of the following arguments:

• run: Run: The full Run object generated by the application on the given example.
• example: Example: The full dataset Example, including the example inputs, outputs (if available), and metdata (if available).
• inputs: dict: A dictionary of the inputs corresponding to a single example in a dataset.
• outputs: dict: A dictionary of the outputs generated by the application on the given inputs.
• reference_outputs/referenceOutputs: dict: A dictionary of the reference outputs associated with the example, if available.

For most use cases you'll only need inputs, outputs, and reference_outputs. run and example are useful only if you need some extra trace or example metadata outside of the actual inputs and outputs of the application.

When using JS/TS these should all be passed in as part of a single object argument.

Evaluator output

Custom evaluators are expected to return one of the following types:

Python and JS/TS

• dict: dicts of the form {"score" | "value": ..., "key": ...} allow you to customize the metric type ("score" for numerical and "value" for categorical) and metric name. This if useful if, for example, you want to log an integer as a categorical metric.

Python only

• int | float | bool: this is interepreted as an continuous metric that can be averaged, sorted, etc. The function name is used as the name of the metric.
• str: this is intepreted as a categorical metric. The function name is used as the name of the metric.
• list[dict]: return multiple metrics using a single function.

Additional examples

Python

Requires langsmith>=0.2.0

from langsmith import evaluate, wrappers
from langsmith.schemas import Run, Example
from openai import AsyncOpenAI
# Assumes you've installed pydantic.
from pydantic import BaseModel

# We can still pass in Run and Example objects if we'd like
def correct_old_signature(run: Run, example: Example) -> dict:
    """Check if the answer exactly matches the expected answer."""
    return {"key": "correct", "score": run.outputs["answer"] == example.outputs["answer"]}

# Just evaluate actual outputs
def concision(outputs: dict) -> int:
    """Score how concise the answer is. 1 is the most concise, 5 is the least concise."""
    return min(len(outputs["answer"]) // 1000, 4) + 1

# Use an LLM-as-a-judge
oai_client = wrappers.wrap_openai(AsyncOpenAI())

async def valid_reasoning(inputs: dict, outputs: dict) -> bool:
    """Use an LLM to judge if the reasoning and the answer are consistent."""

    instructions = """\

Given the following question, answer, and reasoning, determine if the reasoning for the \
answer is logically valid and consistent with question and the answer."""

    class Response(BaseModel):
        reasoning_is_valid: bool

    msg = f"Question: {inputs['question']}\nAnswer: {outputs['answer']}\nReasoning: {outputs['reasoning']}"
    response = await oai_client.beta.chat.completions.parse(
        model="gpt-4o-mini",
        messages=[{"role": "system", "content": instructions,}, {"role": "user", "content": msg}],
        response_format=Response
    )
    return response.choices[0].message.parsed.reasoning_is_valid

def dummy_app(inputs: dict) -> dict:
    return {"answer": "hmm i'm not sure", "reasoning": "i didn't understand the question"}

results = evaluate(
    dummy_app,
    data="dataset_name",
    evaluators=[correct_old_signature, concision, valid_reasoning]
)

TypeScript

import { Client } from "langsmith";
import { evaluate } from "langsmith/evaluation";
import { Run, Example } from "langsmith/schemas";
import OpenAI from "openai";

// Type definitions
interface AppInputs {
    question: string;
}

interface AppOutputs {
    answer: string;
    reasoning: string;
}

interface Response {
    reasoning_is_valid: boolean;
}

// Old signature evaluator
function correctOldSignature(run: Run, example: Example) {
    return {
        key: "correct",
        score: run.outputs?.["answer"] === example.outputs?.["answer"],
    };
}

// Output-only evaluator
function concision({ outputs }: { outputs: AppOutputs }) {
    return {
        key: "concision",
        score: Math.min(Math.floor(outputs.answer.length / 1000), 4) + 1,
    };
}

// LLM-as-judge evaluator
const openai = new OpenAI();

async function validReasoning({
    inputs,
    outputs
}: {
    inputs: AppInputs;
    outputs: AppOutputs;
}) {
    const instructions = `      Given the following question, answer, and reasoning, determine if the reasoning for the       answer is logically valid and consistent with question and the answer.`;

    const msg = `Question: ${inputs.question}
Answer: ${outputs.answer}\nReasoning: ${outputs.reasoning}`;

    const response = await openai.chat.completions.create({
        model: "gpt-4",
        messages: [
            { role: "system", content: instructions },
            { role: "user", content: msg }
        ],
        response_format: { type: "json_object" },
        functions: [{
        name: "parse_response",
        parameters: {
            type: "object",
            properties: {
            reasoning_is_valid: {
                type: "boolean",
                description: "Whether the reasoning is valid"
            }
            },
            required: ["reasoning_is_valid"]
        }
        }]
    });

    const parsed = JSON.parse(response.choices[0].message.content ?? "{}") as Response;

    return {
        key: "valid_reasoning",
        score: parsed.reasoning_is_valid ? 1 : 0
    };
}

// Example application
function dummyApp(inputs: AppInputs): AppOutputs {
    return {
        answer: "hmm i'm not sure",
        reasoning: "i didn't understand the question"
    };
}

const results = await evaluate(dummyApp, {
        data: "dataset_name",
        evaluators: [correctOldSignature, concision, validReasoning],
        client: new Client()
});

Related

• Evaluate aggregate experiment results: Define summary evaluators, which compute metrics for an entire experiment.
• Run an evaluation comparing two experiments: Define pairwise evaluators, which compute metrics by comparing two (or more) experiments against each other.