Using Evaluators

Evaluators are a way of validating that your AI task is running as expected. Simply put, an evaluator in relation to an AI task is a function that runs on the result - e.g. (input, output, expected) -> score.

Setup

Phoenix is vendor agnostic and thus doesn't require you to use any particular evals library. Because of this, the eval libraries for Phoenix are distributed as separate packages. The Phoenix eval libraries are very lightweight and provide many utilities to make evaluation simpler.

Python

pip install arize-phoenix-evals arize-phoenix-client

TypeScript

Phoenix supports two main types of evaluators: LLM Evaluators (which use an LLM as a judge) and Code Evaluators (which use deterministic functions). You can also define evaluators as simple functions that return a score. See Running Evaluators in Experiments for a complete example.

LLM Evaluators

LLM Evaluators are functions where an LLM as a judge performs the scoring of your AI task. LLM Evaluators are useful when you cannot express the scoring as simply a block of code (e.x. is the answer relevant to the question). With Phoenix you can either:

• Use and extend a pre-built evaluator

• Create a custom evaluator using the evals library

• Create your own LLM evaluator using your own tooling

Pre-built LLM Evaluators

Phoenix provides LLM evaluators out of the box. These evaluators are vendor agnostic and can be instantiated with any LLM provider:

Python

from phoenix.evals.metrics.hallucination import HallucinationEvaluator
from phoenix.evals import LLM

hallucination_evaluator = HallucinationEvaluator(llm=LLM(provider="openai", model="gpt-4o-mini"))

TypeScript

Note that pre-built evaluators rarely will work well for your specific AI task and should be used as starting points. Proceed with caution.

Custom LLM Evaluators

Phoenix eval libraries provide building blocks for you to build your own LLM-as-a-judge evaluators. You can create custom classification evaluators that use an LLM to classify outputs into categories with optional scores.

Python

from phoenix.evals import ClassificationEvaluator
from phoenix.evals import LLM

# Define a prompt template with mustache placeholders
HELPFULNESS_TEMPLATE = """Rate how helpful the response is to the question.

Question: {{input}}
Response: {{output}}

"helpful" means the response directly addresses the question.
"not_helpful" means the response does not address the question."""

# Define the classification choices (labels mapped to scores)
choices = {"not_helpful": 0, "helpful": 1}

# Create the custom evaluator
helpfulness_evaluator = ClassificationEvaluator(
    name="helpfulness",
    prompt_template=HELPFULNESS_TEMPLATE,
    llm=LLM(provider="openai", model="gpt-4o-mini"),
    choices=choices
)

TypeScript

Code Evaluators

Code evaluators are functions that evaluate the output of your LLM task that don't use another LLM as a judge. An example might be checking for whether or not a given output contains a link - which can be implemented as a RegEx match.

The simplest way to create a code evaluator is to write a function. By default, a function of one argument will be passed the output of an experiment run. These evaluators can either return a boolean or numeric value which will be recorded as the evaluation score.

Simple Code Evaluators

Python

Imagine our experiment is testing a task that is intended to output a numeric value from 1-100. We can write a simple evaluator to check if the output is within the allowed range:

def in_bounds(x):
    return 1 <= x <= 100

By simply passing the in_bounds function to run_experiment, we will automatically generate evaluations for each experiment run for whether or not the output is in the allowed range.

TypeScript

Code Evaluators with Multiple Parameters

More complex evaluations can use additional information. These values can be accessed by defining a function with specific parameter names which are bound to special values:

Parameter name	Description	Example
input	experiment run input	def eval(input): ...
output	experiment run output	def eval(output): ...
expected	example output	def eval(expected): ...
reference	alias for expected	def eval(reference): ...
metadata	experiment metadata	def eval(metadata): ...

These parameters can be used in any combination and any order to write custom complex evaluators!

Python

import json
import editdistance  # pip install editdistance

def edit_distance(output, expected) -> int:
    return editdistance.eval(
        json.dumps(output, sort_keys=True), 
        json.dumps(expected, sort_keys=True)
    )

TypeScript

Customizing Code Evaluators with create_evaluator

For better integration with the Experiments UI, use the create_evaluator function (or decorator in Python) to set display properties like the evaluator name and kind.

Python

from phoenix.evals import create_evaluator
import re

@create_evaluator(name="contains_link", kind="CODE")
def contains_link(output):
    pattern = r"https?://[^\s]+"
    return bool(re.search(pattern, output))

@create_evaluator(name="wordiness", kind="CODE")
def wordiness(expected, output):
    return len(output.split()) < len(expected.split())

TypeScript

Running Evaluators in Experiments

Evaluators are passed as a list to the evaluators parameter in run_experiment. You can use any combination of LLM evaluators, code evaluators, or simple functions.

Python

from phoenix.experiments import run_experiment
from phoenix.evals import create_evaluator

@create_evaluator(name="has_greeting", kind="CODE")
def has_greeting(output):
    return any(word in output.lower() for word in ["hello", "hi", "hey"])

def exact_match(output, expected):
    return output.strip() == expected.strip()

experiment = run_experiment(
    dataset=my_dataset,
    task=my_task,
    evaluators=[has_greeting, exact_match]
)

TypeScript