Run Experiments

Phoenix supports two workflows for experiments. Use the Playground UI to iterate quickly, or the SDK to run experiments in code.

Run Experiments with the SDK

Run experiments programmatically with tasks and evaluators in code.

Run Experiments in the UI

Configure prompts and evaluators in the Playground and compare results.

Run Experiments with the SDK

Setup

Make sure you have the Phoenix client and the instrumentors needed for the experiment setup. For this example we will use the OpenAI instrumentor to trace the LLM calls.

Python
TypeScript

pip install arize-phoenix-client arize-phoenix-otel openinference-instrumentation-openai openai datasets duckdb pandas

npm install @arizeai/phoenix-client

The key steps of running an experiment are:

Define/upload a Dataset (e.g. a dataframe)

Each record of the dataset is called an Example

Define a task

A task is a function that takes each Example and returns an output

Define Evaluators

An Evaluator is a function that evaluates the output for each Example

Run the experiment

We’ll start by initializing the Phoenix client to connect to your deployed Phoenix instance.

from phoenix.client import Client

# Initialize client - automatically reads from environment variables:
# PHOENIX_BASE_URL and PHOENIX_API_KEY (if using Phoenix Cloud)
client = Client()

# Or explicitly configure for your Phoenix instance:
# client = Client(base_url="https://your-phoenix-instance.com", api_key="your-api-key")

Load a Dataset

A dataset can be as simple as a list of strings inside a dataframe. More sophisticated datasets can be also extracted from traces based on actual production data. Here we just have a small list of questions that we want to ask an LLM about the NBA games: Create pandas dataframe

import pandas as pd

df = pd.DataFrame(
    {
        "question": [
            "Which team won the most games?",
            "Which team won the most games in 2015?",
            "Who led the league in 3 point shots?",
        ]
    }
)

The dataframe can be sent to Phoenix via the Client. input_keys and output_keys are column names of the dataframe, representing the input/output to the task in question. Here we have just questions, so we left the outputs blank: Upload dataset to Phoenix

dataset = client.datasets.create_dataset(
    name="nba-questions",
    dataframe=df,
    input_keys=["question"],
    output_keys=[],
)

Each row of the dataset is called an Example.

Create a Task

A task is any function/process that returns a JSON serializable output. Task can also be an async function, but we used sync function here for simplicity. If the task is a function of one argument, then that argument will be bound to the input field of the dataset example.

def task(x):
    return ...

For our example here, we’ll ask an LLM to build SQL queries based on our question, which we’ll run on a database and obtain a set of results: Set Up Database

import duckdb
from datasets import load_dataset

data = load_dataset("suzyanil/nba-data")["train"]
conn = duckdb.connect(database=":memory:", read_only=False)
conn.register("nba", data.to_pandas())

Set Up Prompt and LLM

from textwrap import dedent

import openai

# Create OpenAI client (separate from Phoenix client)
openai_client = openai.Client()
columns = conn.query("DESCRIBE nba").to_df().to_dict(orient="records")

LLM_MODEL = "gpt-4o"

columns_str = ",".join(column["column_name"] + ": " + column["column_type"] for column in columns)
system_prompt = dedent(f"""
You are a SQL expert, and you are given a single table named nba with the following columns:
{columns_str}\n
Write a SQL query corresponding to the user's
request. Return just the query text, with no formatting (backticks, markdown, etc.).""")


def generate_query(question):
    response = openai_client.chat.completions.create(
        model=LLM_MODEL,
        messages=[
            {"role": "system", "content": system_prompt},
            {"role": "user", "content": question},
        ],
    )
    return response.choices[0].message.content


def execute_query(query):
    return conn.query(query).fetchdf().to_dict(orient="records")


def text2sql(question):
    results = error = None
    query = None
    try:
        query = generate_query(question)
        results = execute_query(query)
    except duckdb.Error as e:
        error = str(e)
    return {"query": query, "results": results, "error": error}

Define task as a Function Recall that each row of the dataset is encapsulated as Example object. Recall that the input keys were defined when we uploaded the dataset:

def task(x):
    return text2sql(x["question"])

More complex task inputs More complex tasks can use additional information. These values can be accessed by defining a task function with specific parameter names which are bound to special values associated with the dataset example:

Parameter name	Description	Example
`input`	example input	`def task(input): ...`
`expected`	example output	`def task(expected): ...`
`reference`	alias for `expected`	`def task(reference): ...`
`metadata`	example metadata	`def task(metadata): ...`
`example`	`Example` object	`def task(example): ...`

A task can be defined as a sync or async function that takes any number of the above argument names in any order!

Define Evaluators

An evaluator is any function that takes the task output and return an assessment. Here we’ll simply check if the queries succeeded in obtaining any result from the database:

def no_error(output) -> bool:
    return not bool(output.get("error"))


def has_results(output) -> bool:
    return bool(output.get("results"))

Run an Experiment

Instrument OpenAI Instrumenting the LLM will also give us the spans and traces that will be linked to the experiment, and can be examined in the Phoenix UI:

from openinference.instrumentation.openai import OpenAIInstrumentor

from phoenix.otel import register

tracer_provider = register()
OpenAIInstrumentor().instrument(tracer_provider=tracer_provider)

Run the Task and Evaluators Running an experiment is as easy as calling run_experiment with the components we defined above. The results of the experiment will be show up in Phoenix:

Python
TypeScript

experiment = client.experiments.run_experiment(
    dataset=dataset,
    task=task,
    evaluators=[no_error, has_results]
)

import { createDataset } from "@arizeai/phoenix-client/datasets";
import { asExperimentEvaluator, runExperiment } from "@arizeai/phoenix-client/experiments";
import type { EvaluatorParams } from "@arizeai/phoenix-client/types/experiments";

const { datasetId } = await createDataset({
  name: "names-dataset",
  description: "a simple dataset of names",
  examples: [
    { input: { name: "John" }, output: { text: "Hello, John!" }, metadata: {} },
    { input: { name: "Jane" }, output: { text: "Hello, Jane!" }, metadata: {} },
  ],
});

const task = async (example) => `hello ${example.input.name}`;

const evaluators = [
  asExperimentEvaluator({
    name: "matches",
    kind: "CODE",
    evaluate: async ({ output, expected }: EvaluatorParams) => {
      const matches = output === expected?.text;
      return {
        label: matches ? "matches" : "does not match",
        score: matches ? 1 : 0,
        explanation: matches ? "output matches expected" : "output does not match expected",
        metadata: {},
      };
    },
  }),
  asExperimentEvaluator({
    name: "contains-hello",
    kind: "CODE",
    evaluate: async ({ output }: EvaluatorParams) => {
      const matches = typeof output === "string" && output.includes("hello");
      return {
        label: matches ? "contains hello" : "does not contain hello",
        score: matches ? 1 : 0,
        explanation: matches ? "output contains hello" : "output does not contain hello",
        metadata: {},
      };
    },
  }),
];

const experiment = await runExperiment({
  dataset: { datasetId },
  task,
  evaluators,
});

Add More Evaluations

If you want to attach more evaluations to the same experiment after the fact, you can do so with `evaluate_experiment`.

evaluators = [
    # add evaluators here
]
experiment = client.experiments.evaluate_experiment(
    experiment=experiment,
    evaluators=evaluators
)

If you no longer have access to the original experiment object, you can retrieve it from Phoenix using the get_experiment client method.

experiment_id = "experiment-id" # set your experiment ID here
experiment = client.experiments.get_experiment(experiment_id=experiment_id)
evaluators = [
    # add evaluators here
]
experiment = client.experiments.evaluate_experiment(
    experiment=experiment,
    evaluators=evaluators
)

Dry Run

Sometimes we may want to do a quick sanity check on the task function or the evaluators before unleashing them on the full dataset. run_experiment() and evaluate_experiment() both are equipped with a dry_run= parameter for this purpose: it executes the task and evaluators on a small subset without sending data to the Phoenix server. Setting dry_run=True selects one sample from the dataset, and setting it to a number, e.g. dry_run=3, selects multiple. The sampling is also deterministic, so you can keep re-running it for debugging purposes.

Run Experiments in the UI

Phoenix lets you run experiments directly from the UI using a dataset and prompt(s) in the Playground. The results are tracked as experiments attached to the dataset so you can compare runs over time.

Load a Dataset

Open Datasets and select the dataset you want to use.
Open the Playground and choose your dataset from the dataset selector.

Configure a Prompt

Define your prompt template and model settings.
If your dataset inputs are nested, set the Prompt variable path in the dataset settings (gear icon).

Run an Experiment

Click Run in the Playground. Phoenix runs your prompt across every dataset example and records results as an experiment. If your dataset has evaluators attached, Phoenix also scores each run and records the results as annotations.

Set a Baseline Experiment

You can mark one experiment on a dataset as the baseline — a persistent reference point that comparisons are measured against. Unlike selecting a baseline transiently in the compare view, this choice is saved with the dataset and survives navigation until you change or remove it. To set or remove a baseline from the experiments table:

Open a dataset and go to its Experiments tab.
Select a single experiment (baseline actions are only available when exactly one experiment is selected).
Open the experiment’s action menu (or use the selection toolbar) and choose Mark as baseline. To clear it, open the menu again and choose Remove baseline.

The current baseline is indicated by a baseline badge next to the experiment in the table. Setting a new baseline replaces any existing one — a dataset has at most one baseline at a time.

View Dataset Metrics

The dataset page has a Metrics tab that charts how your experiments trend over time, so you can spot regressions without opening each experiment. Open a dataset and select the Metrics tab (next to Experiments and Versions) to see bar charts across the dataset’s most recent experiments for:

Run latency — how long runs take
Error rate — the share of failed runs
Cost — token spend per experiment
Token usage — prompt and completion tokens

Use it to confirm a new prompt or model isn’t quietly getting slower, more expensive, or more error-prone than earlier experiments on the same dataset.

Review Failure Modes

After the run completes, open the experiment to understand where the prompt or model is underperforming.

Use the results table to sort and filter by evaluator scores (if you have evaluators attached).
Open examples with low scores (or incorrect categorical labels) to see the full input, output, and reference data.
If an issue is unclear, open the associated traces to inspect tool calls, model parameters, and intermediate steps.

If you do not have evaluators attached yet, start by scanning outputs and references for patterns, then add evaluators to encode those failure modes. This workflow helps you identify recurring failure patterns before you change your prompt or model.

Define Evaluators for the Issues You See

Once you identify a failure mode, encode it as an evaluator so Phoenix can track it across future experiments.

In the Playground experiment toolbar, open Evaluators.
Add an evaluator (LLM evaluator or built-in code evaluator).
Map its inputs from dataset fields, test it on a few examples, and save it.
Make sure the evaluator is selected, then re-run your experiment so it produces annotation columns in the results table.

Evaluator input mappings can reference input, output, reference, and metadata fields from your dataset examples. Use LLM evaluators for judgment-heavy issues like relevance, tone, or correctness. Use built-in code evaluators for deterministic checks like regex matching, exact match, or distance metrics. For more detail on evaluator configuration, see Dataset Evaluators and Using Evaluators.

Experiment Annotations and Optimization Direction

Evaluators configured in the UI produce annotations that are attached to experiment runs. These annotations power the experiment results table, comparison views, and summaries. In addition to the score itself, annotations carry metadata that helps Phoenix interpret the result. One of the most important metadata fields is optimization direction, which tells Phoenix whether higher is better, lower is better, or if there is no ordering. Phoenix uses this to visually indicate which runs are better or worse when comparing experiments. Experiment annotations with optimization direction

Experiment annotations with optimization direction

Optimization direction is set by the evaluator output configuration:

Maximize: higher scores indicate better outcomes (for example, faithfulness or correctness).
Minimize: lower scores indicate better outcomes (for example, distance or error rate).
None: no ordering (for example, categorical labels or freeform notes).

Get Started

Tracing

Evaluation

Datasets & Experiments

Prompts

Settings

Concepts

Resources

Run Experiments

Run Experiments with the SDK

Run Experiments in the UI

Run Experiments with the SDK

Setup

Load a Dataset

Create a Task

Define Evaluators

Run an Experiment

Add More Evaluations

If you want to attach more evaluations to the same experiment after the fact, you can do so with `evaluate_experiment`.

Dry Run

Run Experiments in the UI

Load a Dataset

Configure a Prompt

Run an Experiment

Set a Baseline Experiment

View Dataset Metrics

Review Failure Modes

Define Evaluators for the Issues You See

Experiment Annotations and Optimization Direction

Run Experiments with the SDK

Run Experiments in the UI

​Run Experiments with the SDK

​Setup

​Load a Dataset

​Create a Task

​Define Evaluators

​Run an Experiment

​Add More Evaluations

​If you want to attach more evaluations to the same experiment after the fact, you can do so with evaluate_experiment.

​Dry Run

​Run Experiments in the UI

​Load a Dataset

​Configure a Prompt

​Run an Experiment

​Set a Baseline Experiment

​View Dataset Metrics

​Review Failure Modes

​Define Evaluators for the Issues You See

​Experiment Annotations and Optimization Direction

Run Experiments with the SDK

Setup

Load a Dataset

Create a Task

Define Evaluators

Run an Experiment

Add More Evaluations

If you want to attach more evaluations to the same experiment after the fact, you can do so with `evaluate_experiment`.

Dry Run

Run Experiments in the UI

Load a Dataset

Configure a Prompt

Run an Experiment

Set a Baseline Experiment

View Dataset Metrics

Review Failure Modes

Define Evaluators for the Issues You See

Experiment Annotations and Optimization Direction