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Why use Code Evals?

When your evaluation criteria is deterministic and clear, code-based evaluators provide a consistent and efficient way to assess results. They are useful when you need to check for objective conditions, such as whether a keyword appears, a URL is valid, or a format follows a rule. Arize offers off-the-shelf code evaluators for common evaluation tasks. When you need more control, you can create custom evaluators that align with your unique business logic or quality criteria.

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Create a Code Eval

To create a code evaluator, choose Code Evaluator when creating a new Evaluation Task. Now, the evaluator can be created in just 3 steps:
  1. Name the task and define the data it will be run on.
  1. Sampling Rate (%): Define the percentage of data the task should run on (0–100).
    1. Sampling is applied at the highest evaluator scope in the task:
      session > trace > span
    2. Lower-level evaluators will run on all matching data within that sampled set
  2. Task Filters allow you to specify the data this task will run on. This matches spans, or traces/sessions that contain matching spans.
  3. When running on historical data, the maximum number of items is based on the highest eval scope
  1. Provide a unique Eval Column Name for the evaluator in plaintext. Ensure that this name is distinct from other evaluators across all tasks. Here, you can also set Evaluator Scope and Filters.
  2. Define any required parameters for the selected Code Evaluator
Code Evals Demo

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Arize Managed Code Evals

Arize manages a set of off-the-shelf code evaluators on your behalf. Simply select the evaluator name from a drop-down and the evaluator code will be provided. Users can customize the evaluators by specifying the arguments that should be passed in as parameters. Currently, we support all of the evaluators below and new evaluators can be added upon request.
Evaluator What it’s Used ForParameters
Matches RegexChecks whether the text matches a specified regex pattern
  • span attribute: which span attribute to look at
  • pattern: The regex pattern used for matching against the span attribute value.
EvalDescriptionParameters
Matches RegexChecks whether the text matches a specified regex pattern
  • span attribute: which span attribute to look at
  • pattern: The regex pattern used for matching against the span attribute value.
JSON ParseableChecks whether the LLM data is a valid JSON-parsable string
  • span attribute: which span attribute to look at
Contains any KeywordChecks whether any specified keywords are present in the LLM data
  • span attribute: which span attribute to look at
  • keywords: A list of keyword strings to search for in the span attribute. If any keyword matches, the evaluator will flag the data as a match.
Contains all Keywordschecks that all keywords are present
  • span attribute: which span attribute to look at
  • keywords: A list of keyword strings to search for in the span attribute. If any keyword matches, the evaluator will flag the data as a match.

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Custom Code Evaluators

Custom Code Evaluators are only available in Arize AX Enterprise. Request a demo here.
Custom Code Evals allow you to define your own evaluation logic in Python (with JavaScript coming soon) to score and label LLM traces based on span attributes. This is ideal for use cases that require highly customized and deterministic rules—such as business logic validation, structured output parsing, or expected keyword presence. Once you select CustomArizeEvaluator from the “Select an Eval” drop-down, you’ll define the logic in the right-hand panel of the task creation interface.
Custom Code Evals
Step 1: Imports Start by importing the necessary classes and functions you’ll need in your evaluator. This is easiest to do in full screen view, using the expand button on the top right of each code cell. Currently we support the packages listed below. If you need an additional package installed, please notify the customer support team and we’ll do our best to address your requirements! 
numpy
pandas
scipy
pyarrow
arize[Datasets]==7.25.7
pydantic==2.11.7
jellyfish==1.2.0
Step 2: Test in Code While it’s possible to write the code in the UI, it’s typically easier to iterate in a python script or Colab notebook. We provide the necessary starter code with a Test in Code button. The starter code will port over the span attributes, evaluator class and import sections from your task, with buttons to copy the code snippets and run the code over your own data.
Step 3: Update Evaluator Class and Span Attributes Once you’re seeing the desired results with your evaluation code, we recommend copy-pasting the updated CodeEvaluator child class into the UI, along with the span attributes and imports, if needed. You’re now ready to kick off your task!