---
title: "Create a dataset"
description: "Create a dataset in the Judgment platform and define the schema every example must follow."
sidebar:
  label: "Create a dataset"
seo:
  title: "Create a Dataset | Judgment How-to"
  description: "Name a dataset, define its schema, and verify that it is ready for examples and offline tests."
---

Create a dataset when you need a durable collection of examples, production
traces, or sessions. At the end of this guide, the dataset exists with a schema
that is ready for its first row.

## Prerequisites

- A Judgment project where you can create datasets
- The fields each example needs
- A decision about whether tests will score saved traces, whole sessions, or
  fresh agent executions

## Create and define the dataset

1. ### Create the dataset

    Open **Datasets** and select **New Dataset**. Enter a unique name such as
    `Support regressions`, then select **Create**.

    The initial dialog creates an empty dataset. Schema definition happens on
    the dataset page.

2. ### Open the new dataset

    Select the dataset from the Datasets table. The empty page prompts you to
    define the fields before adding examples.

3. ### Define the schema

    Select **Define schema**. Start from a template or choose **Custom**:

    - **Trace only** stores one saved trace per row.
    - **Session only** stores one saved session per row.
    - **LLM Eval** maps trace input and output into structured fields and keeps
      the trace link.
    - **Full Trace** also maps trace metadata such as duration, cost, tags, and
      behaviors.

    For a custom input set, add fields such as `input` and
    `expected_output`. Use `string`, `number`, `boolean`, or `json` for literal
    data. Add one `trace` or `session` field when platform-started tests should
    score saved evidence.

    A fresh-agent SDK test may start from literal fields alone because the SDK
    attaches a newly generated trace for each example.

4. ### Save the schema

    Select **Save schema**. Check field names and types before adding the first
    example: once a dataset has ever contained an example, its schema is
    locked, even if every example is later removed.

## Verify the dataset

Open **Edit schema** and confirm that every field and type matches the intended
row shape. The dataset is ready when the schema is visible and the Examples tab
shows no rows yet.

## Troubleshooting

- If **Define schema** is unavailable, confirm that the dataset is not a
  read-only legacy dataset.
- If **Edit schema** has become **View schema**, the dataset has already held an
  example. Create a new dataset when the row contract must change.
- If a test config says that judges have nothing to evaluate, add a `trace` or
  `session` field before adding the first example, or run the agent through the
  SDK so every example receives a fresh trace.
- If trace mapping reports a type mismatch, use a field type that matches the
  selected trace field or correct the schema before it locks.

## Next step

[Add production traces to the
dataset](/documentation/datasets/production-traces), or use the [dataset SDK
reference](/sdk-reference/python/datasets/dataset_factory/DatasetFactory) to
add structured examples programmatically.