---
title: "Create holdout"
method: POST
path: "/api/v2/projects/{projectKey}/environments/{environmentKey}/holdouts"
tags: ["Holdouts (beta)"]
---

# Create holdout

`POST /api/v2/projects/{projectKey}/environments/{environmentKey}/holdouts`

Create a new holdout in the specified project.

## Path parameters

- `projectKey` string, string, required — The project key
- `environmentKey` string, string, required — The environment key

## Request body

- HoldoutPostRequest
  - `name` string — A human-friendly name for the holdout
  - `key` string — A key that identifies the holdout
  - `description` string — Description of the holdout
  - `randomizationunit` string — The chosen randomization unit for the holdout base experiment
  - `attributes` string[] — The attributes that the holdout iteration's results can be sliced by
  - `holdoutamount` string — Audience allocation for the holdout
  - `primarymetrickey` string — The key of the primary metric for this holdout
  - `metrics` MetricInput[] — Details on the metrics for this experiment
    - `key` string, required — The metric key
    - `isGroup` boolean — Whether this is a metric group (true) or a metric (false). Defaults to false
    - `primary` boolean — Deprecated, use <code>primarySingleMetricKey</code> and <code>primaryFunnelKey</code>. Whether this is a primary metric (true) or a secondary metric (false)
  - `prerequisiteflagkey` string — The key of the flag that the holdout is dependent on
  - `maintainerId` string — Maintainer id

## Response `201`

Holdout response

- HoldoutRep
  - `_id` string, required
  - `status` 'created' | 'enabled' | 'running' | 'ended', required
  - `description` string
  - `holdoutAmount` string, required — The percentage of traffic allocated to this holdout.
  - `createdAt` integer, required
  - `updatedAt` integer, required
  - `baseExperiment` Experiment, required
    - `_id` string — The experiment ID
    - `key` string, required — The experiment key
    - `name` string, required — The experiment name
    - `description` string — The experiment description
    - `_maintainerId` string, required — The ID of the member who maintains this experiment.
    - `_creationDate` integer, required
    - `environmentKey` string, required
    - `archivedDate` integer
    - `tags` string[] — Tags for the experiment
    - `_links` object, required — The location and content type of related resources
    - `holdoutId` string — The holdout ID
    - `currentIteration` IterationRep
      - `_id` string — The iteration ID
      - `hypothesis` string, required — The expected outcome of this experiment
      - `status` 'not_started' | 'running' | 'stopped', required — The status of the iteration: <code>not_started</code>, <code>running</code>, <code>stopped</code>
      - `createdAt` integer, required
      - `startedAt` integer
      - `endedAt` integer
      - `winningTreatmentId` string — The ID of the treatment chosen when the experiment stopped
      - `winningReason` string — The reason you stopped the experiment
      - `canReshuffleTraffic` boolean — Whether the experiment may reassign traffic to different variations when the experiment audience changes (true) or must keep all traffic assigned to its initial variation (false).
      - `flags` object — Details on the flag used in this experiment
      - `reallocationFrequencyMillis` integer — The cadence (in milliseconds) to update the allocation. Only present for multi-armed bandits.
      - `version` integer — The current version that the iteration is on
      - `primaryMetric` DependentMetricOrMetricGroupRep
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `primarySingleMetric` MetricV2Rep
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `primaryFunnel` DependentMetricGroupRepWithMetrics
        - `key` string, required — A unique key to reference the metric group
        - `name` string, required — A human-friendly name for the metric group
        - `kind` 'funnel' | 'standard' | 'guardrail', required — The type of the metric group
        - `_links` object, required — The location and content type of related resources
        - `metrics` MetricInGroupRep[] — The metrics in the metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `randomizationUnit` string — The unit of randomization for this iteration
      - `attributes` string[] — The available attribute filters for this iteration
      - `treatments` TreatmentRep[] — Details on the variations you are testing in the experiment
        - `_id` string — The treatment ID. This is the variation ID from the flag.
        - `name` string, required — The treatment name. This is the variation name from the flag.
        - `allocationPercent` string, required — The percentage of traffic allocated to this treatment during the iteration
        - `baseline` boolean — Whether this treatment is the baseline to compare other treatments against
        - `parameters` ParameterRep[] — Details on the flag and variation used for this treatment
          - `variationId` string
          - `flagKey` string
      - `secondaryMetrics` MetricV2Rep[] — Deprecated, use <code>metrics</code> instead. Details on the secondary metrics for this experiment.
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `metrics` DependentMetricOrMetricGroupRep[] — Details on the metrics for this experiment
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `layerSnapshot` LayerSnapshotRep
        - `key` string, required — Key of the layer the experiment was part of
        - `name` string, required — Layer name at the time this experiment iteration was stopped
        - `reservationPercent` integer, required — Percent of layer traffic that was reserved in the layer for this experiment iteration
        - `otherReservationPercent` integer, required — Percent of layer traffic that was reserved for other experiments in the same environment, when this experiment iteration was stopped
    - `draftIteration` IterationRep
      - `_id` string — The iteration ID
      - `hypothesis` string, required — The expected outcome of this experiment
      - `status` 'not_started' | 'running' | 'stopped', required — The status of the iteration: <code>not_started</code>, <code>running</code>, <code>stopped</code>
      - `createdAt` integer, required
      - `startedAt` integer
      - `endedAt` integer
      - `winningTreatmentId` string — The ID of the treatment chosen when the experiment stopped
      - `winningReason` string — The reason you stopped the experiment
      - `canReshuffleTraffic` boolean — Whether the experiment may reassign traffic to different variations when the experiment audience changes (true) or must keep all traffic assigned to its initial variation (false).
      - `flags` object — Details on the flag used in this experiment
      - `reallocationFrequencyMillis` integer — The cadence (in milliseconds) to update the allocation. Only present for multi-armed bandits.
      - `version` integer — The current version that the iteration is on
      - `primaryMetric` DependentMetricOrMetricGroupRep
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `primarySingleMetric` MetricV2Rep
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `primaryFunnel` DependentMetricGroupRepWithMetrics
        - `key` string, required — A unique key to reference the metric group
        - `name` string, required — A human-friendly name for the metric group
        - `kind` 'funnel' | 'standard' | 'guardrail', required — The type of the metric group
        - `_links` object, required — The location and content type of related resources
        - `metrics` MetricInGroupRep[] — The metrics in the metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `randomizationUnit` string — The unit of randomization for this iteration
      - `attributes` string[] — The available attribute filters for this iteration
      - `treatments` TreatmentRep[] — Details on the variations you are testing in the experiment
        - `_id` string — The treatment ID. This is the variation ID from the flag.
        - `name` string, required — The treatment name. This is the variation name from the flag.
        - `allocationPercent` string, required — The percentage of traffic allocated to this treatment during the iteration
        - `baseline` boolean — Whether this treatment is the baseline to compare other treatments against
        - `parameters` ParameterRep[] — Details on the flag and variation used for this treatment
          - `variationId` string
          - `flagKey` string
      - `secondaryMetrics` MetricV2Rep[] — Deprecated, use <code>metrics</code> instead. Details on the secondary metrics for this experiment.
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `metrics` DependentMetricOrMetricGroupRep[] — Details on the metrics for this experiment
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `layerSnapshot` LayerSnapshotRep
        - `key` string, required — Key of the layer the experiment was part of
        - `name` string, required — Layer name at the time this experiment iteration was stopped
        - `reservationPercent` integer, required — Percent of layer traffic that was reserved in the layer for this experiment iteration
        - `otherReservationPercent` integer, required — Percent of layer traffic that was reserved for other experiments in the same environment, when this experiment iteration was stopped
    - `previousIterations` IterationRep[] — Details on the previous iterations for this experiment.
      - `_id` string — The iteration ID
      - `hypothesis` string, required — The expected outcome of this experiment
      - `status` 'not_started' | 'running' | 'stopped', required — The status of the iteration: <code>not_started</code>, <code>running</code>, <code>stopped</code>
      - `createdAt` integer, required
      - `startedAt` integer
      - `endedAt` integer
      - `winningTreatmentId` string — The ID of the treatment chosen when the experiment stopped
      - `winningReason` string — The reason you stopped the experiment
      - `canReshuffleTraffic` boolean — Whether the experiment may reassign traffic to different variations when the experiment audience changes (true) or must keep all traffic assigned to its initial variation (false).
      - `flags` object — Details on the flag used in this experiment
      - `reallocationFrequencyMillis` integer — The cadence (in milliseconds) to update the allocation. Only present for multi-armed bandits.
      - `version` integer — The current version that the iteration is on
      - `primaryMetric` DependentMetricOrMetricGroupRep
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `primarySingleMetric` MetricV2Rep
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `primaryFunnel` DependentMetricGroupRepWithMetrics
        - `key` string, required — A unique key to reference the metric group
        - `name` string, required — A human-friendly name for the metric group
        - `kind` 'funnel' | 'standard' | 'guardrail', required — The type of the metric group
        - `_links` object, required — The location and content type of related resources
        - `metrics` MetricInGroupRep[] — The metrics in the metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `randomizationUnit` string — The unit of randomization for this iteration
      - `attributes` string[] — The available attribute filters for this iteration
      - `treatments` TreatmentRep[] — Details on the variations you are testing in the experiment
        - `_id` string — The treatment ID. This is the variation ID from the flag.
        - `name` string, required — The treatment name. This is the variation name from the flag.
        - `allocationPercent` string, required — The percentage of traffic allocated to this treatment during the iteration
        - `baseline` boolean — Whether this treatment is the baseline to compare other treatments against
        - `parameters` ParameterRep[] — Details on the flag and variation used for this treatment
          - `variationId` string
          - `flagKey` string
      - `secondaryMetrics` MetricV2Rep[] — Deprecated, use <code>metrics</code> instead. Details on the secondary metrics for this experiment.
        - `key` string, required — The metric key
        - `_versionId` string — The version ID of the metric
        - `name` string, required — The metric name
        - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
      - `metrics` DependentMetricOrMetricGroupRep[] — Details on the metrics for this experiment
        - `key` string, required — A unique key to reference the metric or metric group
        - `_versionId` string, required — The version ID of the metric or metric group
        - `name` string, required — A human-friendly name for the metric or metric group
        - `kind` 'pageview' | 'click' | 'custom' | 'funnel' | 'standard' | 'guardrail', required — If this is a metric, then it represents the kind of event the metric tracks. If this is a metric group, then it represents the group type
        - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
        - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
        - `_links` object, required — The location and content type of related resources
        - `isGroup` boolean, required — Whether this is a metric group or a metric
        - `metrics` MetricInGroupRep[] — An ordered list of the metrics in this metric group
          - `key` string, required — The metric key
          - `_versionId` string — The version ID of the metric
          - `name` string, required — The metric name
          - `kind` 'pageview' | 'click' | 'custom', required — The kind of event the metric tracks
          - `isNumeric` boolean — For custom metrics, whether to track numeric changes in value against a baseline (<code>true</code>) or to track a conversion when an end user takes an action (<code>false</code>).
          - `unitAggregationType` 'sum' | 'average' — The type of unit aggregation to use for the metric
          - `eventKey` string — The event key sent with the metric. Only relevant for custom metrics.
          - `_links` object, required — The location and content type of related resources
          - `nameInGroup` string — Name of the metric when used within the associated metric group. Can be different from the original name of the metric. Required if and only if the metric group is a <code>funnel</code>.
          - `randomizationUnits` string[] — The randomization units for the metric
      - `layerSnapshot` LayerSnapshotRep
        - `key` string, required — Key of the layer the experiment was part of
        - `name` string, required — Layer name at the time this experiment iteration was stopped
        - `reservationPercent` integer, required — Percent of layer traffic that was reserved in the layer for this experiment iteration
        - `otherReservationPercent` integer, required — Percent of layer traffic that was reserved for other experiments in the same environment, when this experiment iteration was stopped
  - `experiments` RelatedExperimentRep[]
    - `key` string
    - `name` string
    - `environment` string

## Other responses

- `400` — Invalid request

## Changes

- **2026-02-28** `69c5c9aafe78` — 9 warning, 28 info
  - added the new `not_started` enum value to the `baseExperiment/currentIteration/status` response property for the response status `201`
  - added the new `not_started` enum value to the `baseExperiment/draftIteration/status` response property for the response status `201`
  - added the new `not_started` enum value to the `baseExperiment/previousIterations/items/status` response property for the response status `201`
  - added the new `running` enum value to the `baseExperiment/currentIteration/status` response property for the response status `201`
  - …33 more
- **2025-07-08** `b2724c5174d1` — 9 warning
  - added the new `guardrail` enum value to the `baseExperiment/currentIteration/metrics/items/kind` response property for the response status `201`
  - added the new `guardrail` enum value to the `baseExperiment/currentIteration/primaryFunnel/kind` response property for the response status `201`
  - added the new `guardrail` enum value to the `baseExperiment/currentIteration/primaryMetric/kind` response property for the response status `201`
  - added the new `guardrail` enum value to the `baseExperiment/draftIteration/metrics/items/kind` response property for the response status `201`
  - …5 more

[Change history](https://skmtc.dev/launchdarkly/apis/launchdarkly-rest-api/changes/api/v2/projects/:projectKey/environments/:environmentKey/holdouts/post.md)

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[API](https://skmtc.dev/launchdarkly/apis/launchdarkly-rest-api.md) · [All operations](https://skmtc.dev/launchdarkly/apis/launchdarkly-rest-api/llms.txt) · [OpenAPI document](https://skmtc-service-production.skmtc.workers.dev/v1/apis/launchdarkly/launchdarkly-rest-api/revisions/69c5c9aafe78/schema)