Start an experiment

Starts a draft experiment, or resumes a paused one: each of its paywall locations starts splitting end users between the arms, and each feature flag it sets takes the value of the end user's arm.

Guide: Feature flags and A/B tests

Errors

Status	`type`	`code`	When
409	`conflict`	`experiment_already_running`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_concluded`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_archived`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_not_startable`	When a placed paywall has no deployed build, a location runs another experiment, or another running or paused experiment sets one of its flags.

It can also fail with the errors every operation shares, such as unauthenticated; see Errors.

More in Experiments

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Guide: Feature flags and A/B tests

Errors

Status	`type`	`code`	When
409	`conflict`	`experiment_already_running`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_concluded`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_archived`	When the experiment is running, concluded or archived.
409	`conflict`	`experiment_not_startable`	When a placed paywall has no deployed build, a location runs another experiment, or another running or paused experiment sets one of its flags.

It can also fail with the errors every operation shares, such as unauthenticated; see Errors.

More in Experiments

Examples

curl -X POST "https://api.voidhash.com/api/v1/projects/proj_tz4a98xxat96iws9zmbrgj3a/experiments/exp_c6v8b3n5q0w2e7h7d2k9m4x1/start" \  -H "Authorization: Bearer vh_sk_..."

{  "id": "exp_c6v8b3n5q0w2e7h7d2k9m4x1",  "projectId": "proj_tz4a98xxat96iws9zmbrgj3a",  "name": "Onboarding paywall pricing",  "description": "The monthly-first paywall against the annual-first one after onboarding.",  "hypothesis": "Leading with the annual plan brings more new subscriptions.",  "status": "running",  "primaryMetricEventName": "$subscription.created",  "secondaryMetricEventNames": [],  "paywallLocationIds": [    "pw_loc_m4x1c6v8b3n5q0w2e7h7d2k9"  ],  "testedFeatureFlagIds": [],  "winningVariantId": null,  "featureFlagId": "ff_k9m4x1c6v8b3n5q0w2e7h7d2",  "isArchived": false,  "startedAt": "2026-03-02T09:00:00.000Z",  "endedAt": null,  "createdAt": "2026-02-27T15:20:00.000Z",  "updatedAt": "2026-03-02T09:00:00.000Z"}

{  "error": {    "type": "invalid_request",    "code": "string",    "message": "string",    "param": "string"  }}

{  "error": {    "type": "unauthenticated",    "code": "invalid_api_key",    "message": "This secret key is not valid."  }}

{  "error": {    "type": "permission_denied",    "code": "permission_denied",    "message": "Insufficient permissions."  }}

{  "error": {    "type": "not_found",    "code": "project_not_found",    "message": "There is no project proj_tz4a98xxat96iws9zmbrgj3a, or this credential cannot use it.",    "param": "projectId"  }}

{  "error": {    "type": "conflict",    "code": "experiment_already_running",    "message": "Experiment exp_c6v8b3n5q0w2e7h7d2k9m4x1 is already running."  }}

{  "error": {    "type": "internal",    "code": "internal_error",    "message": "Something went wrong on our side. Try again, and contact support with the Request-Id header if it keeps failing."  }}

Start an experiment

Errors

More in Experiments

Start an experiment

Errors

More in Experiments

Authorization

Path parameters

Examples

Response body

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`

Start an experiment

Start an experiment

experiment_already_running

experiment_concluded

experiment_archived

experiment_not_startable

200application/json

400application/json

401application/json

403application/json

404application/json

409application/json

500application/json

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`

`application/json`