Start an experiment
Starts a draft experiment, or resumes a paused one: each of its paywall locations starts splitting end users between the arms.
Guide: Feature flags and A/B tests
AuthorizationBearer <token>
A project secret key: vh_sk_... for production, vh_sk_test_... for test. Keep it on your servers.
In: header
Path parameters
projectId*string
The project's id, proj_.... A secret key works only in the project it belongs to.
experimentId*string
The experiment's id, exp_....
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" ], "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." }}Response body
application/json
application/json
application/json
application/json
application/json
application/json
application/json
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 or a location runs another experiment. |
It can also fail with the errors every operation shares, such as unauthenticated; see Errors.
More in Experiments
- List experiments
GET - Create an experiment
POST - Retrieve an experiment
GET - Update an experiment
PATCH - Archive an experiment
POST - Unarchive an experiment
POST - Pause an experiment
POST - Conclude an experiment
POST - List an experiment's arms
GET - Replace an experiment's arms
PUT - Retrieve an experiment's results
GET
Something wrong or missing on this page? Tell us by email or on Discord.