Start experiment

Start an experiment. The experiment is started exactly as it is configured; this endpoint accepts no attributes, and a request body carrying any is rejected rather than ignored. Set the run window, duration, or variants with PATCH /api/v2/experiments/{experiment_id} before starting. An unconfigured draft returns HTTP 409. Configure either warehouse_exposure_configuration or datadog_flag_configuration, plus the required experiment fields, before starting. Start validation errors can include meta.configuration_pointer to identify a field on the experiment to correct. For a flag-backed experiment this enables the linked feature flag's environment, clears any stored variant override on it, and starts the allocation's rollout. The request is idempotent: an experiment that is already running or ready for a decision still returns 204, so a retry after a timeout is safe. One exception: an experiment scheduled to start is accepted only when it is backed by your own feature flag; a Datadog-flag experiment in that state returns 409 because its stored state and flag allocation disagree. Cancel and conclude are not idempotent and return 409 when repeated.