# Retrieve experiment usage `GET /api/v1/experiment-usage` How many DISTINCT visitors were first exposed to any experiment in the store within a window. It is the number that says whether the programme is reaching enough traffic for its verdicts to mean anything. NOT A BILLING FIGURE. Store visits are decoupled from pricing on every plan, so this caps nothing and charges nothing. No visitor id crosses the wire, here or anywhere on this family: the rows behind the count are keyed by a shopper's device identity and only the count is published. The window is half-open and both ends are echoed, since the end is the server clock and two calls a minute apart would otherwise differ with nothing explaining why. ## Query parameters - `since` (string (date-time)) — Start of the window, RFC3339. Defaults to a trailing thirty days, and a lookback longer than two years is clamped to two. A value that is not RFC3339 is a 400 rather than a fallback to the default, so a client with a malformed timestamp never reads a thirty-day figure as its own window. The response echoes both ends of the window actually used; the end is the server clock and cannot be set. ## Responses - `200` — Success - `400` — The request was refused before any state changed. `code` is one of: `invalid_body`, a write body this route will not take. `reason` partitions it and `field` names the key when one key is at fault. `invalid_query`, a query parameter, including limit and after. `invalid_text`, a NUL byte or bytes that are not valid UTF-8 anywhere in the path, the query or the body. Strip control characters before sending. `idempotency_key_required`, a write sent without the Idempotency-Key header. `invalid_request`, an Idempotency-Key longer than 255 bytes. Routes add their own codes for rules only they know. Switch with a default arm. - `401` — No credential, or one this API does not accept. `code` is always `unauthorized`. THE BODY IS DELIBERATELY UNINFORMATIVE. An expired key, a revoked key, a publishable key, a key belonging to another merchant and a key that never existed are all refused with the same bytes, so this response cannot be used to probe which keys exist. Check the key's state in the dashboard rather than inferring it here. Send the key as `Authorization: Bearer ` or as `X-API-Key: `. It is never accepted in a query string. - `403` — The key lacks the read_experiments scope. `code` is `insufficient_scope` and the message names the scope to ask the merchant for. - `429` — Too many requests. `code` is `rate_limited`. Two limits apply independently: one on the credential and the route family, one on the client address. The headers describe whichever has less left, so honouring Retry-After always clears the window that bound. - `default` — Any status this operation does not list, in the same envelope. A 5xx means the request may or may not have applied. Retry it with the SAME Idempotency-Key: that is the only way to find out without risking a duplicate, and it is what the key is for. A few 4xx conditions arrive here rather than as a listed status because they depend on the merchant's plan or on a module being wired: 402 when a quota or a plan limit is reached, and 503 when a capability the route needs is not configured on this deployment. Both carry a `code` naming which. ## Example ```bash curl --request GET \ --url 'https://api.mercemur.com/api/v1/experiment-usage' \ --header 'Authorization: Bearer ' ```