# Set a discount's application method

`PUT /api/v1/discounts/{discountId}/application-method`

## Path parameters

- `discountId` (string, required)

## Headers

- `Idempotency-Key` (string, required) — A unique key per logical write. Replaying a request with the same key returns the first response byte for byte instead of applying the write twice.

## Request body

Required.

- `target_type` (order | items | shipping_methods, required) — Required, with no default. shipping_methods discounts the SHIPPING and leaves the goods at full price; order and items both discount the subtotal today, because per-line allocation is not implemented. This is the field that decides which total the discount comes off, which is why omitting it is refused rather than defaulted.
- `allocation` (across | each) — Defaults to across. Stored and published so a client can read back what it set; nothing on the money path branches on it today.
- `percentage_basis_points` (integer) — Sets a percentage method. Basis points, so 15% is 1500. 1..10000. Mutually exclusive with amount_minor, and a currency_code beside it is refused: a ratio has none.
- `amount_minor` (integer) — Sets a fixed method, in minor units. Mutually exclusive with percentage_basis_points, and requires currency_code in the same object. On a buy-get method it is PER DISCOUNTED UNIT, not per order.
- `currency_code` (string) — Required with amount_minor and refused without it. Stored on this table, unlike on the discount itself, but still asserted: it must equal the store's base currency or the write is refused with 409 currency_mismatch, because the amount is subtracted from the cart total without conversion.
- `apply_to_quantity` (integer) — The SWITCH that makes this a Buy X Get Y method: with it set, the money path computes a buy-get discount and never runs the standard one. Discounted target units granted per trigger. Must be sent together with buy_rules_min_quantity, and needs buy and target rules on the discount, or the offer computes zero.
- `buy_rules_min_quantity` (integer) — Units matching the discount's buy rules required to trigger the offer. Only read on a buy-get method, so it is refused without apply_to_quantity rather than stored where nothing would read it.
- `max_quantity` (integer) — Cap on total discounted units. Only read on a buy-get method, so it is refused without apply_to_quantity. Omit for uncapped.

## 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 <key>` or as `X-API-Key: <key>`. It is
never accepted in a query string.
- `403` — The key lacks the write_discounts scope. `code` is `insufficient_scope` and the message names the scope to ask the merchant for.
- `404` — The resource this url addresses does not exist, or never did on this store.

`resource` names the kind of thing that was missing and is the half to branch
on: the code is `not_found` on newer routes and a per-entity spelling on
routes that shipped before that rule, and the two mean the same thing.

A 404 is ALWAYS about the url. An id inside the request body that resolves to
nothing is 422 `unprocessable_reference` instead, so the two cases stay
distinguishable. An id that belongs to a different merchant reads exactly like
one that never existed.
- `409` — The request is well formed and the store's current state refuses it.

EVERY WRITE CAN ANSWER TWO OF THESE, whatever it does:
`idempotency_key_reused`, this Idempotency-Key already served a different
method, path or body. The key namespace is per store and not per endpoint, so a
key built from a business id collides across routes; use a fresh key per
logical write.
`idempotency_key_in_progress`, an identical request is still running. Retry
after a short delay; exactly one of the racing calls will have applied.

`not_configured` means the merchant has not set something up yet, with
`resource` naming what. The remaining codes are the per-route state rules named
on the operation.
- `413` — The request body exceeded the cap named in the message. `code` is
`payload_too_large`. Nothing was read and nothing was recorded, so the same
Idempotency-Key may be reused once the body is smaller.
- `415` — The body was not declared as `application/json`. `code` is
`unsupported_media_type`.

This API reads ONE media type. The `+json` structured suffixes are refused
too, because they name semantics (merge-patch in particular) this API does
not implement, and reading such a body as plain JSON would be a silent
misreading. A write that carries NO body needs no Content-Type at all.
- `422` — An id INSIDE the request resolves to nothing on this store. `code` is
`unprocessable_reference`, `resource` names the kind of thing that did not
resolve and `field` names where it arrived, so the fix is mechanical.

Distinct from 404, which is about the url, and from 400, which is about the
bytes. The message stays generic about WHY the id did not resolve: an id owned
by another merchant must read identically to one that never existed.
- `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 PUT \
  --url 'https://api.mercemur.com/api/v1/discounts/{discountId}/application-method' \
  --header 'Authorization: Bearer <token>'
```