> ## Documentation Index > Fetch the complete documentation index at: https://docs.arcuserp.com/llms.txt > Use this file to discover all available pages before exploring further. # Apply an order-level adjustment (discount, fee, or coupon) > Applies an order-level adjustment -- a discount, fee, or coupon -- to an existing order. The adjustment is recorded in `order_adjustments` and the order totals are recalculated immediately. Adjustments can be added to orders in `draft` or `open` status; terminal orders (cancelled, fulfilled, posted-invoice) reject with 422. Supported `effect_type` values: `discount`, `fee`. Supported `discount_type` values: `percentage`, `fixed_amount`. To apply a coupon by code instead, use `POST /v1/orders/{id}/apply-coupon`. Idempotent via `Idempotency-Key` header. Requires `orders:write` scope. **Layer 1 isolation:** `entity_id` is resolved server-side from your API key and the order's ownership is pre-flight verified before any mutation. Callers do not pass `entity_id` in the body; any supplied value is ignored in favor of the key-resolved value. Cross-tenant attempts (POSTing a foreign order UUID) return HTTP 404 `order_not_found`. ## OpenAPI ````yaml /openapi.yaml post /orders/{id}/adjustments openapi: 3.1.0 info: title: Arcus ERP Public API version: 1.0.0 description: > Arcus ERP public REST API. Designed for external integrations and data migration. **Authentication.** Bearer token (API key) via the `Authorization` header. Format: `Authorization: Bearer ark_live_ent__` (or `ark_test_*` for sandbox). API keys are issued per-entity in **Settings > Developers > API Keys**. **Entity scoping.** The entity is encoded in the API key prefix; routes are flat (e.g. `/v1/accounts`, `/v1/orders`, `/v1/products`). A small set of platform endpoints (migration, reconciliation, events, webhook endpoints, API keys) use the `/v1/entities/{entity_id}/...` form -- those are noted in their tags. **Key capabilities.** - Related-resource hydration via `?expand[]=` (see `x-arcus-expand` on each resource). - Cursor-based pagination (`starting_after` / `ending_before` / `limit`). - Idempotency via the `Idempotency-Key` header. - Webhook events for asynchronous notification. - Conditional requests / ETag for cache validation. servers: - url: https://api.arcuserp.com/v1 description: Arcus ERP API (accepts both live `ark_live_*` and test `ark_test_*` keys) - url: https://dev-api.arcuserp.com/v1 description: >- Dev sandbox API (test-only data, accepts `ark_test_*` keys against dev RDS) security: [] paths: /orders/{id}/adjustments: parameters: - name: id in: path required: true schema: type: string format: uuid description: UUID of the order to apply the adjustment to. post: tags: - Orders summary: Apply an order-level adjustment (discount, fee, or coupon) description: > Applies an order-level adjustment -- a discount, fee, or coupon -- to an existing order. The adjustment is recorded in `order_adjustments` and the order totals are recalculated immediately. Adjustments can be added to orders in `draft` or `open` status; terminal orders (cancelled, fulfilled, posted-invoice) reject with 422. Supported `effect_type` values: `discount`, `fee`. Supported `discount_type` values: `percentage`, `fixed_amount`. To apply a coupon by code instead, use `POST /v1/orders/{id}/apply-coupon`. Idempotent via `Idempotency-Key` header. Requires `orders:write` scope. **Layer 1 isolation:** `entity_id` is resolved server-side from your API key and the order's ownership is pre-flight verified before any mutation. Callers do not pass `entity_id` in the body; any supplied value is ignored in favor of the key-resolved value. Cross-tenant attempts (POSTing a foreign order UUID) return HTTP 404 `order_not_found`. operationId: applyOrderAdjustment parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: required: true content: application/json: schema: type: object required: - name - effect_type - discount_type - value properties: coupon_id: type: string format: uuid description: >- Optional. If provided, applies the named coupon instead of a manual adjustment. name: type: string description: >- Display label for the adjustment (e.g. "Summer Promo 10%", "Rush Fee"). effect_type: type: string enum: - discount - fee description: >- Whether this adjustment reduces (`discount`) or increases (`fee`) the order total. discount_type: type: string enum: - percentage - fixed_amount description: How `value` is interpreted. value: type: number description: >- Numeric value. For `percentage`, 0--100. For `fixed_amount`, a dollar amount. notes: type: string description: Optional internal notes. applied_by: type: string format: uuid description: >- UUID of the user applying the adjustment. Defaults to API key owner if omitted. example: name: Summer Promo 10% effect_type: discount discount_type: percentage value: 10 notes: Applied via API integration - summer campaign responses: '200': description: Adjustment applied and order totals recalculated. content: application/json: schema: type: object description: The created adjustment row with updated order total. '400': $ref: '#/components/responses/Error' description: >- Validation failed (missing required field, invalid effect_type/discount_type). '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '404': $ref: '#/components/responses/Error' '422': $ref: '#/components/responses/Error' description: Order is in a terminal state that does not allow adjustments. security: - ApiKeyAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header required: false description: | Client-generated unique key for idempotent POST/PATCH/DELETE operations. Alias for the Idempotency parameter. Max 255 chars. On retry with the same key, the original response is returned without re-executing the operation. Keys expire after 24 hours. schema: type: string maxLength: 255 responses: Error: description: Error response (400/401/403/404/409/422/429/500) content: application/json: schema: $ref: '#/components/schemas/ErrorEnvelope' schemas: ErrorEnvelope: type: object description: | Canonical error response envelope. All API errors use this shape. required: - error - code properties: error: type: string description: Machine-readable error key example: not_found code: type: string description: Machine-readable error code (often same as error) example: not_found type: type: string enum: - validation_error - permission_error - not_found - conflict - rate_limit - internal - expand_error - not_implemented example: not_found hint: type: string description: Human-readable one-sentence explanation (English) example: >- The requested order does not exist or does not belong to this entity. param: type: string description: The parameter that caused the error, if applicable example: expand[0] required: type: string description: The scope required (only on insufficient_scope errors) example: accounts:read request_id: type: string description: >- Unique request ID for support tracing (maps to CloudWatch log stream) example: req_abc123 securitySchemes: ApiKeyAuth: type: http scheme: bearer description: | API key issued per entity via Settings > Developers > API Keys. Each key carries scopes (e.g. orders:read, products:write). Bearer token format: Authorization: Bearer ark_live_ent__ Test keys use ark_test_ent__. Both are issued per entity via Settings > Developers > API Keys. ````