> ## 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. # Add an order item to a package > Adds an order line item (or partial quantity) to a package. Either `order_item_id` OR `product_id` must be supplied; if both are provided the `order_item_id` is authoritative. Two invariants enforced BEFORE the INSERT (canonical writer `utils/fulfillment-helpers.mjs::addItemToPackage`): 1. **Item-on-order** -- the product must be on the parent order. Returns 422 `product_not_on_order` with envelope `{error, code, type:'validation_error', param:'product_id', hint, product_id, order_id}` when the product is not on any order_items row for the order. 2. **Quantity cap** -- the requested quantity must not exceed `SUM(ordered_qty) - SUM(elsewhere_packed)` across ALL packages of the order for this product. Returns 422 `package_quantity_exceeds_ordered` with envelope `{error, code, type:'validation_error', param:'quantity', hint, ordered_qty, already_packed_qty, remaining_qty, product_id}`. Multi-line same-product orders: the first non-fully-packed line wins. Both refusal paths write to the parent order activity log. Requires `fulfillment:write` scope. ## OpenAPI ````yaml /openapi.yaml post /packages/{id}/items 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: /packages/{id}/items: parameters: - name: id in: path required: true schema: type: string format: uuid post: tags: - Fulfillment summary: Add an order item to a package description: > Adds an order line item (or partial quantity) to a package. Either `order_item_id` OR `product_id` must be supplied; if both are provided the `order_item_id` is authoritative. Two invariants enforced BEFORE the INSERT (canonical writer `utils/fulfillment-helpers.mjs::addItemToPackage`): 1. **Item-on-order** -- the product must be on the parent order. Returns 422 `product_not_on_order` with envelope `{error, code, type:'validation_error', param:'product_id', hint, product_id, order_id}` when the product is not on any order_items row for the order. 2. **Quantity cap** -- the requested quantity must not exceed `SUM(ordered_qty) - SUM(elsewhere_packed)` across ALL packages of the order for this product. Returns 422 `package_quantity_exceeds_ordered` with envelope `{error, code, type:'validation_error', param:'quantity', hint, ordered_qty, already_packed_qty, remaining_qty, product_id}`. Multi-line same-product orders: the first non-fully-packed line wins. Both refusal paths write to the parent order activity log. Requires `fulfillment:write` scope. operationId: addPackageItem parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: required: true content: application/json: schema: type: object required: - quantity properties: order_item_id: type: string format: uuid description: Order line ID. Either order_item_id or product_id required. product_id: type: string format: uuid description: >- Product ID. The canonical writer resolves the matching order_item by (order_id, product_id, entity_id). quantity: type: integer minimum: 1 serial_numbers: type: array items: type: string responses: '200': description: Package item added content: application/json: schema: $ref: '#/components/schemas/PackageItem' '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '404': $ref: '#/components/responses/Error' '409': $ref: '#/components/responses/Error' '422': description: > Item-on-order or qty-cap invariant refusal. The `code` field carries the machine tag: * `product_not_on_order` -- product is not on the parent order * `package_quantity_exceeds_ordered` -- requested qty exceeds remaining cap content: application/json: schema: type: object required: - error - code - type - hint properties: error: type: string code: type: string enum: - product_not_on_order - package_quantity_exceeds_ordered type: type: string enum: - validation_error param: type: string hint: type: string product_id: type: string format: uuid order_id: type: string format: uuid ordered_qty: type: integer already_packed_qty: type: integer remaining_qty: type: integer 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 schemas: PackageItem: type: object description: >- A line item placed inside a package (one order_item can be split across packages). properties: id: type: string format: uuid package_id: type: string format: uuid order_item_id: type: string format: uuid product_id: type: string format: uuid nullable: true quantity: type: integer description: Qty of this order_item packed into this package. sku: type: string nullable: true title: type: string nullable: true serial_numbers: type: array items: type: string nullable: true created_at: type: string format: date-time readOnly: true 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 responses: Error: description: Error response (400/401/403/404/409/422/429/500) content: application/json: schema: $ref: '#/components/schemas/ErrorEnvelope' 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. ````