> ## 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. # Send a purchase order to vendor > Emails a purchase order to its vendor using the entity's purchase-order template, or records it as sent externally without emailing when the vendor was contacted outside Arcus; a revision resend uses a dedicated revision template, and a stamp-only send records `sent_at` without dispatching a new email. Requires `purchasing:write` scope, and is refused with 422 when the order is pending approval, cancelled, or missing a vendor email address. ## OpenAPI ````yaml /openapi.yaml post /purchase-orders/{id}/send-to-vendor 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. **Changelog.** Entries are dated and name every published contract whose MEANING moved, not only the ones whose field names changed. The narrative version of the same entries, written for integrators, is published at https://arcuserp.mintlify.app/changelog. **2026-09-22 (planned 2026-09-21), REORDER-BUYER-TRUTH: demand changed what it MEANS on three published contracts, with no field renamed.** An integrator that pins field names sees no breakage and different numbers, which is why this entry exists. - **Demand now counts build consumption.** `demand_avg_per_day` on the public product, kit and inventory-balance objects, and `daily_demand` / `demand_basis` / `net_suggested_qty` on `GET /v1/purchasing/reorder-report`, are composed from fulfilled sales lines PLUS posted `build_consume` inventory draws: each physical decrement of a product counts exactly once. The previous rule adopted an internal-consumption basis only when the sales blend was exactly zero, so a product both sold AND consumed into work orders planned as if the build draws did not exist. Products drawn into work orders move; on one production-shaped dataset five did, the largest from 0.05/day to 14.95/day. - **`demand_basis` now carries four values, not two:** `sales`, `sales_and_builds`, `builds` and `consumption`. A consumer with a two-branch reader (anything that is not `consumption` is `sales`) silently hides the two new ones. - **`current_demand_units` on `GET /v1/inventory/balances/:id` moved, by a second rule.** It counts committed-but-unshipped CUSTOMER demand, and it now counts a kit component's own line rather than its parent kit line, and excludes non-sales documents. On one production dataset 269 of 1,043 balance rows changed, 227 of them downward; the largest single move was 1,941 to 25, on a product whose open PURCHASE order line had been reported as customer demand. Re-baseline anything that alerts or reorders off this field. - `run_rate_30/90/180/365` and `blended_daily` on the same balance object move for the first reason above. - **Added, not changed:** `GET /v1/purchasing/reorder-report` accepts `demand_window` and returns the unit, cover-through and reorder-point-provenance fields documented on that operation; `order_multiple` is accepted and returned on the product-vendor doors. - **Volume note.** The `inventory.low_stock`, `inventory.out_of_stock` and `inventory.back_in_stock` webhook events are population-gated on `on_hand <= reorder_point`, and reorder points move with the demand above, so subscribers should expect a one-time step change in event volume around the release. 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: /purchase-orders/{id}/send-to-vendor: parameters: - name: id in: path required: true schema: type: string format: uuid post: tags: - Purchasing summary: Send a purchase order to vendor description: >- Emails a purchase order to its vendor using the entity's purchase-order template, or records it as sent externally without emailing when the vendor was contacted outside Arcus; a revision resend uses a dedicated revision template, and a stamp-only send records `sent_at` without dispatching a new email. Requires `purchasing:write` scope, and is refused with 422 when the order is pending approval, cancelled, or missing a vendor email address. operationId: sendPurchaseOrderToVendorV1 parameters: - name: Idempotency-Key in: header required: false schema: type: string maxLength: 255 description: Optional idempotency key for safe retries (Stripe-style). requestBody: required: false content: application/json: schema: type: object properties: to_email: type: string format: email description: | Override the destination email. When omitted, defaults to the vendor account's `email`. Required (either here or on the vendor) unless `stamp_only=true`. stamp_only: type: boolean default: false description: | When `true`, stamps `sent_at` + `last_notified_revision` without dispatching an email (the email has already been sent through a separate path). Skips the `to_email` / vendor-email guard. send_mode: type: string enum: - email - external default: email description: > `email` (default) sends the vendor an email via the canonical `sendDocument` helper. `external` marks the PO as sent WITHOUT dispatching an email (the vendor was contacted outside Arcus) -- same atomic `draft`->`open` + `sent_at` stamp + broadcast, no communication log row, audited as `po_marked_sent_external`, and the vendor-email guard is skipped. Mutually independent of `stamp_only`; both skip the email dispatch. template_key_override: type: string enum: - po_revised description: | When set to `po_revised`, uses the revision-send template instead of the default purchase-order template. Combine with `revision_number` and `changed_fields` for context. revision_number: type: integer description: | The revision number being sent (e.g. `2` for the second send after the initial first-send `1`). Used only when `template_key_override='po_revised'`. changed_fields: type: string description: | Human-readable summary of what changed between revisions (e.g. "line quantity, due date"). Rendered in the revision email template. Used only when `template_key_override='po_revised'`. responses: '200': description: Purchase order sent (or stamped). Returns the canonical send result. content: application/json: schema: type: object properties: po_id: type: string format: uuid po_number: type: string sent_to: type: string format: email sent_at: type: string format: date-time revision_send: type: boolean description: >- Present and `true` only when `template_key_override='po_revised'`. revision_number: type: integer description: Echoes the request body when this was a revision send. '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '404': description: Purchase order not found for this entity. content: application/json: schema: $ref: '#/components/responses/Error' '422': description: | Vendor email missing (`vendor_email_missing`), PO awaiting approval (`po_pending_approval`), or PO cancelled (`po_cancelled`). content: application/json: schema: $ref: '#/components/responses/Error' security: - ApiKeyAuth: [] components: 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. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.