Batch upsert invoices by external ID

Create or update multiple invoices by external system ID. Supports up to 100 invoices per request with stale update protection and immutable field guardrails. --- **Related endpoints** - `GET /invoices` — List invoices - `POST /invoices` — Create invoice - `GET /invoices/{id}` — Get invoice - `PATCH /invoices/{id}` — Update invoice - `DELETE /invoices/{id}` — Delete invoice - `POST /invoices/{id}/margin-estimate` — Estimate invoice net by payment rail - `POST /invoices/{id}/refund` — Refund invoice - `POST /invoices/{id}/reject` — Reject invoice **Common errors** - `400 invalid_request` — malformed payload or failed validation. - `401 authentication_error` — missing, invalid, expired, or revoked credential. Codes: `authentication_failed`, `invalid_api_key`, `expired_api_key`, `api_key_revoked`, `session_invalid`, `merchant_required`. Carries a `WWW-Authenticate: Bearer` challenge. - `403 authorization_error` — the caller is identified but the action is denied. Codes include `insufficient_permissions`, `ip_not_allowed`, `merchant_mismatch`, and `origin_not_allowed`. **Idempotency** Pass an `Idempotency-Key` header (UUID v4 recommended) to make retries safe. Keys are valid for 24 hours; see [the idempotency guide](/docs/fundamentals/idempotency).

curl -X PUT "https://api.revkeen.com/v2/invoices/external/batch" \
  -H "x-api-key: $REVKEEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "source": "practicehub",
    "invoices": [
      {
        "external_id": "INV-12345",
        "customer_external_ref": {
          "source": "string",
          "id": "string"
        },
        "customer_uuid": "00000000-0000-0000-0000-000000000000",
        "invoice_number": "string",
        "total_minor": 10000,
        "subtotal_minor": 0,
        "tax_minor": 0,
        "discount_minor": 0,
        "currency": "USD",
        "invoice_date": "string",
        "due_date": "string",
        "status": "draft",
        "line_items": [
          {
            "description": null,
            "quantity": null,
            "unit_amount_minor": null,
            "product_id": null
          }
        ],
        "notes": "string",
        "metadata": {},
        "subscription_terms": {
          "collection_method": null,
          "start_mode": null,
          "start_date": null,
          "duration_type": null,
          "duration_count": null,
          "end_date": null,
          "first_payment_behavior": null
        },
        "external_type": "appointment",
        "external_updated_at": "2026-01-01T00:00:00Z"
      }
    ]
  }'

{  "created": 0,  "updated": 0,  "unchanged": 0,  "skipped": 0,  "blocked": 0,  "failed": [    {      "external_id": "string",      "error": "string",      "reason_code": "string"    }  ],  "drift_alerted": 0,  "void_propagated": 0}