> ## 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. # Promote this currency to the entity's base > Atomically promotes this currency to `is_base=true` and demotes the previously-base currency to `is_base=false` in a single transaction. Exactly one base currency per entity is invariant. Requires the `currencies:write` scope. ## OpenAPI ````yaml /openapi.yaml post /currencies/{id}/set-base 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: /currencies/{id}/set-base: parameters: - name: id in: path required: true schema: type: string format: uuid post: tags: - Currencies summary: Promote this currency to the entity's base description: | Atomically promotes this currency to `is_base=true` and demotes the previously-base currency to `is_base=false` in a single transaction. Exactly one base currency per entity is invariant. Requires the `currencies:write` scope. operationId: setBaseCurrencyV1 responses: '200': description: Promoted currency content: application/json: schema: $ref: '#/components/schemas/Currency' '404': $ref: '#/components/responses/Error' security: - ApiKeyAuth: - currencies:write components: schemas: Currency: type: object description: > A currency that the entity transacts in. USD is seeded on entity creation with `is_base=true`. Wave B multi-currency foundation: exchange_rates link non-base currencies to the base. Soft-delete refuses with HTTP 409 if the row is the entity's base currency; promote a different currency first via POST /currencies/{id}/set-base. Scope: currencies:read / currencies:write / currencies:delete. Rule 23 SSOT: `is_base` is flipped only by canonical setBaseCurrency (atomically demotes the previous base in the same transaction). properties: id: type: string format: uuid readOnly: true object: type: string example: currency entity_id: type: string format: uuid readOnly: true code: type: string minLength: 3 maxLength: 3 description: ISO 4217 currency code, UPPERCASE 3 letters. example: USD name: type: string description: Human-readable currency name. 1..80 chars. symbol: type: string nullable: true description: Display symbol. Optional; up to 8 chars. decimals: type: integer default: 2 description: Display precision. JPY = 0, BHD = 3, most = 2. is_active: type: boolean default: true is_base: type: boolean default: false description: >- True for exactly one currency per entity. Read-only; set via set-base. metadata: type: object description: Free-form caller metadata. Default {}. deleted_at: type: string format: date-time nullable: true readOnly: true created_at: type: string format: date-time readOnly: true updated_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. ````