# Add a tax ID to a customer > The customers.tax_ids.add operation: POST /api/v1/customers/{id}/tax_ids in the REST API and the customers_tax_ids_add tool in the MCP server. Adds a tax ID to a customer after checking it with its register (VIES for EU VAT IDs, HMRC for UK VAT numbers, the Swiss UID register; Norwegian and US numbers by format; other types are kept `unverified`). The answer is stored with the ID: status, source, reason, consultation number and time (spec §9.4). A malformed number is refused (`tax_id_invalid`); one the register doesn't know is stored as `invalid` and doesn't make the customer a business. Dry runs simulate the check. At most 5 tax IDs per customer; the same number twice is refused (`customer_tax_id_exists`). The type is inferred again (spec §13.1). Emits `customer.updated`. ## At a glance - **REST:** `POST /api/v1/customers/{id}/tax_ids` - **MCP tool:** `customers_tax_ids_add` - **Operation ID:** `customers.tax_ids.add` - **Scope:** Brand - **Permission:** `customers:write` - **Risk tier:** `write` - **Dry run:** Supported (send `Ownrecord-Dry-Run: true`) - **Idempotency:** `Idempotency-Key` required - **Modes:** Test and live - **Authentication:** API key with the `customers:write` scope - **Since:** M4 ## Request ### Path parameters | Field | Type | Description | | ----- | ---------- | ---------------------------- | | `id` | `cus_…` ID | The customer's ID (`cus_…`). | ### Body A JSON object (`Content-Type: application/json`): | Field | Type | Required | Description | | --------- | ------ | -------- | -------------------------------------------------------------------------------------------------------------------------------------------------- | | `type` | string | Yes | The tax ID's type: `eu_vat`, `gb_vat`, `ch_uid`/`ch_vat`, `no_org`/`no_vat` or `us_ein`. Other types are kept but can't be checked (`unverified`). | | `value` | string | Yes | The number as the customer gave it, with or without the country prefix and separators. | | `country` | string | Yes | The country that issued the tax ID (ISO 3166-1 alpha-2; `EL` = Greece). | ## Response `201 Created` with a JSON object: | Field | Type | Description | | ------------------------------------------ | --------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `id` | `cus_…` ID | | | `object` | `"customer"` | | | `livemode` | boolean | | | `entityId` | `ent_…` ID | | | `brandId` | `brd_…` ID | | | `email` | string | | | `name` | string or null | | | `businessName` | string or null | | | `locale` | string or null | | | `type` | one of `individual`, `business`, `unknown` | Inferred (spec §13.1): `business` with a tax ID that isn't rejected, or with a business name when declared a business. Tax is decided per sale: only a `valid` tax ID makes a sale business-to-business. | | `declaredType` | one of `individual`, `business`, `unknown` | What you declared. | | `billingAddress` | object or null | | | `taxIds` | array of object | | | `taxIds[].id` | string | Identifies the tax ID on the customer: its canonical number, e.g. `ATU12345678`. | | `taxIds[].type` | string | `eu_vat`, `gb_vat`, `ch_uid`/`ch_vat`, `no_org`/`no_vat`, `us_ein`, or another type (unverified). | | `taxIds[].value` | string | The number as given. | | `taxIds[].country` | string | | | `taxIds[].number` | string or null | The canonical number (null for types Ownrecord can't check). | | `taxIds[].verification` | object | The latest check (spec §9.4). | | `taxIds[].verification.status` | one of `valid`, `invalid`, `unavailable`, `format_only`, `unverified` | `valid`, `invalid`, `unavailable` (the register couldn't answer), `format_only` (no register: format and checksum) or `unverified` (a type Ownrecord can't check). | | `taxIds[].verification.source` | string | Who decided: VIES, HMRC, CH_UID, FORMAT (format and checksum only) or NONE. | | `taxIds[].verification.reason` | string or null | Why, e.g. `not_registered` or `service_unavailable`. | | `taxIds[].verification.checkedAt` | date-time | | | `taxIds[].verification.consultationNumber` | string or null | The register's proof of the check (VIES request identifier, HMRC consultation number). | | `taxIds[].verification.simulated` | boolean | A fixture answer (development and test environments), not a register's. | | `taxIds[].history` | array of object | Earlier checks, newest first (at most 10). | | `taxIds[].history[].status` | one of `valid`, `invalid`, `unavailable`, `format_only`, `unverified` | `valid`, `invalid`, `unavailable` (the register couldn't answer), `format_only` (no register: format and checksum) or `unverified` (a type Ownrecord can't check). | | `taxIds[].history[].source` | string | Who decided: VIES, HMRC, CH_UID, FORMAT (format and checksum only) or NONE. | | `taxIds[].history[].reason` | string or null | Why, e.g. `not_registered` or `service_unavailable`. | | `taxIds[].history[].checkedAt` | date-time | | | `taxIds[].history[].consultationNumber` | string or null | The register's proof of the check (VIES request identifier, HMRC consultation number). | | `taxIds[].history[].simulated` | boolean | A fixture answer (development and test environments), not a register's. | | `taxIds[].addedAt` | date-time | | | `taxCountryLocked` | object or null | Where the customer's purchases are taxed (spec §9.3): set at the first successful payment, moved only when a top-up's evidence resolves to another country. | | `taxExempt` | object or null | Tax exemption (spec §9.5). Not applied yet: exemption certificates come in a later release. | | `processorRefs` | array of object | The customer at each payment processor used (set by checkout). | | `processorRefs[].processorConnectionId` | `pcon_…` ID | | | `processorRefs[].customerRef` | string | | | `risk` | object | | | `risk.level` | one of `normal`, `elevated`, `blocked` | | | `risk.reasons` | array of string | | | `risk.firstPaymentAt` | date-time or null | | | `risk.totalPaid` | money | | | `portalLastLoginAt` | date-time or null | | | `testClockId` | `clk_…` ID or null | Test mode: the test clock the customer was created on (spec §16.13). | | `metadata` | object | | | `createdAt` | date-time | | | `updatedAt` | date-time | | | `deletedAt` | date-time or null | When the customer was deleted (soft delete); null while active. | ## Examples ### Add an Austrian VAT ID ```http POST /api/v1/customers/cus_01m4ty3c2b5qx7mhd9k8w6s0aa/tax_ids Authorization: Bearer or_test_… Idempotency-Key: 3f1c9a52-7d0e-4b8f-9a61-0c2d5e8b7f14 Content-Type: application/json { "type": "eu_vat", "value": "ATU 0000 0000", "country": "AT" } ``` Response (`201`): ```json { "id": "cus_01m4ty3c2b5qx7mhd9k8w6s0aa", "object": "customer", "livemode": false, "entityId": "ent_01m4tx5nj8emwbyf7y9vcf47x2", "brandId": "brd_01m4tx5nj8emwbyf7y9vcf47x4", "email": "billing@alpenapps.example", "name": "Lena Huber", "businessName": "Alpen Apps GmbH", "locale": "de-AT", "type": "business", "declaredType": "business", "billingAddress": { "line1": "Mariahilfer Straße 1", "line2": null, "city": "Wien", "postalCode": "1060", "region": null, "country": "AT" }, "taxIds": [ { "id": "ATU00000000", "type": "eu_vat", "value": "ATU 0000 0000", "country": "AT", "number": "ATU00000000", "verification": { "status": "valid", "source": "VIES", "reason": null, "checkedAt": "2026-10-14T09:00:00.000Z", "consultationNumber": "WAPIAAAAZ1x2y3", "simulated": false }, "history": [], "addedAt": "2026-10-14T09:00:00.000Z" } ], "taxCountryLocked": null, "taxExempt": null, "processorRefs": [], "risk": { "level": "normal", "reasons": [], "firstPaymentAt": null, "totalPaid": { "amount": "0", "currency": "EUR" } }, "portalLastLoginAt": null, "testClockId": null, "metadata": { "userId": "u_4821" }, "createdAt": "2026-10-14T09:00:00.000Z", "updatedAt": "2026-10-14T09:00:00.000Z", "deletedAt": null } ``` ## Errors | Code | HTTP status | When | What to do | | ------------------------------------------------------------------------------------------------ | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | [`validation_failed`](https://ownrecord.io/docs/reference/errors.md#validation_failed) | 400 | A field is missing or malformed. | Fix the fields listed in `details.issues` and send the request again. | | [`tax_id_invalid`](https://ownrecord.io/docs/reference/errors.md#tax_id_invalid) | 400 | A tax ID's number isn't valid for its type and country (format, checksum, another country's prefix, a Northern Ireland XI number); `details.reason` says which. Nothing is stored. Types Ownrecord can't check (e.g. `au_abn`) are stored as `unverified` instead. | Check the number and its country for typing errors (`details.reason` says what is wrong), then send it again. A Northern Ireland business enters its GB VAT number. | | [`resource_not_found`](https://ownrecord.io/docs/reference/errors.md#resource_not_found) | 404 | This brand has no such customer in this mode (customers belong to one brand and one mode). | Check the ID and that your credentials are scoped to the right organization, entity, brand and mode. | | [`customer_deleted`](https://ownrecord.io/docs/reference/errors.md#customer_deleted) | 400 | The customer was deleted. | Create a new customer for this buyer. Deleted customers stay readable for the records they belong to, but can't be changed. | | [`customer_tax_id_exists`](https://ownrecord.io/docs/reference/errors.md#customer_tax_id_exists) | 409 | The customer already has this tax ID (`details.tax_id_id`). | Nothing to do: the tax ID is already on the customer (`details.tax_id_id`). To check it again, remove it and add it back. | | [`customer_tax_id_limit`](https://ownrecord.io/docs/reference/errors.md#customer_tax_id_limit) | 400 | The customer already has 5 tax IDs. | Remove a tax ID the customer no longer uses before adding another (at most 5 per customer). | | [`version_conflict`](https://ownrecord.io/docs/reference/errors.md#version_conflict) | 409 | The customer changed at the same moment; retry. | Fetch the object again and retry the change. | | [`scope_required`](https://ownrecord.io/docs/reference/errors.md#scope_required) | 400 | The request names no brand (an org or entity key without `Ownrecord-Brand`). | Call the operation with credentials or a target at its level: an organization, a legal entity, a brand or the platform (`details.required_scope`). | | [`livemode_not_allowed`](https://ownrecord.io/docs/reference/errors.md#livemode_not_allowed) | 403 | You have no live-mode access, so you can't act on live customers. | Use test mode, or credentials for the target's mode. Developers need live access granted by an organization owner or admin. | | [`permission_denied`](https://ownrecord.io/docs/reference/errors.md#permission_denied) | 403 | You can't change this brand's customers (`customers:write`: owners, admins and support). | Ask an organization owner or admin for a role or scope that allows this action. | It can also return these [common errors](https://ownrecord.io/docs/reference.md#common-errors): [`api_version_unsupported`](https://ownrecord.io/docs/reference/errors.md#api_version_unsupported), [`invalid_json`](https://ownrecord.io/docs/reference/errors.md#invalid_json), [`unsupported_media_type`](https://ownrecord.io/docs/reference/errors.md#unsupported_media_type), [`authentication_required`](https://ownrecord.io/docs/reference/errors.md#authentication_required), [`api_key_invalid`](https://ownrecord.io/docs/reference/errors.md#api_key_invalid), [`idempotency_key_required`](https://ownrecord.io/docs/reference/errors.md#idempotency_key_required), [`idempotency_key_reused`](https://ownrecord.io/docs/reference/errors.md#idempotency_key_reused), [`idempotency_in_progress`](https://ownrecord.io/docs/reference/errors.md#idempotency_in_progress), [`approval_required`](https://ownrecord.io/docs/reference/errors.md#approval_required), [`approval_rejected`](https://ownrecord.io/docs/reference/errors.md#approval_rejected), [`approval_expired`](https://ownrecord.io/docs/reference/errors.md#approval_expired), [`stale_approval`](https://ownrecord.io/docs/reference/errors.md#stale_approval), [`approval_policy_denied`](https://ownrecord.io/docs/reference/errors.md#approval_policy_denied), [`rate_limited`](https://ownrecord.io/docs/reference/errors.md#rate_limited), [`internal_error`](https://ownrecord.io/docs/reference/errors.md#internal_error).