# `POST /entity`

Protected. Requires a valid session cookie (`credentials: 'include'`).

Create entity

Creates an entity in a company customer group and writes `entityhistory` (`Erstellt`).
Optional `number`: omit to allocate the next customer number (`type` entity).


## Body (JSON)

| Field | Required | Type | Description |
|-------|----------|------|-------------|
| `customergroupId` | yes | integer |  |
| `name` | yes | string |  |
| `firstName` | no | string |  |
| `number` | no | string | Customer number; omit to auto-allocate |

```json
{
  "customergroupId": 1,
  "name": "Müller",
  "firstName": "Anna",
  "number": "10042"
}
```

## Response `201`

Created entity

```json
{
  "entity": {
    "id": 1,
    "customergroupId": 1,
    "firstName": "…",
    "name": "…",
    "number": "…",
    "createdAt": "2026-08-10T00:00:00.000Z",
    "updatedAt": "2026-08-10T00:00:00.000Z"
  }
}
```

## Errors

| Status | Description |
|--------|-------------|
| 400 | Name/number required or no company selected — `{"error":"…"}` |
| 401 | Unauthorized — `{"error":"…"}` |
| 403 | Not authorized — `{"error":"…"}` |
| 404 | Company or customer group not found — `{"error":"…"}` |
| 409 | Number already in use — `{"error":"…"}` |

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