> ## Documentation Index
> Fetch the complete documentation index at: https://terminal49-feat-trade-intel-sdk.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Create container custom field

> Create or update a custom field value on a container in the Terminal49 API. Attach internal metadata like reference codes, priority, or cost centers.

Creates or updates a custom field on a container. If a custom field with the specified `api_slug` already exists, it will be updated.

## Path parameters

| Parameter | Required | Description |
| - | - | - |
| `container_id` | Yes | The ID of the container |

## Request body

| Parameter | Required | Description |
| - | - | - |
| `data.type` | Yes | Must be `custom_field` |
| `data.attributes.api_slug` | Yes | The slug of the custom field definition |
| `data.attributes.value` | Yes | The value to set (type depends on the definition's data type) |

The container is implied by the path, so do not send `data.relationships.entity` on this endpoint.

## Authorization

Requires `update` permission on the container.

## Response

Returns `201 Created` with the custom field resource on success.

## Behavior

* Uses `find_or_initialize_by` internally, so it creates if missing or updates if it exists
* Values are validated against the definition's data type
* For enum fields, values are validated against the definition's options

## Example request

```json theme={null}
{
  "data": {
    "type": "custom_field",
    "attributes": {
      "api_slug": "customer_reference_number",
      "value": "ABC124"
    }
  }
}
```

## Example response

```json theme={null}
{
  "data": {
    "id": "YOUR_CUSTOM_FIELD_ID",
    "type": "custom_field",
    "attributes": {
      "api_slug": "customer_reference_number",
      "value": "ABC124",
      "display_value": "ABC124"
    },
    "relationships": {
      "entity": {
        "data": {
          "id": "YOUR_CONTAINER_ID",
          "type": "container"
        }
      },
      "definition": {
        "data": {
          "id": "YOUR_DEFINITION_ID",
          "type": "custom_field_definition"
        }
      }
    }
  }
}
```


## OpenAPI

````yaml post /containers/{container_id}/custom_fields
openapi: 3.0.0
info:
  title: Terminal49 API Reference
  version: 0.2.0
  contact:
    name: Terminal49 API support
    url: https://www.terminal49.com
    email: support@terminal49.com
  description: >-
    The Terminal 49 API offers a convenient way to programmatically track your
    shipments from origin to destination.


    Please enter your API key into the "Variables" tab before using these
    endpoints within Postman.
  x-label: Beta
  termsOfService: https://www.terminal49.com/terms
servers:
  - url: https://api.terminal49.com/v2
    description: Production
security:
  - authorization: []
tags:
  - name: Containers
  - name: Custom Field Definitions
  - name: Custom Field Options
  - name: Custom Fields
  - name: Shipments
  - name: Locations
  - name: Events
  - name: Tracking Requests
  - name: Webhooks
  - name: Webhook Notifications
  - name: Ports
  - name: Metro Areas
  - name: Terminals
  - name: Routing (Paid)
  - name: Documents
  - name: Email Submissions
  - name: Document Schemas
  - name: Search
  - name: Parties
  - name: Trade Intelligence
    description: >-
      Trade intelligence answers questions about who imports what into the US,
      from where, on which carriers, and how that changes over time. The data is
      US import bill-of-lading records (US Customs vessel manifests): every
      ocean container landed at a US port (plus US-bound cargo landed at
      Vancouver, BC) from January 2022 to today, refreshed daily. It does not
      include air freight, exports, or domestic moves.


      **Access.** Trade intelligence is enabled per account. Accounts without it
      receive `403 Forbidden` with `{"error": "Trade intelligence is not enabled
      for this account"}`; contact sales@terminal49.com to enable it. Requests
      are limited to roughly 120 per minute per account (`429` with `{"error":
      "Rate limit exceeded; retry in a minute"}`).


      **Format.** These endpoints accept and return plain JSON, not JSON:API.
      Send `Content-Type: application/json` on `POST` requests and authenticate
      exactly like every other v2 endpoint (`Authorization: Token
      YOUR_API_KEY`).


      **Typical flow.** Resolve a name or product first, then analyze:
      `companies/search` or `commodities/search` give you the exact
      `company_name` or `hs4` to pass to `companies/profile`, `importers/top`,
      `trends`, or `breakdown`. Container and bill of lading numbers go straight
      to `containers/lookup` and `bills_of_lading/lookup`.


      **Reading the numbers.**


      - **Volume is physical containers.** Each box is counted once, even when
      it appears on several bills of lading. A container carrying several HS
      codes counts once per code and companies can share containers, so do not
      add volumes across HS codes or companies; take market totals from `trends`
      or `breakdown` without a company filter.

      - **Estimated value is a modelled USD estimate**, not the declared customs
      value.

      - **The current month is partial.** `trends` and `breakdown` include it by
      default; `companies/profile` and `importers/top` use full calendar months.
      Compare full months against the same months a year earlier.

      - **Absent volume is not small volume.** Importers can ask US Customs to
      withhold their name from manifests, and much large-retail freight is
      booked under forwarders or suppliers. Treat a company's figures as a
      floor, never as its size.

      - **One company, many names.** Company names are normalized but split by
      state, and large importers use several names (distribution, merchandising,
      and DC entities). Related entities are separate companies; report them
      separately. `companies/search` ranks by match quality, not by size.

      - **Companies are the consignee or notify party on the bill.** Forwarders,
      NVOCCs, and customs brokers appear alongside cargo owners. A high
      notify-party share, or LOGISTICS, FREIGHT, SHIPPING, CUSTOMS, or BROKERAGE
      in the name, usually indicates a logistics provider.

      - **Carriers, countries, and ports use the manifests' spellings**, and
      carriers appear under spelling variants (for example `CMA CGM` and `CMA
      CGM AMERICA LLC`). Filter with substrings and merge variants before
      computing shares.

      - **HS4 descriptions are truncated.** Check `common_goods` from
      `commodities/search` before relying on a code.
paths:
  /containers/{container_id}/custom_fields:
    parameters:
      - schema:
          type: string
        name: container_id
        in: path
        required: true
        description: Container ID
    post:
      tags:
        - Containers
      summary: Create a container custom field
      operationId: post-containers-custom-fields
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                data:
                  type: object
                  required:
                    - type
                    - attributes
                  properties:
                    type:
                      type: string
                      enum:
                        - custom_field
                    attributes:
                      type: object
                      required:
                        - api_slug
                        - value
                      properties:
                        api_slug:
                          type: string
                        value:
                          $ref: '#/components/schemas/custom_field_value'
            example:
              data:
                type: custom_field
                attributes:
                  api_slug: customer_reference_number
                  value: ABC124
      responses:
        '201':
          description: Created
          content:
            application/json:
              schema:
                type: object
                properties:
                  data:
                    $ref: '#/components/schemas/custom_field'
                  links:
                    $ref: '#/components/schemas/link-self'
components:
  schemas:
    custom_field_value:
      description: Raw custom field value (type depends on definition)
      nullable: true
      oneOf:
        - type: string
        - type: number
        - type: boolean
        - type: array
          items:
            type: string
        - type: object
    custom_field:
      title: Custom field
      type: object
      properties:
        id:
          type: string
          format: uuid
        type:
          type: string
          enum:
            - custom_field
        attributes:
          type: object
          properties:
            api_slug:
              type: string
            value:
              $ref: '#/components/schemas/custom_field_value'
            display_value:
              type: string
              nullable: true
              description: Formatted value for display
          required:
            - api_slug
        relationships:
          type: object
          properties:
            entity:
              type: object
              properties:
                data:
                  type: object
                  properties:
                    type:
                      type: string
                      enum:
                        - shipment
                        - container
                        - tracking_request
                    id:
                      type: string
                      format: uuid
                  required:
                    - type
                    - id
            definition:
              type: object
              properties:
                data:
                  type: object
                  properties:
                    type:
                      type: string
                      enum:
                        - custom_field_definition
                    id:
                      type: string
                      format: uuid
                  required:
                    - type
                    - id
      required:
        - id
        - type
    link-self:
      title: link
      type: object
      properties:
        self:
          type: string
          format: uri
  securitySchemes:
    authorization:
      name: Authorization
      type: apiKey
      in: header
      description: >-
        Use a Terminal49 API key in the `Authorization` header with the `Token`
        prefix.


        `Authorization: Token YOUR_API_KEY`

````

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