> ## Documentation Index
> Fetch the complete documentation index at: https://agenticadvertisingorg-snap-format-preview-links.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Format References

> Normative reference for format_id (structured identifier object) vs format (full definition object) in AdCP.

AdCP uses two related but distinct concepts when working with creative formats. Their names are similar, which causes a recurring implementation error — this page defines both precisely and names the two failure modes.

## format\_id — structured reference object

`format_id` is **always a JSON object**, never a plain string. It identifies a format by the agent that declared it and the format's local slug:

```json theme={null}
{
  "agent_url": "https://creative.adcontextprotocol.org",
  "id": "display_300x250"
}
```

Optional dimension and duration fields extend it for parameterized template formats:

```json theme={null}
{
  "agent_url": "https://creative.adcontextprotocol.org",
  "id": "display_static",
  "width": 300,
  "height": 250
}
```

`format_id` is a **pointer** — it names a format without carrying its definition.

## format — full definition object

`format` is the complete specification of a creative format. It is returned by `list_creative_formats` and related tasks. A `format` object contains a `format_id` as one of its properties, plus the asset requirements, render specs, and all other metadata:

```json theme={null}
{
  "format_id": {
    "agent_url": "https://creative.adcontextprotocol.org",
    "id": "display_300x250"
  },
  "name": "Display Banner 300×250",
  "assets": [
    { "asset_id": "image", "asset_type": "image", "item_type": "individual", "required": true }
  ],
  "renders": [
    { "role": "primary", "dimensions": { "width": 300, "height": 250 } }
  ]
}
```

A `format` is a **definition** — it describes what the format requires and how it renders.

## Contrast at a glance

| Concept              | Field name   | JSON type                      | Use                                                                                  |
| -------------------- | ------------ | ------------------------------ | ------------------------------------------------------------------------------------ |
| Format identifier    | `format_id`  | `object` — `{ agent_url, id }` | Pointer. Used in creative manifests, request filters, placement declarations.        |
| Format definition    | `format`     | `object` — full spec           | Returned by `list_creative_formats`. Contains `format_id` as a nested field.         |
| Array of identifiers | `format_ids` | `array` of `format_id` objects | Filter parameter on `list_creatives`, `list_creative_formats`, and related requests. |
| Array of definitions | `formats`    | `array` of `format` objects    | Response field on `list_creative_formats`.                                           |

## Two named failure modes

### Anti-pattern A — string in a `format_id` slot

Wrong — `format_id` must be an object, not a plain string:

```json theme={null}
{ "format_id": "display_300x250" }
```

Correct:

```json theme={null}
{ "format_id": { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_300x250" } }
```

AJV error: `format_id must be of type object`. Cause: the `.id` string inside the `format_id` object looks like a self-contained name and is sometimes extracted and used directly.

### Anti-pattern B — format\_id object in a `format` / `formats` slot

Wrong — a `formats[]` element must be a full definition object, not a bare `format_id`:

```json theme={null}
{
  "formats": [
    { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_300x250" }
  ]
}
```

Correct:

```json theme={null}
{
  "formats": [
    {
      "format_id": { "agent_url": "https://creative.adcontextprotocol.org", "id": "display_300x250" },
      "name": "Display Banner 300×250",
      "assets": [ { "asset_id": "image", "asset_type": "image", "item_type": "individual", "required": true } ]
    }
  ]
}
```

AJV error: `formats[0] must have required property 'name'`. Cause: `format_id` and `format` are both objects; the shorter object is sometimes put in the slot that expects the larger one.

## See also

* [Creative Formats](/docs/creative/formats) — full format object structure, asset types, and render specs
* [Template Format IDs](/docs/creative/template-format-ids) — parameterized format IDs for dimension-variable templates