> ## Documentation Index
> Fetch the complete documentation index at: https://docs.getmatter.com/llms.txt
> Use this file to discover all available pages before exploring further.

# IDs & Object Types

> How resources are identified in the Matter API.

Every resource in the API has a prefixed string ID and an `object` field that tells you its type.

## ID format

IDs are opaque strings with a type prefix:

| Resource        | Prefix | Example     |
| --------------- | ------ | ----------- |
| Account         | `act_` | `act_k8x2m` |
| Item            | `itm_` | `itm_r9f3a` |
| Annotation      | `ann_` | `ann_m2k8v` |
| Tag             | `tag_` | `tag_n5j2x` |
| Author          | `aut_` | `aut_p4w7q` |
| Reading Session | `rs_`  | `rs_k8x2m`  |

The prefix tells you the resource type at a glance, which is helpful when debugging or reading logs.

## Object types

Every response includes an `object` field identifying the resource type:

```json theme={null}
{
  "object": "item",
  "id": "itm_r9f3a",
  "title": "How to Do Great Work"
}
```

For lists:

```json theme={null}
{
  "object": "list",
  "results": [
    { "object": "item", "id": "itm_r9f3a" },
    { "object": "item", "id": "itm_x7k2p" }
  ],
  "has_more": false,
  "next_cursor": null
}
```

This makes it safe to handle polymorphic responses — you can always check `object` to know what you're looking at.

## Timestamps

All timestamps are ISO 8601 strings in UTC:

```
2026-03-30T18:30:00Z
```

Use these fields for sync, sorting, and display. See [incremental sync](/api/pagination#incremental-sync) for how to use `updated_at` to efficiently stay in sync.