---
url: 'https://adk.nht.io/api/@nhtio/adk/common/interfaces/ReaderDescriptor.md'
description: >-
  A tagged, serialisable description of *where* a reader's bytes live — never
  the bytes themselves.
---

# Interface: ReaderDescriptor

Defined in: [src/lib/contracts/reader\_descriptor.ts:40](https://github.com/NHTIO/ADK/blob/v1.20260713.1/src/src/lib/contracts/reader_descriptor.ts#L40)

A tagged, serialisable description of *where* a reader's bytes live — never the bytes themselves.

## Remarks

Reader-backed primitives ([@nhtio/adk!Media](../classes/Media.md), [@nhtio/adk!SpooledArtifact](../../spooled_artifact/classes/SpooledArtifact.md)) serialise as
**handles**. On `encode()`, the reader emits this descriptor via its optional `describe()` method; on
`decode()`, the registered resolver for `tag` re-binds the `locator` to a live reader (re-injecting
the ambient `Disk`/OPFS root/`fetch` the locator alone cannot carry).

* `tag` — the resolver key (e.g. `"spool:flydrive"`, `"media:in-memory"`, `"media:fetch"`). Namespaced
  `media:` / `spool:` so the two reader families never collide in their respective registries.
* `locator` — the JSON pointer the resolver needs: a storage key, a URL, or (for in-memory readers
  that own their bytes) the buffer itself, base64-encoded.

## Properties

| Property                                | Type                                              | Description                                                                        | Defined in                                                                                                                                  |
| --------------------------------------- | ------------------------------------------------- | ---------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- |
|  `locator` | [`LocatorValue`](../type-aliases/LocatorValue.md) | JSON-expressible pointer the resolver consumes to re-open the byte source.         | [src/lib/contracts/reader\_descriptor.ts:44](https://github.com/NHTIO/ADK/blob/v1.20260713.1/src/src/lib/contracts/reader_descriptor.ts#L44) |
|  `tag`         | `string`                                          | Resolver key identifying which registered resolver re-binds this handle on decode. | [src/lib/contracts/reader\_descriptor.ts:42](https://github.com/NHTIO/ADK/blob/v1.20260713.1/src/src/lib/contracts/reader_descriptor.ts#L42) |