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

# POST /precheckAssets

> Step 1 of the three-step upload flow. Send SHA-256 hashes of every file in your deck; get back the subset the backend doesn't already have.

<Note>
  For most use cases, the [`slideless` CLI](/cli/overview) handles the upload flow end-to-end — you rarely need to call this endpoint directly. This reference is for custom tooling that can't run Node.
</Note>

## The three-step upload flow

Slideless uses a **content-addressed** upload protocol. Each file is stored by its SHA-256 hash, so unchanged files across deck versions dedupe automatically.

1. **`POST /precheckAssets`** — send hashes, get back which ones are missing. (This page.)
2. **[`POST /uploadPresentationAsset`](/api-reference/upload-asset)** — upload each missing blob (multipart, streamed).
3. **[`POST /commitPresentationVersion`](/api-reference/commit-presentation-version)** — commit the manifest, bump the version.

## Endpoint

```
POST https://europe-west1-slideless-ai.cloudfunctions.net/precheckAssets
```

## Auth

| Header          | Value                       |
| --------------- | --------------------------- |
| `Authorization` | `Bearer cko_…` (or `cka_…`) |
| `Content-Type`  | `application/json`          |

Required scope: `presentations:write`.

## Request body

| Field            | Type       | Required | Description                                                                  |
| ---------------- | ---------- | -------- | ---------------------------------------------------------------------------- |
| `hashes`         | `string[]` | yes      | SHA-256 hashes (64 lowercase hex chars each). Max 2000 per call.             |
| `presentationId` | string     | no       | Check against this existing presentation's blob store. Requires ownership.   |
| `sessionId`      | string     | no       | Carry a session from a prior call. Mutually exclusive with `presentationId`. |

Call patterns:

* **New presentation (first call):** omit both `presentationId` and `sessionId`. Response includes `sessionId` + reserved `presentationId` to use for `uploadPresentationAsset` and `commitPresentationVersion`.
* **Update existing presentation:** pass `presentationId`. Backend checks dedup against that presentation's blob store.
* **Subsequent calls in a new-presentation flow:** pass the `sessionId` you got from the first call.

Example:

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{
  "hashes": [
    "4edd6a4b20278337c9cd35ef2b71e375522abd1c7563301dca7eebac31b5665e",
    "c3d6ce7c672be2d98d555bed4504b944c9730ec33baf35b778cc049cf928ccb0"
  ]
}
```

## Response (200)

### New-presentation flow (no presentationId in request)

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{
  "success": true,
  "data": {
    "missing": [
      "4edd6a4b20278337c9cd35ef2b71e375522abd1c7563301dca7eebac31b5665e",
      "c3d6ce7c672be2d98d555bed4504b944c9730ec33baf35b778cc049cf928ccb0"
    ],
    "sessionId": "019dba77-...",
    "presentationId": "019dba77-4a1d-716e-8993-61e7c5771a7b"
  }
}
```

### Update flow (presentationId in request)

```json theme={"theme":{"light":"github-light","dark":"github-dark"}}
{
  "success": true,
  "data": {
    "missing": ["c3d6ce7c672be2d98d555bed4504b944c9730ec33baf35b778cc049cf928ccb0"]
  }
}
```

| Field            | Description                                                                                         |
| ---------------- | --------------------------------------------------------------------------------------------------- |
| `missing`        | Subset of input hashes that need uploading. Everything else is already in the store.                |
| `sessionId`      | Only present on new-presentation flow. Pass on subsequent calls. Sessions auto-expire after 1 hour. |
| `presentationId` | Only present on new-presentation flow. The reserved presentationId for the final commit.            |

## Errors

| Status | Code                | Cause                                                                                     |
| ------ | ------------------- | ----------------------------------------------------------------------------------------- |
| `400`  | `invalid-argument`  | `hashes` isn't an array, >2000 entries, or entries don't match the sha256 regex           |
| `401`  | `unauthenticated`   | Missing/invalid API key                                                                   |
| `403`  | `permission-denied` | Key lacks `presentations:write`, or caller is not the owner or an active dev collaborator |
| `404`  | `not-found`         | `presentationId` or `sessionId` doesn't exist (sessions expire after 1 hour)              |

## Next

* **[POST /uploadPresentationAsset](/api-reference/upload-asset)** — upload missing blobs (step 2).
* **[POST /commitPresentationVersion](/api-reference/commit-presentation-version)** — commit the manifest (step 3).