# What is object storage?

Object storage keeps data as whole objects in buckets. Each object is addressed
by a key over HTTP. It does not store data as files in folders or as disk
blocks.

**A Runtime sandbox mounts your bucket from Amazon S3, Cloudflare R2 or Google
Cloud Storage as a folder. The sandbox never holds the bucket's key.** You
store the key once as a secret. The host signs the sandbox's requests on the
way out. Code in the sandbox reads and writes the files, and it cannot copy the
credential ([mount your own bucket](/docs/storage#mount-your-own-bucket)).

## How it is organised

Amazon's S3 guide describes the model with three terms. An **object** "is a
file and any metadata that describes the file". A **bucket** "is a container
for objects". A **key** "is the unique identifier for an object within a
bucket". Object storage has no real directories. `photos/puppy.jpg` is one key
that happens to contain a slash.

## Object, block and file storage

| Kind   | Addressed by           | Changed by                 | Typical use                             |
| ------ | ---------------------- | -------------------------- | --------------------------------------- |
| Object | Bucket and key, HTTP   | Replacing the whole object | Datasets, backups, media, build outputs |
| Block  | A disk's block numbers | Writing any block          | A machine's root disk, databases        |
| File   | A path in a tree       | Writing part of a file     | Shared folders, home directories        |

Object stores trade in-place edits for scale and price. S3 promises
strong read-after-write consistency. Cloudflare R2 stores data for $0.015 per
GB-month and does not charge for [egress](/glossary/egress-fees).

## Object storage and sandboxes

An agent's input data usually already lives in a bucket. Mounting the bucket
saves a download on every run. The host keeps the key, so a prompt injection
in that data cannot send the credential anywhere. Runtime stores its own state
on disks instead. That state includes a sandbox's files, a [volume](/docs/storage)
that outlives sandboxes, and snapshots copied off their server
([snapshots survive their server](/docs/storage#snapshots-survive-their-server)).
Runtime does not sell an object store of its own today. It mounts yours.

## Related

- [How to mount an S3 bucket in a sandbox](/how-to/mount-an-s3-bucket)
- [What are egress fees?](/glossary/egress-fees)
- [What is a sandbox snapshot?](/glossary/sandbox-snapshot)
- [What is workload identity?](/glossary/workload-identity)

## Sources

Checked 25 October 2026.

- [What is Amazon S3?, AWS](https://docs.aws.amazon.com/AmazonS3/latest/userguide/Welcome.html)
- [Cloudflare R2 pricing](https://developers.cloudflare.com/r2/pricing/)