Bucket layout

A version is a directory holding its _manifest.json. Artifacts writes each upload's files under a directory of its own inside it, named in the manifest's dir, so a retried or superseded upload never touches another's files:

<bucket>/<location path>/<artifact>/<version>/<dir>/<file>    # location path: empty for created buckets
o41art-7f3k2q9d/experiments/hoonshot/1234/step-00000500/u3f9c2a7d1e04b6a8c5f2/params.safetensors
                                                   .../u3f9c2a7d1e04b6a8c5f2/state.json
                                                   .../_manifest.json   # written last

The crate writing to a raw s3://, gs://, az:// or local location puts the files beside the manifest and writes dir: null. Every reader, the crate's included, takes the files from <version>/<dir>/ when dir is set and from <version>/ otherwise.

The manifest

{
  "format": "o41-artifacts/1",
  "artifact": "experiments/hoonshot/1234",
  "version": "step-00000500",
  "files": [
    { "name": "params.safetensors", "size": 2793406464 },
    { "name": "state.json", "size": 912 }
  ],
  "meta": { "step": 500 },
  "source": null,
  "dir": "u3f9c2a7d1e04b6a8c5f2",
  "committed": "2026-10-06T11:42:10Z"
}

source is the full path a fork's first version was forked from, null otherwise.

Commit

• A version is committed when its manifest exists and the service has marked it so. The service completes every upload, checks every size, writes _manifest.json, then commits.
• A version without a manifest is a cut-off write: never listed, never the latest.
• Pending uploads expire after 24 hours: they are aborted and what was written is deleted. Created buckets also abort unfinished multipart uploads after 2 days.
• Buckets are self-describing: an artifact's catalog can be rebuilt from its manifests.