CoreWeave AI Object Storage now supports conditional writes. Use HTTP precondition headers to make writes atomic and prevent accidental overwrites without requiring client-side locking.Documentation Index
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Overview
Conditional writes extend the precondition support introduced with RenameObject in January 2026 toPutObject, CompleteMultipartUpload, and CopyObject. Attach preconditions to these requests to make writes atomic and prevent accidental overwrites. The operation succeeds only if the precondition is met; otherwise the server returns a 412 Precondition Failed error and the object is not modified.
Two headers are supported:
| Header | Value | Behavior |
|---|---|---|
If-None-Match | * | Write only if no object exists at the key. Prevents accidental overwrites of existing data. |
If-Match | ETag string | Write only if the object’s current ETag matches. Enables atomic compare-and-swap updates. |
Use cases
| Use case | Description |
|---|---|
| Safe checkpoint writes | Use If-None-Match: * to write a checkpoint only once, even when multiple training workers race to write the same key. |
| Atomic model promotion | Use If-Match with the current ETag to swap a model file only if it has not changed since you last read it. |
| Idempotent initialization | Use If-None-Match: * to create configuration objects or seed files without risk of overwriting existing data. |
Constraints
If-None-Matchonly accepts the value*. Passing an ETag value returns400 InvalidRequest.- If two concurrent requests use
If-None-Match: *for the same key, one succeeds and the other returns409 ConditionalRequestConflict. Retry on 409.
Learn more
- See the guide for using conditional requests for AWS CLI and Boto3 examples
- See the API reference for Conditional writes and S3 PutObject for more information
- See RenameObject (January 30, 2026) for the same precondition pattern with additional source, destination, and time-based conditions