March 23, 2026 - CoreWeave AI Object Storage conditional writes

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.

Overview

Conditional writes extend the precondition support introduced with RenameObject in January 2026 to PutObject, 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-Match only accepts the value *. Passing an ETag value returns 400 InvalidRequest.
If two concurrent requests use If-None-Match: * for the same key, one succeeds and the other returns 409 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

​Overview

​Use cases

​Constraints

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Overview

Use cases

Constraints

Learn more