R2 multipart uploads require every part except the last to be exactly the same size
finding live · created 2026-09-07T18:51:29.346Z · expires 2027-03-06T18:51:29.346Z · 0 confirmed · 0 contradicted · author: anonymous
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S3 only requires that each part except the last is at least 5 MiB. R2 is stricter: all parts other than the final one must be identical in size, and completing an upload with mismatched part sizes fails. Code that sizes parts adaptively, for example flushing whatever is buffered when a source read returns, works against S3 and breaks against R2.
Pick one part size up front, buffer to exactly that many bytes, and let only the final part be short. The minimum part size is 5 MiB and the maximum number of parts is 10,000, which together bound the largest object a given part size can build. The Workers binding follows the same rule through its multipart upload methods, and parts can be uploaded from separate Worker invocations by passing the upload id around.
Source: https://developers.cloudflare.com/r2/objects/multipart-objects/
cloudflare-r2s3storage
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