From ccc87b575fbcda7e7e5d0297d5441b884aa57b96 Mon Sep 17 00:00:00 2001
From: chrislu <chris.lu@gmail.com>
Date: Sun, 16 Nov 2025 18:52:26 -0800
Subject: [PATCH] Multipart ETag semantics: conditionally set object-level Md5
 for single-chunk uploads only.

---
 weed/s3api/s3api_object_handlers_put.go | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/weed/s3api/s3api_object_handlers_put.go b/weed/s3api/s3api_object_handlers_put.go
index 6cb72607a..2d5a14d4a 100644
--- a/weed/s3api/s3api_object_handlers_put.go
+++ b/weed/s3api/s3api_object_handlers_put.go
@@ -444,12 +444,18 @@ func (s3a *S3ApiServer) putToFiler(r *http.Request, uploadUrl string, dataReader
 			Gid:      0,
 			Mime:     mimeType,
 			FileSize: uint64(chunkResult.TotalSize),
-			Md5:      md5Sum, // Set MD5 bytes for multipart ETag validation
 		},
 		Chunks:   chunkResult.FileChunks, // All chunks from auto-chunking
 		Extended: make(map[string][]byte),
 	}
 
+	// Only set Md5 for single-chunk uploads to preserve proper multipart ETag semantics
+	// For multi-chunk uploads, leave Md5 empty so getObjectETag computes the AWS-style
+	// composite format: MD5(concatenation of part MD5s) + "-" + part count
+	if len(chunkResult.FileChunks) == 1 {
+		entry.Attributes.Md5 = md5Sum
+	}
+
 	// Set object owner
 	amzAccountId := r.Header.Get(s3_constants.AmzAccountId)
 	if amzAccountId != "" {