diff --git a/weed/s3api/s3api_object_handlers.go b/weed/s3api/s3api_object_handlers.go
index 8a3715560..0d27dcf69 100644
--- a/weed/s3api/s3api_object_handlers.go
+++ b/weed/s3api/s3api_object_handlers.go
@@ -1182,7 +1182,16 @@ func (s3a *S3ApiServer) setResponseHeaders(w http.ResponseWriter, entry *filer_p
 		for k, v := range entry.Extended {
 			// Skip internal SeaweedFS headers
 			if !strings.HasPrefix(k, "xattr-") && !s3_constants.IsSeaweedFSInternalHeader(k) {
-				w.Header()[k] = []string{string(v)}
+				// Support backward compatibility: migrate old lowercase format to canonical format
+				// OLD: "x-amz-meta-foo" → NEW: "X-Amz-Meta-foo"
+				headerKey := k
+				if strings.HasPrefix(strings.ToLower(k), "x-amz-meta-") && !strings.HasPrefix(k, s3_constants.AmzUserMetaPrefix) {
+					// Old format detected - normalize to canonical prefix
+					suffix := strings.ToLower(k[len("x-amz-meta-"):])
+					headerKey = s3_constants.AmzUserMetaPrefix + suffix
+					glog.V(4).Infof("Migrating old user metadata header %q to %q in response", k, headerKey)
+				}
+				w.Header()[headerKey] = []string{string(v)}
 			}
 		}
 	}
diff --git a/weed/s3api/s3api_object_handlers_copy.go b/weed/s3api/s3api_object_handlers_copy.go
index 28f7abcd5..ce65534b4 100644
--- a/weed/s3api/s3api_object_handlers_copy.go
+++ b/weed/s3api/s3api_object_handlers_copy.go
@@ -700,15 +700,29 @@ func processMetadataBytes(reqHeader http.Header, existing map[string][]byte, rep
 	if replaceMeta {
 		for header, values := range reqHeader {
 			if strings.HasPrefix(header, s3_constants.AmzUserMetaPrefix) {
+				// AWS S3 stores user metadata keys in lowercase
+				// Go's HTTP server canonicalizes headers (e.g., x-amz-meta-foo → X-Amz-Meta-Foo)
+				// Preserve the canonical prefix "X-Amz-Meta-" but lowercase the user-defined suffix
+				suffix := strings.ToLower(header[len(s3_constants.AmzUserMetaPrefix):])
+				normalizedKey := s3_constants.AmzUserMetaPrefix + suffix
 				for _, value := range values {
-					metadata[header] = []byte(value)
+					metadata[normalizedKey] = []byte(value)
 				}
 			}
 		}
 	} else {
+		// Copy existing metadata as-is (already normalized during storage)
+		// Support both old format (x-amz-meta-*) and new format (X-Amz-Meta-*) for backward compatibility
 		for k, v := range existing {
 			if strings.HasPrefix(k, s3_constants.AmzUserMetaPrefix) {
+				// New format (X-Amz-Meta-foo) - copy as-is
 				metadata[k] = v
+			} else if strings.HasPrefix(strings.ToLower(k), "x-amz-meta-") {
+				// Old format (x-amz-meta-foo) - migrate to new format
+				suffix := strings.ToLower(k[len("x-amz-meta-"):])
+				normalizedKey := s3_constants.AmzUserMetaPrefix + suffix
+				metadata[normalizedKey] = v
+				glog.V(4).Infof("Migrating old user metadata key %q to %q", k, normalizedKey)
 			}
 		}
 	}
diff --git a/weed/s3api/s3api_object_handlers_put.go b/weed/s3api/s3api_object_handlers_put.go
index a08d3e04c..d8181f3dd 100644
--- a/weed/s3api/s3api_object_handlers_put.go
+++ b/weed/s3api/s3api_object_handlers_put.go
@@ -480,12 +480,14 @@ func (s3a *S3ApiServer) putToFiler(r *http.Request, uploadUrl string, dataReader
 	// Copy user metadata and standard headers
 	for k, v := range r.Header {
 		if len(v) > 0 && len(v[0]) > 0 {
-			if strings.HasPrefix(k, "X-Amz-Meta-") {
+			if strings.HasPrefix(k, s3_constants.AmzUserMetaPrefix) {
 				// AWS S3 stores user metadata keys in lowercase
 				// Go's HTTP server canonicalizes headers (e.g., x-amz-meta-foo → X-Amz-Meta-Foo)
-				// but S3 expects lowercase, so convert back to lowercase for storage
-				lowerKey := strings.ToLower(k)
-				entry.Extended[lowerKey] = []byte(v[0])
+				// Preserve the canonical prefix "X-Amz-Meta-" but lowercase the user-defined suffix
+				// This ensures the key is still detectable via prefix checks elsewhere
+				suffix := strings.ToLower(k[len(s3_constants.AmzUserMetaPrefix):])
+				normalizedKey := s3_constants.AmzUserMetaPrefix + suffix
+				entry.Extended[normalizedKey] = []byte(v[0])
 			} else if k == "Cache-Control" || k == "Expires" || k == "Content-Disposition" {
 				entry.Extended[k] = []byte(v[0])
 			}
diff --git a/weed/server/filer_server_handlers_write_autochunk.go b/weed/server/filer_server_handlers_write_autochunk.go
index 3b9689858..59b83501b 100644
--- a/weed/server/filer_server_handlers_write_autochunk.go
+++ b/weed/server/filer_server_handlers_write_autochunk.go
@@ -546,10 +546,12 @@ func SaveAmzMetaData(r *http.Request, existing map[string][]byte, isReplace bool
 		if strings.HasPrefix(header, s3_constants.AmzUserMetaPrefix) {
 			// AWS S3 stores user metadata keys in lowercase
 			// Go's HTTP server canonicalizes headers (e.g., x-amz-meta-foo → X-Amz-Meta-Foo)
-			// but S3 expects lowercase, so convert back to lowercase for storage
-			lowerHeader := strings.ToLower(header)
+			// Preserve the canonical prefix "X-Amz-Meta-" but lowercase the user-defined suffix
+			// This ensures the key is still detectable via prefix checks elsewhere
+			suffix := strings.ToLower(header[len(s3_constants.AmzUserMetaPrefix):])
+			normalizedKey := s3_constants.AmzUserMetaPrefix + suffix
 			for _, value := range values {
-				metadata[lowerHeader] = []byte(value)
+				metadata[normalizedKey] = []byte(value)
 			}
 		}
 	}