diff --git a/common/buf/copy.go b/common/buf/copy.go
index f8d4577e3..654bf0a42 100644
--- a/common/buf/copy.go
+++ b/common/buf/copy.go
@@ -40,8 +40,10 @@ type SizeCounter struct {
 	Size int64
 }
 
+// CopyOption is an option for copying data.
 type CopyOption func(*copyHandler)
 
+// IgnoreReaderError is a CopyOption that ignores errors from reader. Copy will continue in such case.
 func IgnoreReaderError() CopyOption {
 	return func(handler *copyHandler) {
 		handler.onReadError = append(handler.onReadError, func(err error) error {
@@ -50,6 +52,7 @@ func IgnoreReaderError() CopyOption {
 	}
 }
 
+// IgnoreWriterError is a CopyOption that ignores errors from writer. Copy will continue in such case.
 func IgnoreWriterError() CopyOption {
 	return func(handler *copyHandler) {
 		handler.onWriteError = append(handler.onWriteError, func(err error) error {
@@ -58,6 +61,7 @@ func IgnoreWriterError() CopyOption {
 	}
 }
 
+// UpdateActivity is a CopyOption to update activity on each data copy operation.
 func UpdateActivity(timer signal.ActivityUpdater) CopyOption {
 	return func(handler *copyHandler) {
 		handler.onData = append(handler.onData, func(MultiBuffer) {
@@ -66,6 +70,7 @@ func UpdateActivity(timer signal.ActivityUpdater) CopyOption {
 	}
 }
 
+// CountSize is a CopyOption that sums the total size of data copied into the given SizeCounter.
 func CountSize(sc *SizeCounter) CopyOption {
 	return func(handler *copyHandler) {
 		handler.onData = append(handler.onData, func(b MultiBuffer) {
@@ -97,8 +102,7 @@ func copyInternal(reader Reader, writer Writer, handler *copyHandler) error {
 	}
 }
 
-// Copy dumps all payload from reader to writer or stops when an error occurs.
-// ActivityTimer gets updated as soon as there is a payload.
+// Copy dumps all payload from reader to writer or stops when an error occurs. It returns nil when EOF.
 func Copy(reader Reader, writer Writer, options ...CopyOption) error {
 	handler := new(copyHandler)
 	for _, option := range options {