3 files changed, 18 insertions, 8 deletions
diff --git a/object/commit/append.go b/object/commit/append.go
index d5258b97..02d69058 100644
--- a/object/commit/append.go
+++ b/object/commit/append.go
@@ -33,7 +33,7 @@ func (commit *Commit) AppendWithoutHeader(dst []byte) ([]byte, error) {
 
 	dst = append(dst, byte('\n'))
 
-	if commit.ChangeID != "" {
+	if len(commit.ChangeID) != 0 {
 		dst = append(dst, []byte("change-id ")...)
 		dst = append(dst, commit.ChangeID...)
 		dst = append(dst, byte('\n'))
@@ -41,7 +41,7 @@ func (commit *Commit) AppendWithoutHeader(dst []byte) ([]byte, error) {
 
 	for _, h := range commit.ExtraHeaders {
 		// GIGO on empty keys and such.
-		dst = append(dst, []byte(h.Key)...)
+		dst = append(dst, h.Key...)
 		dst = append(dst, byte(' '))
 		dst = append(dst, h.Value...)
 		dst = append(dst, byte('\n'))
diff --git a/object/commit/commit.go b/object/commit/commit.go
index 6a89bce9..a8a247bf 100644
--- a/object/commit/commit.go
+++ b/object/commit/commit.go
@@ -14,12 +14,12 @@ type Commit struct {
 	Author       signature.Signature
 	Committer    signature.Signature
 	Message      []byte
-	ChangeID     string
+	ChangeID     []byte
 	ExtraHeaders []ExtraHeader
 }
 
 // ExtraHeader represents an extra header in a Git object.
 type ExtraHeader struct {
-	Key   string
+	Key   []byte
 	Value []byte
 }
diff --git a/object/commit/parse.go b/object/commit/parse.go
index 20353e14..74f607f1 100644
--- a/object/commit/parse.go
+++ b/object/commit/parse.go
@@ -13,6 +13,16 @@ import (
 var ErrInvalidCommit = errors.New("object/commit: invalid commit")
 
 // Parse decodes a commit object body.
+//
+// The returned commit aliases body:
+// its Message, ChangeID, and extra-header fields,
+// along with the byte fields of its signatures,
+// share body's backing array.
+// The commit inherits body's lifetime
+// and must not be mutated unless body may be.
+// Use [Commit.Clone] for an independent copy.
+//
+// Labels: Life-Parent, Mut-No.
 func Parse(body []byte, objectFormat id.ObjectFormat) (*Commit, error) {
 	c := new(Commit)
 
@@ -95,7 +105,7 @@ func Parse(body []byte, objectFormat id.ObjectFormat) (*Commit, error) {
 				return nil, fmt.Errorf("%w: unexpected change-id header at offset %d", ErrInvalidCommit, lineStart)
 			}
 
-			c.ChangeID = string(value)
+			c.ChangeID = value
 		case "gpgsig", "gpgsig-sha256":
 			if state != parseStateExtra {
 				return nil, fmt.Errorf("%w: unexpected %s header at offset %d", ErrInvalidCommit, key, lineStart)
@@ -119,8 +129,8 @@ func Parse(body []byte, objectFormat id.ObjectFormat) (*Commit, error) {
 			}
 
 			c.ExtraHeaders = append(c.ExtraHeaders, ExtraHeader{
-				Key:   string(key),
-				Value: append([]byte(nil), value...),
+				Key:   key,
+				Value: value,
 			})
 		}
 	}
@@ -141,7 +151,7 @@ func Parse(body []byte, objectFormat id.ObjectFormat) (*Commit, error) {
 		panic("unreachable parse state")
 	}
 
-	c.Message = append([]byte(nil), body[i:]...)
+	c.Message = body[i:]
 
 	return c, nil
 }