diff --git a/git-codereview/hook.go b/git-codereview/hook.go
index 921fd91..31dff11 100644
--- a/git-codereview/hook.go
+++ b/git-codereview/hook.go
@@ -113,33 +113,59 @@
 // It adds a Change-Id line to the bottom of the commit message
 // if there is not one already.
 func hookCommitMsg(args []string) {
-	// Add Change-Id to commit message if needed.
 	if len(args) != 1 {
 		dief("usage: git-codereview hook-invoke commit-msg message.txt\n")
 	}
+
+	b := CurrentBranch()
+	if b.DetachedHead() {
+		// Likely executing rebase or some other internal operation.
+		// Probably a mistake to make commit message changes.
+		return
+	}
+
 	file := args[0]
 	data, err := ioutil.ReadFile(file)
 	if err != nil {
 		dief("%v", err)
 	}
 	data = stripComments(data)
+
+	// Empty message not allowed.
 	if len(bytes.TrimSpace(data)) == 0 {
 		dief("empty commit message")
 	}
-	if bytes.Contains(data, []byte("\nChange-Id: ")) {
-		return
+
+	// Add Change-Id to commit message if not present.
+	edited := false
+	if !bytes.Contains(data, []byte("\nChange-Id: ")) {
+		edited = true
+		n := len(data)
+		for n > 0 && data[n-1] == '\n' {
+			n--
+		}
+		var id [20]byte
+		if _, err := io.ReadFull(rand.Reader, id[:]); err != nil {
+			dief("generating Change-Id: %v", err)
+		}
+		data = append(data[:n], fmt.Sprintf("\n\nChange-Id: I%x\n", id[:])...)
 	}
-	n := len(data)
-	for n > 0 && data[n-1] == '\n' {
-		n--
+
+	// Add branch prefix to commit message if not present and not on master.
+	branch := strings.TrimPrefix(b.OriginBranch(), "origin/")
+	if branch != "master" {
+		prefix := "[" + branch + "] "
+		if !bytes.HasPrefix(data, []byte(prefix)) {
+			edited = true
+			data = []byte(prefix + string(data))
+		}
 	}
-	var id [20]byte
-	if _, err := io.ReadFull(rand.Reader, id[:]); err != nil {
-		dief("generating Change-Id: %v", err)
-	}
-	data = append(data[:n], fmt.Sprintf("\n\nChange-Id: I%x\n", id[:])...)
-	if err := ioutil.WriteFile(file, data, 0666); err != nil {
-		dief("%v", err)
+
+	// Write back.
+	if edited {
+		if err := ioutil.WriteFile(file, data, 0666); err != nil {
+			dief("%v", err)
+		}
 	}
 }
 
diff --git a/git-codereview/hook_test.go b/git-codereview/hook_test.go
index 9465b55..671af94 100644
--- a/git-codereview/hook_test.go
+++ b/git-codereview/hook_test.go
@@ -48,6 +48,42 @@
 	}
 }
 
+func TestHookCommitMsgBranchPrefix(t *testing.T) {
+	gt := newGitTest(t)
+	defer gt.done()
+
+	checkPrefix := func(prefix string) {
+		write(t, gt.client+"/msg.txt", "Test message.\n")
+		testMain(t, "hook-invoke", "commit-msg", gt.client+"/msg.txt")
+		data, err := ioutil.ReadFile(gt.client + "/msg.txt")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.HasPrefix(data, []byte(prefix)) {
+			t.Errorf("after hook-invoke commit-msg on %s, want prefix %q:\n%s", CurrentBranch().Name, prefix, data)
+		}
+	}
+
+	// Create server branch and switch to server branch on client.
+	// Test that commit hook adds prefix.
+	trun(t, gt.server, "git", "checkout", "-b", "dev.cc")
+	trun(t, gt.client, "git", "fetch", "-q")
+	trun(t, gt.client, "git", "change", "dev.cc")
+	checkPrefix("[dev.cc] Test message.\n")
+
+	// Work branch with server branch as upstream.
+	trun(t, gt.client, "git", "change", "ccwork")
+	checkPrefix("[dev.cc] Test message.\n")
+
+	// Master has no prefix.
+	trun(t, gt.client, "git", "change", "master")
+	checkPrefix("Test message.\n")
+
+	// Work branch from master has no prefix.
+	trun(t, gt.client, "git", "change", "work")
+	checkPrefix("Test message.\n")
+}
+
 func TestHookPreCommit(t *testing.T) {
 	gt := newGitTest(t)
 	defer gt.done()