feat(adopt): allow re-adoption of branches

README.md

@@ -206,7 +206,7 @@ gh stack create <name>
 
 Start tracking an existing branch by setting its parent.
 
-By default, adopts the current branch. The parent must be either the trunk or another tracked branch.
+By default, adopts the current branch. The parent must be either the trunk or another tracked branch. If the branch is already tracked, `adopt` updates its parent to the specified branch (or does nothing if the parent is unchanged).
 
 #### adopt Usage
 

cmd/adopt.go

@@ -62,9 +62,15 @@ func runAdopt(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("branch %q does not exist", branchName)
 	}
 
-	// Check if already tracked
-	if _, getParentErr := cfg.GetParent(branchName); getParentErr == nil {
-		return fmt.Errorf("branch %q is already tracked", branchName)
+	// A branch cannot be its own parent
+	if parent == branchName {
+		return fmt.Errorf("cannot adopt: branch %q cannot be its own parent", branchName)
+	}
+
+	// Check if already tracked, capture old parent if so
+	oldParent, alreadyTracked := "", false
+	if p, getParentErr := cfg.GetParent(branchName); getParentErr == nil {
+		oldParent, alreadyTracked = p, true
 	}
 
 	// Validate parent is trunk or tracked
@@ -94,6 +100,14 @@ func runAdopt(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	s := style.New()
+
+	// No-op: already tracked with the same parent
+	if alreadyTracked && oldParent == parent {
+		fmt.Printf("%s Branch %s is already tracked with parent %s\n", s.WarningIcon(), s.Branch(branchName), s.Branch(parent))
+		return nil
+	}
+
 	// Set parent
 	if err := cfg.SetParent(branchName, parent); err != nil {
 		return err
@@ -105,7 +119,10 @@ func runAdopt(cmd *cobra.Command, args []string) error {
 		_ = cfg.SetForkPoint(branchName, forkPoint) //nolint:errcheck // best effort
 	}
 
-	s := style.New()
-	fmt.Printf("%s Adopted branch %s with parent %s\n", s.SuccessIcon(), s.Branch(branchName), s.Branch(parent))
+	if alreadyTracked {
+		fmt.Printf("%s Updated branch %s parent from %s to %s\n", s.SuccessIcon(), s.Branch(branchName), s.Branch(oldParent), s.Branch(parent))
+	} else {
+		fmt.Printf("%s Adopted branch %s with parent %s\n", s.SuccessIcon(), s.Branch(branchName), s.Branch(parent))
+	}
 	return nil
 }

cmd/adopt_test.go

@@ -37,7 +37,7 @@ func TestAdoptBranch(t *testing.T) {
 	}
 }
 
-func TestAdoptRejectsAlreadyTracked(t *testing.T) {
+func TestAdoptReparentsAlreadyTracked(t *testing.T) {
 	dir := setupTestRepo(t)
 
 	cfg, _ := config.Load(dir)
@@ -46,14 +46,23 @@ func TestAdoptRejectsAlreadyTracked(t *testing.T) {
 	trunk, _ := g.CurrentBranch()
 	cfg.SetTrunk(trunk)
 
-	// Create and track a branch
-	g.CreateBranch("tracked-feature")
-	cfg.SetParent("tracked-feature", trunk)
+	// Create two branches; track feature-a under trunk
+	g.CreateBranch("feature-a")
+	cfg.SetParent("feature-a", trunk)
+	g.CreateBranch("feature-b")
+	cfg.SetParent("feature-b", trunk)
+
+	// Simulate what runAdopt does when reparenting feature-b under feature-a
+	if err := cfg.SetParent("feature-b", "feature-a"); err != nil {
+		t.Fatalf("SetParent failed: %v", err)
+	}
 
-	// Trying to get parent should succeed (it's tracked)
-	_, err := cfg.GetParent("tracked-feature")
+	parent, err := cfg.GetParent("feature-b")
 	if err != nil {
-		t.Error("expected branch to be tracked")
+		t.Fatalf("GetParent failed: %v", err)
+	}
+	if parent != "feature-a" {
+		t.Errorf("expected parent %q after reparent, got %q", "feature-a", parent)
 	}
 }
 

e2e/adopt_orphan_test.go

@@ -3,6 +3,84 @@ package e2e_test
 
 import "testing"
 
+func TestAdoptReparentsTrackedBranch(t *testing.T) {
+	env := NewTestEnv(t)
+	env.MustRun("init")
+
+	// Create feat-a and feat-b both off trunk (main)
+	env.MustRun("create", "feat-a")
+	env.CreateCommit("feat-a work")
+
+	env.Git("checkout", "main")
+	env.MustRun("create", "feat-b")
+	env.CreateCommit("feat-b work")
+
+	// feat-b is currently tracked with parent main; reparent onto feat-a
+	result := env.MustRun("adopt", "--branch", "feat-b", "feat-a")
+
+	env.AssertStackParent("feat-b", "feat-a")
+	if !result.ContainsStdout("feat-a") {
+		t.Errorf("expected stdout to mention feat-a, got: %s", result.Stdout)
+	}
+	if !result.ContainsStdout("main") {
+		t.Errorf("expected stdout to mention old parent main, got: %s", result.Stdout)
+	}
+}
+
+func TestAdoptNoOpWhenParentUnchangedE2E(t *testing.T) {
+	env := NewTestEnv(t)
+	env.MustRun("init")
+
+	env.Git("checkout", "-b", "existing-branch")
+	env.CreateCommit("some work")
+	env.MustRun("adopt", "main")
+
+	// Adopt again with same parent: should succeed and print a warning
+	result := env.Run("adopt", "main")
+	if !result.Success() {
+		t.Errorf("expected no-op adopt to succeed, got exit %d: %s", result.ExitCode, result.Stderr)
+	}
+	if !result.ContainsStdout("already tracked") {
+		t.Errorf("expected 'already tracked' in stdout, got: %s", result.Stdout)
+	}
+}
+
+func TestAdoptSelfParentRejected(t *testing.T) {
+	env := NewTestEnv(t)
+	env.MustRun("init")
+
+	env.MustRun("create", "feat-a")
+	env.CreateCommit("a work")
+
+	result := env.Run("adopt", "--branch", "feat-a", "feat-a")
+	if result.Success() {
+		t.Error("expected self-parent adopt to fail, but it succeeded")
+	}
+	if !result.ContainsStderr("own parent") {
+		t.Errorf("expected 'own parent' in stderr, got: %s", result.Stderr)
+	}
+}
+
+func TestAdoptReparentCycleDetected(t *testing.T) {
+	env := NewTestEnv(t)
+	env.MustRun("init")
+
+	// Build trunk → feat-a → feat-b
+	env.MustRun("create", "feat-a")
+	env.CreateCommit("a work")
+	env.MustRun("create", "feat-b")
+	env.CreateCommit("b work")
+
+	// Attempting to reparent feat-a under feat-b would create a cycle
+	result := env.Run("adopt", "--branch", "feat-a", "feat-b")
+	if result.Success() {
+		t.Error("expected cycle detection to fail, but adopt succeeded")
+	}
+	if !result.ContainsStderr("cycle") {
+		t.Errorf("expected 'cycle' in stderr, got: %s", result.Stderr)
+	}
+}
+
 func TestAdoptExistingBranch(t *testing.T) {
 	env := NewTestEnv(t)
 	env.MustRun("init")