fix(push): ensure branches are rebased before pushing

cmd/push.go

@@ -77,6 +77,22 @@ func runPush(cmd *cobra.Command, args []string) error {
 		branches[i], branches[j] = branches[j], branches[i]
 	}
 
+	// Validate all branches are properly rebased onto their parents
+	for _, b := range branches {
+		parent, _ := cfg.GetParent(b.Name) //nolint:errcheck // empty string is fine
+		if parent == "" {
+			continue
+		}
+
+		needsRebase, err := g.NeedsRebase(b.Name, parent)
+		if err != nil {
+			return fmt.Errorf("failed to check rebase status for %s: %w", b.Name, err)
+		}
+		if needsRebase {
+			return fmt.Errorf("branch %q is not rebased onto %q; run 'gh stack cascade' first", b.Name, parent)
+		}
+	}
+
 	// Update PR bases and push
 	for _, b := range branches {
 		parent, _ := cfg.GetParent(b.Name) //nolint:errcheck // empty string is fine

e2e/push_test.go

@@ -40,3 +40,36 @@ func TestPushStack(t *testing.T) {
 		t.Errorf("stack not fully pushed: %s", remoteBranches)
 	}
 }
+
+func TestPushFailsWhenNotRebased(t *testing.T) {
+	env := NewTestEnvWithRemote(t)
+	env.MustRun("init")
+
+	// Create stack: main -> feat-a -> feat-b
+	env.MustRun("create", "feat-a")
+	env.CreateCommit("a work")
+
+	env.MustRun("create", "feat-b")
+	env.CreateCommit("b work")
+
+	// Go back to feat-a and add a new commit
+	// This makes feat-b no longer rebased onto feat-a
+	env.Git("checkout", "feat-a")
+	env.CreateCommit("more a work")
+
+	// Go to feat-b and try to push - should fail
+	env.Git("checkout", "feat-b")
+	result := env.Run("push")
+
+	if result.Success() {
+		t.Error("expected push to fail when branch is not rebased")
+	}
+
+	if !strings.Contains(result.Stderr, "not rebased onto") {
+		t.Errorf("expected error about rebase, got: %s", result.Stderr)
+	}
+
+	if !strings.Contains(result.Stderr, "gh stack cascade") {
+		t.Errorf("expected error to mention 'gh stack cascade', got: %s", result.Stderr)
+	}
+}