diff --git a/process.go b/process.go
index 2c153d8..76cfc5d 100644
--- a/process.go
+++ b/process.go
@@ -166,6 +166,22 @@ func (p *Process) kill() (bool, error) {
 	return true, p.cmd.Process.Kill()
 }
 
+// killTree forcefully terminates the process group when one exists.
+func (p *Process) killTree() (bool, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	if p.Status != StatusRunning {
+		return false, nil
+	}
+
+	if p.cmd == nil || p.cmd.Process == nil {
+		return false, nil
+	}
+
+	return true, syscall.Kill(-p.cmd.Process.Pid, syscall.SIGKILL)
+}
+
 // Shutdown gracefully stops the process: SIGTERM, then SIGKILL after grace period.
 // If GracePeriod was not set (zero), falls back to immediate Kill().
 // If KillGroup is set, signals are sent to the entire process group.
diff --git a/service.go b/service.go
index 13cc019..c0d3c6b 100644
--- a/service.go
+++ b/service.go
@@ -112,7 +112,7 @@ func (s *Service) OnShutdown(ctx context.Context) error {
 	s.mu.RUnlock()
 
 	for _, p := range procs {
-		_ = p.Kill()
+		_, _ = p.killTree()
 	}
 
 	return nil
@@ -165,10 +165,9 @@ func (s *Service) StartWithOptions(ctx context.Context, opts RunOptions) (*Proce
 		cmd.Env = append(cmd.Environ(), opts.Env...)
 	}
 
-	// Detached processes get their own process group
-	if opts.Detach {
-		cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
-	}
+	// Put every subprocess in its own process group so shutdown can terminate
+	// the full tree without affecting the parent process.
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
 
 	// Set up pipes
 	stdout, err := cmd.StdoutPipe()