diff --git a/internal/core/local_runtime/instance.go b/internal/core/local_runtime/instance.go
index 9488c8e72..3af3d719b 100644
--- a/internal/core/local_runtime/instance.go
+++ b/internal/core/local_runtime/instance.go
@@ -127,6 +127,36 @@ func (s *PluginInstance) Stop() {
 // Once the subprocess exists itself, STDOUT always close, which results in `CLOSE STDOUT`
 func (s *PluginInstance) StartStdout() {
 	defer func() {
+		// mark shutdown to stop monitor loop
+		s.shutdown = true
+		// best-effort kill process (and its group on supported platforms)
+		if s.cmd != nil && s.cmd.Process != nil {
+			pid := s.cmd.Process.Pid
+			pgid, getpgidErr, groupKillErr := tryKillProcessGroup(s.cmd)
+			if getpgidErr == nil {
+				if groupKillErr != nil {
+					s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
+						notifier.OnInstanceErrorLog(s, fmt.Errorf("failed to kill process group %d: %s", pgid, groupKillErr.Error()))
+					})
+				}
+			} else {
+				// fallback to single-process kill
+				s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
+					notifier.OnInstanceWarningLog(s, fmt.Sprintf("failed to get pgid for pid %d: %s. fallback to killing process", pid, getpgidErr.Error()))
+				})
+				if err := killProcess(s.cmd); err != nil {
+					s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
+						notifier.OnInstanceErrorLog(s, fmt.Errorf("failed to kill process %d: %s", pid, err.Error()))
+					})
+				}
+			}
+			// reap once
+			if err := s.cmd.Wait(); err != nil {
+				s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
+					notifier.OnInstanceErrorLog(s, fmt.Errorf("failed to reap process %d: %s", pid, err.Error()))
+				})
+			}
+		}
 		// notify shutdown signal
 		s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
 			notifier.OnInstanceShutdown(s)
@@ -171,22 +201,7 @@ func (s *PluginInstance) StartStdout() {
 			)
 		})
 	}
-
-	// once reader of stdout is closed, kill subprocess
-	if err := s.cmd.Process.Kill(); err != nil {
-		// no need to return here, just log the error, it's perhaps the process was exited already
-		// and the kill command fails
-		s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
-			notifier.OnInstanceErrorLog(s, fmt.Errorf("failed to kill subprocess: %s", err.Error()))
-		})
-	}
-
-	// collect subprocess, avoid zombie processes
-	if _, err := s.cmd.Process.Wait(); err != nil {
-		s.WalkNotifiers(func(notifier PluginInstanceNotifier) {
-			notifier.OnInstanceErrorLog(s, fmt.Errorf("failed to reap subprocess: %s", err.Error()))
-		})
-	}
+	// process kill and wait are handled in the deferred cleanup above
 }
 
 // handles stdout data and notify corresponding listeners
diff --git a/internal/core/local_runtime/proc_others.go b/internal/core/local_runtime/proc_others.go
new file mode 100644
index 000000000..dfdc2a385
--- /dev/null
+++ b/internal/core/local_runtime/proc_others.go
@@ -0,0 +1,11 @@
+//go:build !(darwin || linux || freebsd || netbsd || openbsd || windows)
+
+package local_runtime
+
+import "os/exec"
+
+func setProcGroup(cmd *exec.Cmd) {}
+
+func tryKillProcessGroup(cmd *exec.Cmd) (int, error, error) { return 0, nil, nil }
+
+func killProcess(cmd *exec.Cmd) error { if cmd != nil && cmd.Process != nil { return cmd.Process.Kill() }; return nil }
\ No newline at end of file
diff --git a/internal/core/local_runtime/proc_unix.go b/internal/core/local_runtime/proc_unix.go
new file mode 100644
index 000000000..0509de950
--- /dev/null
+++ b/internal/core/local_runtime/proc_unix.go
@@ -0,0 +1,40 @@
+//go:build darwin || linux || freebsd || netbsd || openbsd
+
+package local_runtime
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// setProcGroup places the child into its own process group (Unix only)
+func setProcGroup(cmd *exec.Cmd) {
+	if cmd == nil {
+		return
+	}
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+}
+
+// tryKillProcessGroup tries to kill the process group and returns diagnostic errors.
+// Returns: (pgid, getpgidErr, killErr)
+func tryKillProcessGroup(cmd *exec.Cmd) (int, error, error) {
+	if cmd == nil || cmd.Process == nil {
+		return 0, syscall.EINVAL, nil
+	}
+	pid := cmd.Process.Pid
+	pgid, err := syscall.Getpgid(pid)
+	if err != nil {
+		return 0, err, nil
+	}
+	if err := syscall.Kill(-pgid, syscall.SIGKILL); err != nil {
+		return pgid, nil, err
+	}
+	return pgid, nil, nil
+}
+
+func killProcess(cmd *exec.Cmd) error {
+	if cmd == nil || cmd.Process == nil {
+		return nil
+	}
+	return cmd.Process.Kill()
+}
diff --git a/internal/core/local_runtime/proc_windows.go b/internal/core/local_runtime/proc_windows.go
new file mode 100644
index 000000000..ed12677ce
--- /dev/null
+++ b/internal/core/local_runtime/proc_windows.go
@@ -0,0 +1,23 @@
+//go:build windows
+
+package local_runtime
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// On Windows, do nothing for process group. Consider Job Objects for full tree control.
+func setProcGroup(cmd *exec.Cmd) {}
+
+// tryKillProcessGroup is unsupported on Windows. Return a getpgid-like error for caller to log and fallback.
+func tryKillProcessGroup(cmd *exec.Cmd) (int, error, error) {
+	return 0, fmt.Errorf("process group unsupported on windows"), nil
+}
+
+func killProcess(cmd *exec.Cmd) error {
+	if cmd != nil && cmd.Process != nil {
+		return cmd.Process.Kill()
+	}
+	return nil
+}
diff --git a/internal/core/local_runtime/subprocess.go b/internal/core/local_runtime/subprocess.go
index 2ef6aceb8..b1bdb866a 100644
--- a/internal/core/local_runtime/subprocess.go
+++ b/internal/core/local_runtime/subprocess.go
@@ -26,6 +26,8 @@ func (r *LocalPluginRuntime) getInstanceCmd() (*exec.Cmd, error) {
 			return nil, err
 		}
 		cmd = exec.Command(pythonPath, "-m", r.Config.Meta.Runner.Entrypoint)
+		// ensure child is placed in its own process group on supported platforms
+		setProcGroup(cmd)
 
 	default:
 		return nil, fmt.Errorf("unsupported language: %s", r.Config.Meta.Runner.Language)