diff --git a/internal/sandbox-sidecar/command/command.go b/internal/sandbox-sidecar/command/command.go
index 01067cad..e52487d8 100644
--- a/internal/sandbox-sidecar/command/command.go
+++ b/internal/sandbox-sidecar/command/command.go
@@ -95,9 +95,24 @@ func (b *ChrootCommandBuilder) Build(ctx context.Context, pid int, req sidecarap
 		Chroot:     fmt.Sprintf("/proc/%d/root", pid),
 		Credential: &syscall.Credential{Uid: 0, Gid: 0},
 	}
+	setupProcessGroup(cmd)
 	return cmd, nil
 }
 
+func setupProcessGroup(cmd *exec.Cmd) {
+	if cmd.SysProcAttr == nil {
+		cmd.SysProcAttr = &syscall.SysProcAttr{}
+	}
+	cmd.SysProcAttr.Setpgid = true
+	cmd.Cancel = func() error {
+		err := syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		if errors.Is(err, syscall.ESRCH) {
+			return os.ErrProcessDone
+		}
+		return err
+	}
+}
+
 // --- Input/Output types ---
 
 type CreateCommandInput struct {
diff --git a/internal/sandbox-sidecar/command/command_test.go b/internal/sandbox-sidecar/command/command_test.go
index 70e17aca..872b3477 100644
--- a/internal/sandbox-sidecar/command/command_test.go
+++ b/internal/sandbox-sidecar/command/command_test.go
@@ -24,7 +24,9 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/danielgtaylor/huma/v2"
@@ -59,6 +61,10 @@ func extractSSEData(body string) string {
 	return result.String()
 }
 
+func processAlive(pid int) bool {
+	return syscall.Kill(pid, 0) == nil
+}
+
 var _ = Describe("Command Handlers", func() {
 	var (
 		commandAPI      humatest.TestAPI
@@ -913,6 +919,78 @@ var _ = Describe("Command Handlers", func() {
 		})
 	})
 
+	Describe("process group termination", func() {
+		startWithChild := func(extra map[string]any) (string, int) {
+			f, err := os.CreateTemp("", "sidecar-child-pid-*")
+			Expect(err).NotTo(HaveOccurred())
+			pidFile := f.Name()
+			Expect(f.Close()).To(Succeed())
+			DeferCleanup(func() { _ = os.Remove(pidFile) })
+
+			req := map[string]any{
+				"args": []string{"/bin/sh", "-c", `sleep 300 & echo $! > "$CHILD_PID_FILE"; wait`},
+				"env":  map[string]string{"CHILD_PID_FILE": pidFile},
+			}
+			for k, v := range extra {
+				req[k] = v
+			}
+			body, err := json.Marshal(req)
+			Expect(err).NotTo(HaveOccurred())
+
+			resp := commandAPI.Post("/v1/commands", "Content-Type: application/json", strings.NewReader(string(body)))
+			Expect(resp.Code).To(Equal(http.StatusAccepted))
+			var result sidecarapi.CreateCommandResponse
+			Expect(json.NewDecoder(resp.Body).Decode(&result)).To(Succeed())
+
+			var childPID int
+			Eventually(func() bool {
+				data, readErr := os.ReadFile(pidFile) //nolint:gosec // test-created temp file path
+				if readErr != nil {
+					return false
+				}
+				pid, convErr := strconv.Atoi(strings.TrimSpace(string(data)))
+				if convErr != nil {
+					return false
+				}
+				childPID = pid
+				return true
+			}, "3s", "20ms").Should(BeTrue())
+
+			DeferCleanup(func() { _ = syscall.Kill(childPID, syscall.SIGKILL) })
+			return result.ID, childPID
+		}
+
+		waitExited := func(id string) {
+			Eventually(func() *int {
+				resp := commandAPI.Get(fmt.Sprintf("/v1/commands/%s/status", id))
+				var status sidecarapi.CommandStatusResponse
+				Expect(json.NewDecoder(resp.Body).Decode(&status)).To(Succeed())
+				return status.ExitCode
+			}, "5s").ShouldNot(BeNil())
+		}
+
+		It("kills the whole process tree on DELETE", func() {
+			id, childPID := startWithChild(nil)
+			Expect(processAlive(childPID)).To(BeTrue(), "child should be running before kill")
+
+			resp := commandAPI.Delete(fmt.Sprintf("/v1/commands/%s", id))
+			Expect(resp.Code).To(Equal(http.StatusNoContent))
+
+			waitExited(id)
+			Eventually(func() bool { return processAlive(childPID) }, "3s", "20ms").
+				Should(BeFalse(), "forked child must not survive after the command is killed")
+		})
+
+		It("kills the whole process tree on timeout", func() {
+			id, childPID := startWithChild(map[string]any{"timeoutSeconds": 1})
+			Expect(processAlive(childPID)).To(BeTrue(), "child should be running before timeout")
+
+			waitExited(id)
+			Eventually(func() bool { return processAlive(childPID) }, "3s", "20ms").
+				Should(BeFalse(), "forked child must not survive after the command times out")
+		})
+	})
+
 	Describe("working directory", func() {
 		It("runs command in specified cwd", func() {
 			code, result := postCommand(`{"args": ["/bin/sh", "-c", "pwd"], "cwd": "/tmp"}`)
diff --git a/internal/sandbox-sidecar/command/suite_test.go b/internal/sandbox-sidecar/command/suite_test.go
index 4ed61075..a6627ad6 100644
--- a/internal/sandbox-sidecar/command/suite_test.go
+++ b/internal/sandbox-sidecar/command/suite_test.go
@@ -45,6 +45,7 @@ func (b *DirectCommandBuilder) Build(ctx context.Context, _ int, req sidecarapi.
 	cmd.Env = env
 	cmd.Dir = req.Cwd
 	cmd.WaitDelay = waitDelayGracePeriod
+	setupProcessGroup(cmd)
 	return cmd, nil
 }