diff --git a/internal/sessions/filelock_process_test.go b/internal/sessions/filelock_process_test.go
new file mode 100644
index 000000000..3ba0f897a
--- /dev/null
+++ b/internal/sessions/filelock_process_test.go
@@ -0,0 +1,97 @@
+package sessions
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+	"time"
+)
+
+// TestSessionFileLockBlocksAnotherProcessUntilItExits proves the OS session lock
+// (flock on unix, LockFileEx on Windows) serializes mutations across processes,
+// not just across Store instances in one process, and that it is released when
+// the holder exits without unlocking.
+func TestSessionFileLockBlocksAnotherProcessUntilItExits(t *testing.T) {
+	root := t.TempDir()
+	store := NewStore(StoreOptions{RootDir: root})
+	if _, err := store.Create(CreateInput{SessionID: "s"}); err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := exec.Command(os.Args[0], "-test.run=^TestSessionFileLockHelperProcess$")
+	cmd.Env = append(os.Environ(), "ZERO_SESSIONS_LOCK_HELPER=1", "ZERO_SESSIONS_LOCK_ROOT="+root)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd.Stderr = os.Stderr
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+	exited := false
+	t.Cleanup(func() {
+		if !exited {
+			_ = cmd.Process.Kill()
+			_ = cmd.Wait()
+		}
+	})
+
+	ready, err := bufio.NewReader(stdout).ReadString('\n')
+	if err != nil || strings.TrimSpace(ready) != "ready" {
+		t.Fatalf("helper readiness = %q, err %v", ready, err)
+	}
+
+	// The helper process holds the session lock, so this must block.
+	done := make(chan error, 1)
+	go func() {
+		_, appendErr := store.AppendEvent("s", AppendEventInput{Type: EventMessage, Payload: map[string]any{}})
+		done <- appendErr
+	}()
+	select {
+	case err := <-done:
+		t.Fatalf("AppendEvent completed while another process held the session lock (err=%v)", err)
+	case <-time.After(300 * time.Millisecond):
+		// Expected: still blocked.
+	}
+
+	// The helper exits without unlocking; the OS must release the lock for us.
+	if err := stdin.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err := cmd.Wait(); err != nil {
+		t.Fatalf("helper exit: %v", err)
+	}
+	exited = true
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("AppendEvent after the holder exited: %v", err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("AppendEvent did not proceed after the lock holder exited")
+	}
+}
+
+func TestSessionFileLockHelperProcess(t *testing.T) {
+	if os.Getenv("ZERO_SESSIONS_LOCK_HELPER") != "1" {
+		return
+	}
+	store := NewStore(StoreOptions{RootDir: os.Getenv("ZERO_SESSIONS_LOCK_ROOT")})
+	if _, err := store.lockSession("s"); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+	if _, err := fmt.Fprintln(os.Stdout, "ready"); err != nil {
+		os.Exit(3)
+	}
+	_, _ = io.Copy(io.Discard, os.Stdin)
+	// Deliberately exit without calling the unlock function.
+}
diff --git a/internal/sessions/rewind_test.go b/internal/sessions/rewind_test.go
index b4cdf988f..6ccad7868 100644
--- a/internal/sessions/rewind_test.go
+++ b/internal/sessions/rewind_test.go
@@ -353,9 +353,6 @@ func TestCaptureAndPruneConcurrent(t *testing.T) {
 // separate Store instances on the same RootDir (e.g. CLI rewind vs TUI). While
 // one Store holds the session lock, another Store's AppendEvent must block.
 func TestSessionFileLockSerializesAcrossStores(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("flock semantics differ on windows")
-	}
 	root := t.TempDir()
 	storeA := NewStore(StoreOptions{RootDir: root})
 	if _, err := storeA.Create(CreateInput{SessionID: "s"}); err != nil {