diff --git a/kv/fsm.go b/kv/fsm.go
index aeb0ab5..8aa1057 100644
--- a/kv/fsm.go
+++ b/kv/fsm.go
@@ -107,9 +107,22 @@ func (f *kvFSM) handleTxnRequest(ctx context.Context, r *pb.Request, commitTS ui
 }
 
 func (f *kvFSM) validateConflicts(ctx context.Context, muts []*pb.Mutation, startTS uint64) error {
-	// Debug guard only: real OCC runs at the leader/storage layer, so conflicts
-	// should already be resolved before log application. Keep this stub to make
-	// any unexpected violations visible during development.
+	seen := make(map[string]struct{}, len(muts))
+	for _, mut := range muts {
+		keyStr := string(mut.Key)
+		if _, ok := seen[keyStr]; ok {
+			continue
+		}
+		seen[keyStr] = struct{}{}
+
+		latest, exists, err := f.store.LatestCommitTS(ctx, mut.Key)
+		if err != nil {
+			return errors.WithStack(err)
+		}
+		if exists && latest > startTS {
+			return errors.Wrapf(store.ErrWriteConflict, "key: %s", string(mut.Key))
+		}
+	}
 	return nil
 }
 
diff --git a/kv/fsm_occ_test.go b/kv/fsm_occ_test.go
new file mode 100644
index 0000000..e89cbb1
--- /dev/null
+++ b/kv/fsm_occ_test.go
@@ -0,0 +1,74 @@
+package kv
+
+import (
+	"context"
+	"testing"
+
+	pb "github.com/bootjp/elastickv/proto"
+	"github.com/bootjp/elastickv/store"
+	"github.com/hashicorp/raft"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+)
+
+func TestValidateConflictsDetectsStaleWrite(t *testing.T) {
+	ctx := context.Background()
+	st := store.NewMVCCStore()
+	require.NoError(t, st.PutAt(ctx, []byte("k"), []byte("v1"), 50, 0))
+
+	fsm, ok := NewKvFSM(st).(*kvFSM)
+	require.True(t, ok)
+
+	muts := []*pb.Mutation{{Op: pb.Op_PUT, Key: []byte("k"), Value: []byte("v2")}}
+	err := fsm.validateConflicts(ctx, muts, 40)
+	require.ErrorIs(t, err, store.ErrWriteConflict)
+}
+
+func TestApplyReturnsErrorOnConflict(t *testing.T) {
+	ctx := context.Background()
+	st := store.NewMVCCStore()
+	fsm, ok := NewKvFSM(st).(*kvFSM)
+	require.True(t, ok)
+
+	// First write commits at ts=100.
+	put := &pb.Request{
+		IsTxn: false,
+		Phase: pb.Phase_NONE,
+		Ts:    100,
+		Mutations: []*pb.Mutation{{
+			Op:    pb.Op_PUT,
+			Key:   []byte("k"),
+			Value: []byte("v1"),
+		}},
+	}
+	data, err := proto.Marshal(put)
+	require.NoError(t, err)
+
+	resp := fsm.Apply(&raft.Log{Type: raft.LogCommand, Data: data})
+	require.Nil(t, resp)
+
+	// Stale transaction attempts to commit with startTS=90.
+	conflict := &pb.Request{
+		IsTxn: true,
+		Phase: pb.Phase_COMMIT,
+		Ts:    90,
+		Mutations: []*pb.Mutation{{
+			Op:    pb.Op_PUT,
+			Key:   []byte("k"),
+			Value: []byte("v2"),
+		}},
+	}
+	data, err = proto.Marshal(conflict)
+	require.NoError(t, err)
+
+	resp = fsm.Apply(&raft.Log{Type: raft.LogCommand, Data: data})
+
+	err, ok = resp.(error)
+	require.True(t, ok)
+	require.ErrorIs(t, err, store.ErrWriteConflict)
+
+	// Ensure committed value remains.
+	v, err := st.GetAt(ctx, []byte("k"), ^uint64(0))
+	require.NoError(t, err)
+	require.Equal(t, []byte("v1"), v)
+}