diff --git a/net/observation/handler.go b/net/observation/handler.go
index ab88a055..0bc9e74f 100644
--- a/net/observation/handler.go
+++ b/net/observation/handler.go
@@ -146,10 +146,11 @@ type Observation[C Client] struct {
 	waitForResponse     atomic.Bool
 	observationHandler  *Handler[C]
 
-	private struct {
+	private struct { // members guarded by mutex
 		mutex       sync.Mutex
-		obsSequence uint32    // guarded by mutex
-		lastEvent   time.Time // guarded by mutex
+		obsSequence uint32
+		lastEvent   time.Time
+		etag        []byte
 	}
 }
 
@@ -199,6 +200,12 @@ func (o *Observation[C]) Request() message.Message {
 	return o.req
 }
 
+func (o *Observation[C]) etag() []byte {
+	o.private.mutex.Lock()
+	defer o.private.mutex.Unlock()
+	return o.private.etag
+}
+
 // Cancel remove observation from server. For recreate observation use Observe.
 func (o *Observation[C]) Cancel(ctx context.Context, opts ...message.Option) error {
 	if !o.cleanUp() {
@@ -217,6 +224,10 @@ func (o *Observation[C]) Cancel(ctx context.Context, opts ...message.Option) err
 		}
 	}
 	req.SetToken(o.req.Token)
+	etag := o.etag()
+	if len(etag) > 0 {
+		_ = req.SetETag(etag) // ignore invalid etag
+	}
 	resp, err := o.observationHandler.do(req)
 	if err != nil {
 		return err
@@ -237,11 +248,20 @@ func (o *Observation[C]) wantBeNotified(r *pool.Message) bool {
 
 	o.private.mutex.Lock()
 	defer o.private.mutex.Unlock()
-	if ValidSequenceNumber(o.private.obsSequence, obsSequence, o.private.lastEvent, now) {
-		o.private.obsSequence = obsSequence
-		o.private.lastEvent = now
-		return true
+	if !ValidSequenceNumber(o.private.obsSequence, obsSequence, o.private.lastEvent, now) {
+		return false
 	}
 
-	return false
+	o.private.obsSequence = obsSequence
+	o.private.lastEvent = now
+	if etag, err := r.ETag(); err == nil {
+		if cap(o.private.etag) < len(etag) {
+			o.private.etag = make([]byte, len(etag))
+		}
+		if len(o.private.etag) != len(etag) {
+			o.private.etag = o.private.etag[:len(etag)]
+		}
+		copy(o.private.etag, etag)
+	}
+	return true
 }
diff --git a/tcp/clientobserve_test.go b/tcp/clientobserve_test.go
index 016b42e5..c2bde263 100644
--- a/tcp/clientobserve_test.go
+++ b/tcp/clientobserve_test.go
@@ -113,6 +113,8 @@ func TestConnObserve(t *testing.T) {
 							var etag []byte
 							var errE error
 							if tt.args.etag != nil {
+								// force unexpected etag, Cancel should still succeed but with codes.Content
+								// instead of codes.Valid
 								etag = tt.args.etag
 							} else {
 								etag, errE = message.GetETag(p)
@@ -141,12 +143,13 @@ func TestConnObserve(t *testing.T) {
 					}
 					f()
 				case 1:
-					if tt.args.etag != nil {
-						if etag, errE := r.ETag(); errE == nil && bytes.Equal(tt.args.etag, etag) {
-							errS := w.SetResponse(codes.Valid, message.TextPlain, nil)
-							require.NoError(t, errS)
-							return
-						}
+					p := bytes.NewReader(tt.args.payload)
+					etag, errE := message.GetETag(p)
+					require.NoError(t, errE)
+					if retag, errE := r.ETag(); errE == nil && bytes.Equal(etag, retag) {
+						errS := w.SetResponse(codes.Valid, message.TextPlain, nil)
+						require.NoError(t, errS)
+						return
 					}
 					errS := w.SetResponse(codes.Content, message.TextPlain, bytes.NewReader([]byte("close")))
 					require.NoError(t, errS)
@@ -186,13 +189,7 @@ func TestConnObserve(t *testing.T) {
 			require.NoError(t, err)
 			<-obs.done
 
-			opts := make(message.Options, 0, 1)
-			if tt.args.etag != nil {
-				buf := make([]byte, len(tt.args.etag))
-				opts, _, err = opts.SetBytes(buf, message.ETag, tt.args.etag)
-				require.NoError(t, err)
-			}
-			err = got.Cancel(ctx, opts...)
+			err = got.Cancel(ctx)
 			require.NoError(t, err)
 		})
 	}