diff --git a/internal/quic/conn_send.go b/internal/quic/conn_send.go
index a8d930898..c2d8d146b 100644
--- a/internal/quic/conn_send.go
+++ b/internal/quic/conn_send.go
@@ -60,7 +60,7 @@ func (c *Conn) maybeSend(now time.Time) (next time.Time) {
 		pad := false
 		var sentInitial *sentPacket
 		if c.keysInitial.canWrite() {
-			pnumMaxAcked := c.acks[initialSpace].largestSeen()
+			pnumMaxAcked := c.loss.spaces[initialSpace].maxAcked
 			pnum := c.loss.nextNumber(initialSpace)
 			p := longPacket{
 				ptype:     packetTypeInitial,
@@ -93,7 +93,7 @@ func (c *Conn) maybeSend(now time.Time) (next time.Time) {
 
 		// Handshake packet.
 		if c.keysHandshake.canWrite() {
-			pnumMaxAcked := c.acks[handshakeSpace].largestSeen()
+			pnumMaxAcked := c.loss.spaces[handshakeSpace].maxAcked
 			pnum := c.loss.nextNumber(handshakeSpace)
 			p := longPacket{
 				ptype:     packetTypeHandshake,
@@ -124,7 +124,7 @@ func (c *Conn) maybeSend(now time.Time) (next time.Time) {
 
 		// 1-RTT packet.
 		if c.keysAppData.canWrite() {
-			pnumMaxAcked := c.acks[appDataSpace].largestSeen()
+			pnumMaxAcked := c.loss.spaces[appDataSpace].maxAcked
 			pnum := c.loss.nextNumber(appDataSpace)
 			c.w.start1RTTPacket(pnum, pnumMaxAcked, dstConnID)
 			c.appendFrames(now, appDataSpace, pnum, limit)
diff --git a/internal/quic/conn_send_test.go b/internal/quic/conn_send_test.go
index 822783c41..2205ff2f7 100644
--- a/internal/quic/conn_send_test.go
+++ b/internal/quic/conn_send_test.go
@@ -38,3 +38,46 @@ func TestAckElicitingAck(t *testing.T) {
 	}
 	t.Errorf("after sending %v PINGs, got no ack-eliciting response", count)
 }
+
+func TestSendPacketNumberSize(t *testing.T) {
+	tc := newTestConn(t, clientSide, permissiveTransportParameters)
+	tc.handshake()
+
+	recvPing := func() *testPacket {
+		t.Helper()
+		tc.conn.ping(appDataSpace)
+		p := tc.readPacket()
+		if p == nil {
+			t.Fatalf("want packet containing PING, got none")
+		}
+		return p
+	}
+
+	// Desynchronize the packet numbers the conn is sending and the ones it is receiving,
+	// by having the conn send a number of unacked packets.
+	for i := 0; i < 16; i++ {
+		recvPing()
+	}
+
+	// Establish the maximum packet number the conn has received an ACK for.
+	maxAcked := recvPing().num
+	tc.writeAckForAll()
+
+	// Make the conn send a sequence of packets.
+	// Check that the packet number is encoded with two bytes once the difference between the
+	// current packet and the max acked one is sufficiently large.
+	for want := maxAcked + 1; want < maxAcked+0x100; want++ {
+		p := recvPing()
+		if p.num != want {
+			t.Fatalf("received packet number %v, want %v", p.num, want)
+		}
+		gotPnumLen := int(p.header&0x03) + 1
+		wantPnumLen := 1
+		if p.num-maxAcked >= 0x80 {
+			wantPnumLen = 2
+		}
+		if gotPnumLen != wantPnumLen {
+			t.Fatalf("packet number 0x%x encoded with %v bytes, want %v (max acked = %v)", p.num, gotPnumLen, wantPnumLen, maxAcked)
+		}
+	}
+}
diff --git a/internal/quic/conn_test.go b/internal/quic/conn_test.go
index 058aa7edc..abf7eede7 100644
--- a/internal/quic/conn_test.go
+++ b/internal/quic/conn_test.go
@@ -82,6 +82,7 @@ func (d testDatagram) String() string {
 
 type testPacket struct {
 	ptype             packetType
+	header            byte
 	version           uint32
 	num               packetNumber
 	keyPhaseBit       bool
@@ -599,12 +600,18 @@ func (tc *testConn) readFrame() (debugFrame, packetType) {
 func (tc *testConn) wantDatagram(expectation string, want *testDatagram) {
 	tc.t.Helper()
 	got := tc.readDatagram()
-	if !reflect.DeepEqual(got, want) {
+	if !datagramEqual(got, want) {
 		tc.t.Fatalf("%v:\ngot datagram:  %v\nwant datagram: %v", expectation, got, want)
 	}
 }
 
 func datagramEqual(a, b *testDatagram) bool {
+	if a == nil && b == nil {
+		return true
+	}
+	if a == nil || b == nil {
+		return false
+	}
 	if a.paddedSize != b.paddedSize ||
 		a.addr != b.addr ||
 		len(a.packets) != len(b.packets) {
@@ -622,7 +629,7 @@ func datagramEqual(a, b *testDatagram) bool {
 func (tc *testConn) wantPacket(expectation string, want *testPacket) {
 	tc.t.Helper()
 	got := tc.readPacket()
-	if !reflect.DeepEqual(got, want) {
+	if !packetEqual(got, want) {
 		tc.t.Fatalf("%v:\ngot packet:  %v\nwant packet: %v", expectation, got, want)
 	}
 }
@@ -630,8 +637,10 @@ func (tc *testConn) wantPacket(expectation string, want *testPacket) {
 func packetEqual(a, b *testPacket) bool {
 	ac := *a
 	ac.frames = nil
+	ac.header = 0
 	bc := *b
 	bc.frames = nil
+	bc.header = 0
 	if !reflect.DeepEqual(ac, bc) {
 		return false
 	}
@@ -839,6 +848,7 @@ func parseTestDatagram(t *testing.T, te *testEndpoint, tc *testConn, buf []byte)
 			}
 			d.packets = append(d.packets, &testPacket{
 				ptype:     p.ptype,
+				header:    buf[0],
 				version:   p.version,
 				num:       p.num,
 				dstConnID: p.dstConnID,
@@ -880,6 +890,7 @@ func parseTestDatagram(t *testing.T, te *testEndpoint, tc *testConn, buf []byte)
 			}
 			d.packets = append(d.packets, &testPacket{
 				ptype:       packetType1RTT,
+				header:      hdr[0],
 				num:         pnum,
 				dstConnID:   hdr[1:][:len(tc.peerConnID)],
 				keyPhaseBit: hdr[0]&keyPhaseBit != 0,
diff --git a/internal/quic/endpoint_test.go b/internal/quic/endpoint_test.go
index 2a6daa076..452d26052 100644
--- a/internal/quic/endpoint_test.go
+++ b/internal/quic/endpoint_test.go
@@ -13,7 +13,6 @@ import (
 	"io"
 	"net"
 	"net/netip"
-	"reflect"
 	"testing"
 	"time"
 )
@@ -242,7 +241,7 @@ func (te *testEndpoint) readDatagram() *testDatagram {
 func (te *testEndpoint) wantDatagram(expectation string, want *testDatagram) {
 	te.t.Helper()
 	got := te.readDatagram()
-	if !reflect.DeepEqual(got, want) {
+	if !datagramEqual(got, want) {
 		te.t.Fatalf("%v:\ngot datagram:  %v\nwant datagram: %v", expectation, got, want)
 	}
 }
diff --git a/internal/quic/tls_test.go b/internal/quic/tls_test.go
index 14f74a00a..9c1dd364e 100644
--- a/internal/quic/tls_test.go
+++ b/internal/quic/tls_test.go
@@ -10,7 +10,6 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
-	"reflect"
 	"testing"
 	"time"
 )
@@ -56,7 +55,7 @@ func (tc *testConn) handshake() {
 			fillCryptoFrames(want, tc.cryptoDataOut)
 			i++
 		}
-		if !reflect.DeepEqual(got, want) {
+		if !datagramEqual(got, want) {
 			t.Fatalf("dgram %v:\ngot %v\n\nwant %v", i, got, want)
 		}
 		if i >= len(dgrams) {