diff --git a/wgengine/magicsock/magicsock_test.go b/wgengine/magicsock/magicsock_test.go
index 685fff4da..c57086201 100644
--- a/wgengine/magicsock/magicsock_test.go
+++ b/wgengine/magicsock/magicsock_test.go
@@ -4015,3 +4015,84 @@ func TestConn_receiveIP(t *testing.T) {
 		})
 	}
 }
+
+func Test_lazyEndpoint_InitiationMessagePublicKey(t *testing.T) {
+	tests := []struct {
+		name                       string
+		callWithPeerMapKey         bool
+		maybeEPMatchingKey         bool
+		wantNoteRecvActivityCalled bool
+	}{
+		{
+			name:                       "noteRecvActivity called",
+			callWithPeerMapKey:         true,
+			maybeEPMatchingKey:         false,
+			wantNoteRecvActivityCalled: true,
+		},
+		{
+			name:                       "maybeEP early return",
+			callWithPeerMapKey:         true,
+			maybeEPMatchingKey:         true,
+			wantNoteRecvActivityCalled: false,
+		},
+		{
+			name:                       "not in peerMap early return",
+			callWithPeerMapKey:         false,
+			maybeEPMatchingKey:         false,
+			wantNoteRecvActivityCalled: false,
+		},
+		{
+			name:                       "not in peerMap maybeEP early return",
+			callWithPeerMapKey:         false,
+			maybeEPMatchingKey:         true,
+			wantNoteRecvActivityCalled: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ep := &endpoint{
+				nodeID:    1,
+				publicKey: key.NewNode().Public(),
+			}
+			ep.disco.Store(&endpointDisco{
+				key: key.NewDisco().Public(),
+			})
+
+			var noteRecvActivityCalledFor key.NodePublic
+			conn := newConn(t.Logf)
+			conn.noteRecvActivity = func(public key.NodePublic) {
+				// wireguard-go will call into ParseEndpoint if the "real"
+				// noteRecvActivity ends up JIT configuring the peer. Mimic that
+				// to ensure there are no deadlocks around conn.mu.
+				// See tailscale/tailscale#16651 & http://go/corp#30836
+				_, err := conn.ParseEndpoint(ep.publicKey.UntypedHexString())
+				if err != nil {
+					t.Fatalf("ParseEndpoint() err: %v", err)
+				}
+				noteRecvActivityCalledFor = public
+			}
+			ep.c = conn
+
+			var pubKey [32]byte
+			if tt.callWithPeerMapKey {
+				copy(pubKey[:], ep.publicKey.AppendTo(nil))
+			}
+			conn.peerMap.upsertEndpoint(ep, key.DiscoPublic{})
+
+			le := &lazyEndpoint{
+				c: conn,
+			}
+			if tt.maybeEPMatchingKey {
+				le.maybeEP = ep
+			}
+			le.InitiationMessagePublicKey(pubKey)
+			want := key.NodePublic{}
+			if tt.wantNoteRecvActivityCalled {
+				want = ep.publicKey
+			}
+			if noteRecvActivityCalledFor.Compare(want) != 0 {
+				t.Fatalf("noteRecvActivityCalledFor = %v, want %v", noteRecvActivityCalledFor, want)
+			}
+		})
+	}
+}