diff --git a/cotlib.go b/cotlib.go
index 2897b98..e900a6a 100644
--- a/cotlib.go
+++ b/cotlib.go
@@ -2201,6 +2201,14 @@ func (e *Event) ToXML() ([]byte, error) {
 				buf.WriteString(escapeAttr(c.Callsign))
 				buf.WriteByte('"')
 			}
+			// endpoint is what the TAK server uses to register the
+			// callsign->connection mapping; dropping it broke inbound DM
+			// routing to a self-SA presence (INFR-207 / INFR-206).
+			if c.Endpoint != "" {
+				buf.WriteString(` endpoint="`)
+				buf.WriteString(escapeAttr(c.Endpoint))
+				buf.WriteByte('"')
+			}
 			buf.WriteString("/>\n")
 		}
 		if g := e.Detail.Group; g != nil {
diff --git a/event_schema_error_test.go b/event_schema_error_test.go
index 879f3f4..e29b56a 100644
--- a/event_schema_error_test.go
+++ b/event_schema_error_test.go
@@ -1,3 +1,9 @@
+//go:build !novalidator
+
+// This test exercises the validator schema-init error path (initErr /
+// eventPointSchema), which only exists in the cgo validator build
+// (event_schema.go is !novalidator). Without this tag it fails to compile
+// under -tags novalidator.
 package cotlib
 
 import (
diff --git a/roundtrip_test.go b/roundtrip_test.go
index faa0177..c5fcb24 100644
--- a/roundtrip_test.go
+++ b/roundtrip_test.go
@@ -2,9 +2,50 @@ package cotlib
 
 import (
 	"context"
+	"strings"
 	"testing"
 )
 
+// TestContactEndpointRoundTrip guards INFR-207: ToXML() was a hand-written
+// serializer that emitted <contact callsign> but dropped the endpoint attribute,
+// even though Contact.Endpoint is set. The TAK server keys the callsign->
+// connection mapping off that endpoint, so dropping it silently broke inbound
+// direct-message routing to a self-SA presence (INFR-206). The endpoint must
+// both appear in the serialized XML and survive a parse round-trip.
+func TestContactEndpointRoundTrip(t *testing.T) {
+	orig, err := NewEvent("EP1", "a-f-G-U-C", 1.0, 2.0, 0)
+	if err != nil {
+		t.Fatalf("new event: %v", err)
+	}
+	orig.Detail = &Detail{
+		Contact: &Contact{Callsign: "NERVA", Endpoint: "*:-1:stcp"},
+	}
+	xmlData, err := orig.ToXML()
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	ReleaseEvent(orig)
+
+	if !strings.Contains(string(xmlData), `endpoint="*:-1:stcp"`) {
+		t.Fatalf("ToXML dropped the contact endpoint attribute: %s", xmlData)
+	}
+
+	evt, err := UnmarshalXMLEvent(context.Background(), xmlData)
+	if err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if evt.Detail == nil || evt.Detail.Contact == nil {
+		t.Fatalf("no contact after round-trip: %#v", evt)
+	}
+	if got := evt.Detail.Contact.Endpoint; got != "*:-1:stcp" {
+		t.Errorf("endpoint not preserved through round-trip: got %q want %q", got, "*:-1:stcp")
+	}
+	if got := evt.Detail.Contact.Callsign; got != "NERVA" {
+		t.Errorf("callsign not preserved through round-trip: got %q want %q", got, "NERVA")
+	}
+	ReleaseEvent(evt)
+}
+
 func TestUnmarshalXMLEventRoundTrip(t *testing.T) {
 	orig, err := NewEvent("RT1", "a-f-G", 10.0, 20.0, 0)
 	if err != nil {