diff --git a/internal/platform/canvas_model_stream.go b/internal/platform/canvas_model_stream.go
index 0942cc7..8c5cd96 100644
--- a/internal/platform/canvas_model_stream.go
+++ b/internal/platform/canvas_model_stream.go
@@ -20,6 +20,7 @@ func writeCanvasModelStream(ctx context.Context, w http.ResponseWriter, model ca
 	view := model.view(nil)
 	// A count covers direct members only; an empty canvas is explicit too.
 	counts := map[string]int{}
+	owners := map[string]string{}
 	for _, canvas := range view.Canvases {
 		counts[canvas.ID] = 0
 	}
@@ -29,11 +30,17 @@ func writeCanvasModelStream(ctx context.Context, w http.ResponseWriter, model ca
 			reject(w, 500, "invalid_model_coverage")
 			return
 		}
+		id := modelString(element["id"])
+		if id == "" || owners[id] != "" {
+			reject(w, 500, "invalid_model_identity")
+			return
+		}
+		owners[id] = owner
 		counts[owner]++
 	}
 	header := response{"streamVersion": 1, "kind": "header", "sequence": 0, "snapshotToken": token,
 		"documentFormatVersion": canvasModelFormat, "projectId": model.Root, "canvases": view.Canvases,
-		"versions": view.Versions, "tombstones": view.Tombstones, "removedCanvasIds": view.Removed, "elementCount": len(view.Elements), "canvasElementCounts": counts}
+		"versions": view.Versions, "tombstones": view.Tombstones, "removedCanvasIds": view.Removed, "elementCount": len(view.Elements), "canvasElementCounts": counts, "elementOwners": owners}
 	const maxLine = 8 * 1024 * 1024
 	const maxBytes = 64 * 1024 * 1024
 	sequence, total := 0, 0
diff --git a/internal/platform/canvas_model_stream_test.go b/internal/platform/canvas_model_stream_test.go
index 0174908..b6dc281 100644
--- a/internal/platform/canvas_model_stream_test.go
+++ b/internal/platform/canvas_model_stream_test.go
@@ -46,6 +46,15 @@ func TestCanonicalModelStreamFramesAndSnapshot(t *testing.T) {
 	if !uuidSlugPattern.MatchString(token) || records[0]["projectId"] != modelRootID || !modelEqual(records[0]["versions"], model.Versions) || !modelEqual(records[0]["tombstones"], model.Tombstones) {
 		t.Fatal("stream header lost snapshot identity")
 	}
+	owners := records[0]["elementOwners"].(map[string]any)
+	if len(owners) != len(model.Elements) {
+		t.Fatal("identity manifest must cover unloaded elements too")
+	}
+	for id, element := range model.Elements {
+		if owners[id] != element["ownerCanvasId"] {
+			t.Fatal("identity manifest mixed owners", id)
+		}
+	}
 	seen := map[string]bool{}
 	for i, record := range records {
 		if record["sequence"] != float64(i) || record["snapshotToken"] != token || record["streamVersion"] != float64(1) {
@@ -114,3 +123,15 @@ func TestCanonicalModelStreamDeclaresDirectScopeCountsIncludingEmptyCanvases(t *
 		t.Fatal("invalid owner must fail before publishing a scope certificate")
 	}
 }
+
+func TestCanonicalModelStreamRejectsAmbiguousIdentityManifest(t *testing.T) {
+	for _, id := range []string{"", "a"} {
+		model := modelTestDocument()
+		model.Elements["other"] = modelTestNode(id, modelRootID)
+		writer := httptest.NewRecorder()
+		writeCanvasModelStream(context.Background(), writer, model)
+		if writer.Code != 500 || writer.Flushed {
+			t.Fatal("ambiguous identity was published before rejection")
+		}
+	}
+}