diff --git a/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts b/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
index eddc1dc9..24d30408 100644
--- a/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
+++ b/packages/editor-ext/src/lib/unique-id/unique-id.util.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect } from "vitest";
 import StarterKit from "@tiptap/starter-kit";
 import { addUniqueIdsToDoc } from "./unique-id.util";
 import { UniqueID } from "./unique-id";
+import { TransclusionSource } from "../transclusion/transclusion-source";
 
 // Minimal extension set: StarterKit (paragraph/heading) + the UniqueID config
 // the server uses for the addressing anchors.
@@ -10,12 +11,35 @@ const extensions = [
   UniqueID.configure({ types: ["heading", "paragraph"] }),
 ];
 
+// `transclusionSource` is also an addressed type, but its id is a cross-reference
+// KEY (a transclusionReference / the page_transclusions table resolves a source
+// by it), so it lives in the NO_REASSIGN set: a missing id is filled, a colliding
+// id is NOT reassigned (rewriting it would orphan its references).
+const extensionsWithSource = [
+  StarterKit,
+  // Narrow the content expression to `paragraph+` so the schema builds from
+  // StarterKit alone (the real allow-list references image/table/etc. nodes this
+  // minimal harness doesn't register). The node name — what NO_REASSIGN keys on
+  // — is unchanged.
+  TransclusionSource.extend({ content: "paragraph+" }),
+  UniqueID.configure({
+    types: ["heading", "paragraph", "transclusionSource"],
+  }),
+];
+
 const para = (id: string | undefined, text: string) => ({
   type: "paragraph",
   ...(id !== undefined ? { attrs: { id } } : {}),
   content: [{ type: "text", text }],
 });
 
+const source = (id: string | undefined, text: string) => ({
+  type: "transclusionSource",
+  ...(id !== undefined ? { attrs: { id } } : {}),
+  // The schema requires at least one block child (content expression is `+`).
+  content: [{ type: "paragraph", content: [{ type: "text", text }] }],
+});
+
 const ids = (doc: any): (string | undefined)[] =>
   (doc.content ?? []).map((n: any) => n.attrs?.id);
 
@@ -52,4 +76,28 @@ describe("addUniqueIdsToDoc", () => {
     const out = addUniqueIdsToDoc(doc, extensions);
     expect(ids(out)).toEqual(["x1", "x2"]);
   });
+
+  it("does NOT reassign a colliding transclusionSource id — BOTH keep it (NO_REASSIGN)", () => {
+    // Two sync-block sources sharing an id: rewriting either would orphan the
+    // transclusionReferences / page_transclusions rows that resolve a source by
+    // this key, so the dedupe MUST leave both ids intact. If the NO_REASSIGN
+    // guard is removed, the second source is reassigned a fresh id and this fails.
+    const doc = {
+      type: "doc",
+      content: [source("src", "first"), source("src", "second")],
+    };
+    const out = addUniqueIdsToDoc(doc, extensionsWithSource);
+    const [first, second] = ids(out);
+    expect(first).toBe("src");
+    expect(second).toBe("src");
+  });
+
+  it("still FILLS a missing id on a transclusionSource (only reassignment is suppressed)", () => {
+    // NO_REASSIGN suppresses dedupe of an EXISTING id, not filling a missing one:
+    // a source with no id still needs a key its references can resolve.
+    const doc = { type: "doc", content: [source(undefined, "only")] };
+    const out = addUniqueIdsToDoc(doc, extensionsWithSource);
+    const [id] = ids(out);
+    expect(id).toBeTruthy();
+  });
 });