diff --git a/src/database/models/document-meta.ts b/src/database/models/document-meta.ts
index 739ff2d..21bc208 100644
--- a/src/database/models/document-meta.ts
+++ b/src/database/models/document-meta.ts
@@ -6,7 +6,7 @@ interface IDocumentMeta extends MongooseDocument {
   ownerDid: string | null;
   ownerIdentityDid: string | null;
   portalAddress: string | null;
-  ddocId: string | null; // on-chain id when documentId is an opaque o- id; owner-only, never emitted
+  ddocId: string | null; // on-chain id when documentId is an opaque link id; owner-only, never emitted
   appType?: "ddoc" | "dsheet";
   editLock: string | null; // roomKey-wrapped
   title: string | null; // roomKey-encrypted
diff --git a/src/services/mongodb-store.ts b/src/services/mongodb-store.ts
index 0258ec7..16f23f4 100644
--- a/src/services/mongodb-store.ts
+++ b/src/services/mongodb-store.ts
@@ -14,12 +14,15 @@ const HYDRATION_SLOW_MS = 500;
 const HYDRATION_LARGE_BYTES = 2 * 1024 * 1024;
 const HYDRATION_LARGE_ROWS = 2_000;
 
+// An opaque link id, by exact shape: addresses, hashes and on-chain ids can also start with 0x.
+const LINK_ID_RE = /^0x[1-9A-HJ-NP-Za-km-z]{18}$/;
+
 export const acceptedDdocId = (documentId: string, ddocId: unknown): string | null =>
   typeof ddocId === "string" &&
   ddocId.length > 0 &&
   ddocId.length <= 64 &&
-  documentId.startsWith("o-") &&
-  !ddocId.startsWith("o-")
+  LINK_ID_RE.test(documentId) &&
+  !LINK_ID_RE.test(ddocId)
     ? ddocId
     : null;
 
@@ -528,7 +531,7 @@ export class MongoDBStore {
     const rows: any[] = await DocumentMetaModel.find({
       isPublished: { $ne: true },
       portalAddress: { $ne: null },
-      $or: [{ _id: { $not: /^o-/ } }, { ddocId: { $ne: null } }],
+      $or: [{ _id: { $not: LINK_ID_RE } }, { ddocId: { $ne: null } }],
     })
       .select("portalAddress ddocId")
       .sort({ updatedAt: -1 })
diff --git a/src/tests/services/mongodb-store.test.ts b/src/tests/services/mongodb-store.test.ts
index b34b360..bad5feb 100644
--- a/src/tests/services/mongodb-store.test.ts
+++ b/src/tests/services/mongodb-store.test.ts
@@ -1,7 +1,7 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { DocumentUpdateModel } from "../../database/models/document-update";
 import { CounterModel } from "../../database/models/counter";
-import { MongoDBStore, SessionTerminatedError } from "../../services/mongodb-store";
+import { MongoDBStore, SessionTerminatedError, acceptedDdocId } from "../../services/mongodb-store";
 
 vi.mock("../../database/models", () => {
   const save = vi.fn().mockResolvedValue(undefined);
@@ -313,7 +313,7 @@ describe("listDocumentsForOwner", () => {
     expect(DocumentMetaModel.find).toHaveBeenCalledWith({
       isPublished: { $ne: true },
       portalAddress: { $ne: null },
-      $or: [{ _id: { $not: /^o-/ } }, { ddocId: { $ne: null } }],
+      $or: [{ _id: { $not: /^0x[1-9A-HJ-NP-Za-km-z]{18}$/ } }, { ddocId: { $ne: null } }],
     });
     expect(sort).toHaveBeenCalledWith({ updatedAt: -1 });
     expect(limit).toHaveBeenCalledWith(500);
@@ -794,3 +794,28 @@ describe("lockWireFormat: duplicate-key race", () => {
     await expect(store.lockWireFormat("doc-1")).resolves.toBe(false);
   });
 });
+
+describe("acceptedDdocId", () => {
+  const LINK_ID = "0x" + "Zk".repeat(9);
+  const DDOC_ID = "7sLHvGNqsvNDhRm8tPKZSu";
+
+  it("keeps the on-chain id for a room named by a link id", () => {
+    expect(acceptedDdocId(LINK_ID, DDOC_ID)).toBe(DDOC_ID);
+  });
+
+  it("keeps an on-chain id that itself starts with 0x", () => {
+    expect(acceptedDdocId(LINK_ID, "0xabc")).toBe("0xabc");
+  });
+
+  it("refuses a link id as the on-chain id", () => {
+    expect(acceptedDdocId(LINK_ID, "0x" + "Gh".repeat(9))).toBeNull();
+  });
+
+  it.each([
+    ["a plain room", DDOC_ID],
+    ["a room named by an address", "0x" + "a1".repeat(20)],
+    ["a room with the old o- prefix", "o-" + "Zk".repeat(9)],
+  ])("stores nothing for %s", (_, documentId) => {
+    expect(acceptedDdocId(documentId, DDOC_ID)).toBeNull();
+  });
+});