diff --git a/lib/__tests__/contact-wire.test.ts b/lib/__tests__/contact-wire.test.ts
index 9268c96f0..90c720868 100644
--- a/lib/__tests__/contact-wire.test.ts
+++ b/lib/__tests__/contact-wire.test.ts
@@ -32,6 +32,21 @@ describe('contactToWire', () => {
     expect(contactToWire(cleared, 'create')).toEqual({});
   });
 
+  it('never clears id, uid or addressBookIds on update', () => {
+    const wire = contactToWire({
+      id: undefined, uid: undefined, addressBookIds: undefined, nicknames: undefined,
+    } as Partial<ContactCard>, 'update');
+    expect(wire).toEqual({ nicknames: null });
+
+    const nulls = contactToWire({ id: null, uid: null, addressBookIds: null } as unknown as Partial<ContactCard>, 'update');
+    expect(nulls).toEqual({});
+  });
+
+  it('still sends a uid or addressBookIds that has a value', () => {
+    expect(contactToWire({ uid: 'urn:uuid:1', addressBookIds: { ab: true } }, 'update'))
+      .toEqual({ uid: 'urn:uuid:1', addressBookIds: { ab: true } });
+  });
+
   it('leaves calendar links alone when the update does not mention them', () => {
     expect(contactToWire({ keywords: { vip: true } }, 'update')).toEqual({ keywords: { vip: true } });
   });
diff --git a/lib/__tests__/jmap-contact-client.test.ts b/lib/__tests__/jmap-contact-client.test.ts
index 6dca116fa..17f439475 100644
--- a/lib/__tests__/jmap-contact-client.test.ts
+++ b/lib/__tests__/jmap-contact-client.test.ts
@@ -589,6 +589,34 @@ describe('JMAPClient contact methods', () => {
         .rejects.toThrow('Failed to update contact');
     });
 
+    it.each(['1.0', '0.16'] as const)('leaves a cleared id, uid and addressBookIds out of the update on Stalwart %s', async (version) => {
+      const client = createClient();
+      // What ContactCard/set answers to those keys set to null (1.0 contact/set.rs:
+      // 229-249, 602-607, 669-671; 0.16.23 set.rs:187-199, 524-529, 591-593).
+      const answer = (update: Record<string, unknown>) => {
+        const refuse = (property: string, description: string) =>
+          ({ notUpdated: { 'contact-1': { type: 'invalidProperties', properties: [property], description } } });
+        if ('id' in update) return refuse('id', 'The id property is immutable.');
+        if (update.addressBookIds === null) return refuse('addressBookIds', 'Invalid patch operation for addressBookIds.');
+        // 0.16 takes a null uid and drops the card's UID without a word.
+        if (update.uid === null && version === '1.0') return refuse('uid', 'You cannot change the UID of a contact.');
+        return { updated: { 'contact-1': null } };
+      };
+      const fetchSpy = vi.spyOn(globalThis, 'fetch').mockImplementation(async (_url, init) => {
+        const update = JSON.parse(String(init?.body)).methodCalls[0][1].update['contact-1'];
+        const body = { methodResponses: [['ContactCard/set', answer(update), '0']] };
+        return { ok: true, status: 200, text: () => Promise.resolve(JSON.stringify(body)) } as Response;
+      });
+
+      // A plugin passes the card back with the keys it did not set.
+      await client.updateContact('contact-1', {
+        id: undefined, uid: undefined, addressBookIds: undefined, name: mockContact.name,
+      } as never);
+
+      const sent = JSON.parse(fetchSpy.mock.calls[0][1]?.body as string).methodCalls[0][1].update['contact-1'];
+      expect(sent).toEqual({ name: mockContact.name });
+    });
+
     it('passes a Stalwart 1.0 blob photo through unchanged', async () => {
       const client = createClient();
       const fetchSpy = mockFetch({
diff --git a/lib/__tests__/vcard.test.ts b/lib/__tests__/vcard.test.ts
index 31869a02f..af850edab 100644
--- a/lib/__tests__/vcard.test.ts
+++ b/lib/__tests__/vcard.test.ts
@@ -540,6 +540,46 @@ describe("organization-only cards (issue #701)", () => {
   });
 });
 
+describe("MEMBER export", () => {
+  const group = (members: Record<string, boolean>): ContactCard => ({
+    id: "g1",
+    addressBookIds: {},
+    kind: "group",
+    name: { full: "Team" },
+    members,
+  });
+
+  it("writes a member key that is already a URI as it is", () => {
+    // Groups made in Bulwark key members by uid (urn:uuid:); CardDAV groups
+    // keep whatever MEMBER value the client sent.
+    const vcf = generateVCard([group({ "urn:uuid:0f3c9d2e": true, "mailto:ann@example.com": true })]);
+    const lines = vcf.split("\r\n");
+    expect(lines).toContain("MEMBER:urn:uuid:0f3c9d2e");
+    expect(lines).toContain("MEMBER:mailto:ann@example.com");
+    expect(vcf).not.toContain("urn:uuid:urn:uuid:");
+  });
+
+  it("prefixes a bare key with urn:uuid:", () => {
+    expect(generateVCard([group({ "0f3c9d2e": true, gone: false })]).split("\r\n").filter(l => l.startsWith("MEMBER:")))
+      .toEqual(["MEMBER:urn:uuid:0f3c9d2e"]);
+  });
+
+  it("round-trips members through import and export", () => {
+    const [parsed] = parseVCard([
+      "BEGIN:VCARD",
+      "VERSION:3.0",
+      "FN:Team",
+      "KIND:group",
+      "MEMBER:urn:uuid:member-1",
+      "MEMBER:mailto:ann@example.com",
+      "END:VCARD",
+    ].join("\r\n"));
+    const [reparsed] = parseVCard(generateVCard([parsed]));
+    expect(reparsed.members).toEqual(parsed.members);
+    expect(generateVCard([parsed])).toContain("MEMBER:urn:uuid:member-1");
+  });
+});
+
 describe("round-trip: parse → generate → parse", () => {
   it("produces structurally equivalent data", () => {
     const original = [
diff --git a/lib/jmap/contact-wire.ts b/lib/jmap/contact-wire.ts
index 578e013a7..5e7c8798a 100644
--- a/lib/jmap/contact-wire.ts
+++ b/lib/jmap/contact-wire.ts
@@ -17,6 +17,15 @@ const CLIENT_ONLY_KEYS = [
 /** vCard CALURI / FBURL / CALADRURI, kept flat in the UI. */
 const URI_KEYS = ['calendarUri', 'freeBusyUri', 'schedulingUri', 'source'] as const;
 
+/**
+ * Properties an update never clears. A null `id` fails the whole update ("The
+ * id property is immutable.") and so does a null `addressBookIds` ("Invalid
+ * patch operation for addressBookIds."). A null `uid` is refused by Stalwart
+ * 1.0 ("You cannot change the UID of a contact.") and on 0.16 silently
+ * deletes the card's UID.
+ */
+const NEVER_CLEARED_KEYS = ['id', 'uid', 'addressBookIds'] as const;
+
 const LEGACY_ADDRESS_FIELDS: Array<[keyof ContactAddress, AddressComponent['kind']]> = [
   ['street', 'name'],
   ['locality', 'locality'],
@@ -49,12 +58,14 @@ type Mode = 'create' | 'update';
  * The card as ContactCard/set expects it. On `update`, a key that is present
  * with the value `undefined` means "the user cleared it" and is sent as
  * `null`: JSON drops undefined, and an omitted property keeps its old value.
+ * `id`, `uid` and `addressBookIds` without a value are left out instead.
  */
 export function contactToWire(card: Partial<ContactCard>, mode: Mode): Record<string, unknown> {
   const out: Record<string, unknown> = {};
   for (const [key, value] of Object.entries(card)) {
     if ((CLIENT_ONLY_KEYS as readonly string[]).includes(key)) continue;
     if ((URI_KEYS as readonly string[]).includes(key)) continue;
+    if (mode === 'update' && value == null && (NEVER_CLEARED_KEYS as readonly string[]).includes(key)) continue;
     if (value === undefined) {
       if (mode === 'update') out[key] = null;
       continue;
diff --git a/lib/vcard.ts b/lib/vcard.ts
index ba6c23e87..52c60ff81 100644
--- a/lib/vcard.ts
+++ b/lib/vcard.ts
@@ -1405,7 +1405,11 @@ function generateSingleVCard(contact: ContactCard): string {
   if (contact.members) {
     for (const memberId of Object.keys(contact.members)) {
       if (contact.members[memberId]) {
-        lines.push(`MEMBER:urn:uuid:${memberId}`);
+        // Keys are URIs already for groups made here (members are keyed by
+        // uid, a urn:uuid:) and for groups from CardDAV clients (Stalwart
+        // keeps MEMBER values verbatim); only a bare key gets the prefix.
+        const uri = /^[a-z][a-z0-9+.-]*:/i.test(memberId) ? memberId : `urn:uuid:${memberId}`;
+        lines.push(`MEMBER:${uri}`);
       }
     }
   }