diff --git a/src/device-sync/__tests__/engine/internals.test.ts b/src/device-sync/__tests__/engine/internals.test.ts
index 4fee104b..07ec9348 100644
--- a/src/device-sync/__tests__/engine/internals.test.ts
+++ b/src/device-sync/__tests__/engine/internals.test.ts
@@ -297,6 +297,19 @@ describe('JMAP side', () => {
     expect(accountsWithCapability(session, JMAP_CALENDARS).map((a) => a.id)).toEqual(['a']);
   });
 
+  it("counts only the capability's primary account as personal, like the app's chooser", () => {
+    const server = new FakeJmapServer();
+    server.addAccount('a', { name: 'alice@example.com' });
+    // Another account the server calls personal (a second mailbox of the same login, say).
+    server.addAccount('home', { name: 'alice-home@example.com', isPersonal: true });
+    const session = server.port().session();
+
+    expect(accountsWithCapability(session, JMAP_CONTACTS).map((a) => [a.id, a.primary, a.personal])).toEqual([
+      ['a', true, true],
+      ['home', false, false],
+    ]);
+  });
+
   it('takes the login as the owner address and survives a server without identities', async () => {
     const server = new FakeJmapServer();
     server.addAccount('a', { name: 'Alice@Example.com' });
diff --git a/src/device-sync/jmap/session.ts b/src/device-sync/jmap/session.ts
index caf93550..b6d6743b 100644
--- a/src/device-sync/jmap/session.ts
+++ b/src/device-sync/jmap/session.ts
@@ -12,7 +12,11 @@ export interface CapabilityAccount {
   name: string;
   /** The capability's primary account (the user's own). */
   primary: boolean;
-  /** Collections of personal accounts are selected by default, shared ones are not. */
+  /**
+   * Collections of the personal account are selected by default, those of the others are not. It is the
+   * capability's primary account only, whatever `isPersonal` says: one rule with the app's chooser and
+   * reminder filter, which know only the primary account per authority.
+   */
   personal: boolean;
   readOnly: boolean;
 }
@@ -31,7 +35,7 @@ export function accountsWithCapability(session: JmapSessionView, capability: str
       id,
       name: account.name || id,
       primary: isPrimary,
-      personal: isPrimary || account.isPersonal,
+      personal: isPrimary,
       readOnly: !!account.isReadOnly,
     });
   };