diff --git a/tests/test_contacts_update_preserve.py b/tests/test_contacts_update_preserve.py
new file mode 100644
index 000000000..96440c8fa
--- /dev/null
+++ b/tests/test_contacts_update_preserve.py
@@ -0,0 +1,131 @@
+"""Pin the contact-update preserve logic: partial updates must not wipe
+the other field.
+
+The regression: {uid, emails:[new]} with no phones would skip the
+existing-contact fetch (AND guard) and silently drop phones.  Same in
+reverse.  Fixed by fetching whenever *either* field is missing (OR guard)
+and using `force=True` so a just-added contact isn't missed by a stale
+cache.
+"""
+import asyncio
+
+import pytest
+
+
+def _stub_cc(monkeypatch, contacts_by_uid, *, calls=None):
+    """Install a fake contacts_routes module that serves `contacts_by_uid`.
+
+    Returns a ``calls`` list of (action, *args) tuples for assertions.
+    """
+    import sys
+    import types
+
+    call_log = calls if calls is not None else []
+
+    class FakeCC(types.ModuleType):
+        pass
+
+    mod = FakeCC("routes.contacts_routes")
+    mod._fetch_contacts = lambda force=False: list(contacts_by_uid.values())
+    mod._update_contact = lambda uid, name, emails, phones, **kw: call_log.append(
+        ("update", uid, name, emails, phones)
+    ) or True
+    mod._create_contact = lambda name, email, **kw: call_log.append(
+        ("create", name, email)
+    ) or True
+    mod._delete_contact = lambda uid: call_log.append(("delete", uid)) or True
+
+    monkeypatch.setitem(sys.modules, "routes.contacts_routes", mod)
+    # Also patch _fetch_contacts on the real module (just in case)
+    import routes.contacts_routes as real_cc
+    monkeypatch.setattr(real_cc, "_fetch_contacts", mod._fetch_contacts)
+    monkeypatch.setattr(real_cc, "_update_contact", mod._update_contact)
+
+    return call_log
+
+
+@pytest.fixture
+def contact_bob():
+    return {
+        "uid": "bob-001",
+        "name": "Bob",
+        "emails": ["bob@example.com"],
+        "phones": ["+1-555-0100"],
+        "address": "123 Main",
+    }
+
+
+# ------------------------------------------------------------------
+# Rename-only: both emails + phones survive
+# ------------------------------------------------------------------
+def test_update_rename_preserves_emails_and_phones(monkeypatch, contact_bob):
+    call_log = _stub_cc(monkeypatch, {"bob-001": contact_bob})
+
+    from src.tools.contacts import do_manage_contact
+    result = asyncio.run(
+        do_manage_contact('{"action":"update","uid":"bob-001","name":"Robert"}')
+    )
+
+    assert result.get("output") == "Contact updated."
+    assert len(call_log) == 1
+    _, uid, name, emails, phones = call_log[0]
+    assert name == "Robert"
+    assert emails == ["bob@example.com"]   # preserved
+    assert phones == ["+1-555-0100"]       # preserved
+
+
+# ------------------------------------------------------------------
+# Update only emails → phones survive
+# ------------------------------------------------------------------
+def test_update_emails_only_preserves_phones(monkeypatch, contact_bob):
+    call_log = _stub_cc(monkeypatch, {"bob-001": contact_bob})
+
+    from src.tools.contacts import do_manage_contact
+    result = asyncio.run(
+        do_manage_contact(
+            '{"action":"update","uid":"bob-001","emails":["new@example.com"]}'
+        )
+    )
+
+    assert result.get("output") == "Contact updated."
+    _, uid, name, emails, phones = call_log[0]
+    assert emails == ["new@example.com"]
+    assert phones == ["+1-555-0100"]       # preserved, not wiped
+
+
+# ------------------------------------------------------------------
+# Update only phones → emails survive
+# ------------------------------------------------------------------
+def test_update_phones_only_preserves_emails(monkeypatch, contact_bob):
+    call_log = _stub_cc(monkeypatch, {"bob-001": contact_bob})
+
+    from src.tools.contacts import do_manage_contact
+    result = asyncio.run(
+        do_manage_contact(
+            '{"action":"update","uid":"bob-001","phones":["+1-555-9999"]}'
+        )
+    )
+
+    assert result.get("output") == "Contact updated."
+    _, uid, name, emails, phones = call_log[0]
+    assert phones == ["+1-555-9999"]
+    assert emails == ["bob@example.com"]   # preserved, not wiped
+
+
+# ------------------------------------------------------------------
+# Update both fields explicitly → no fetch needed, both overwritten
+# ------------------------------------------------------------------
+def test_update_both_fields_explicitly_does_not_fetch(monkeypatch, contact_bob):
+    call_log = _stub_cc(monkeypatch, {"bob-001": contact_bob})
+
+    from src.tools.contacts import do_manage_contact
+    result = asyncio.run(
+        do_manage_contact(
+            '{"action":"update","uid":"bob-001","emails":["a@x.com"],"phones":["+0"]}'
+        )
+    )
+
+    assert result.get("output") == "Contact updated."
+    _, uid, name, emails, phones = call_log[0]
+    assert emails == ["a@x.com"]
+    assert phones == ["+0"]