Don't use prefix matching if we can help it

tests/test_contacts_router.py

@@ -90,7 +90,7 @@ class TestCreateContact:
 
         with (
             patch(
-                "app.routers.contacts.ContactRepository.get_by_key_or_prefix",
+                "app.routers.contacts.ContactRepository.get_by_key",
                 new_callable=AsyncMock,
                 return_value=None,
             ),
@@ -123,7 +123,7 @@ class TestCreateContact:
         from fastapi.testclient import TestClient
 
         with patch(
-            "app.routers.contacts.ContactRepository.get_by_key_or_prefix",
+            "app.routers.contacts.ContactRepository.get_by_key",
             new_callable=AsyncMock,
             return_value=None,
         ):
@@ -160,7 +160,7 @@ class TestCreateContact:
 
         with (
             patch(
-                "app.routers.contacts.ContactRepository.get_by_key_or_prefix",
+                "app.routers.contacts.ContactRepository.get_by_key",
                 new_callable=AsyncMock,
                 return_value=existing,
             ),
@@ -220,6 +220,30 @@ class TestGetContact:
 
         assert response.status_code == 404
 
+    def test_get_ambiguous_prefix_returns_409(self):
+        from fastapi.testclient import TestClient
+
+        from app.repository import AmbiguousPublicKeyPrefixError
+
+        with patch(
+            "app.routers.contacts.ContactRepository.get_by_key_or_prefix",
+            new_callable=AsyncMock,
+            side_effect=AmbiguousPublicKeyPrefixError(
+                "abcd12",
+                [
+                    "abcd120000000000000000000000000000000000000000000000000000000000",
+                    "abcd12ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+                ],
+            ),
+        ):
+            from app.main import app
+
+            client = TestClient(app)
+            response = client.get("/api/contacts/abcd12")
+
+        assert response.status_code == 409
+        assert "ambiguous" in response.json()["detail"].lower()
+
 
 class TestMarkRead:
     """Test POST /api/contacts/{public_key}/mark-read."""

tests/test_key_normalization.py

@@ -3,7 +3,7 @@
 import pytest
 
 from app.database import Database
-from app.repository import ContactRepository, MessageRepository
+from app.repository import AmbiguousPublicKeyPrefixError, ContactRepository, MessageRepository
 
 
 @pytest.fixture
@@ -117,3 +117,43 @@ async def test_duplicate_with_same_text_and_null_timestamp_rejected(test_db):
         received_at=received_at,
     )
     assert msg_id2 is None  # duplicate rejected
+
+
+@pytest.mark.asyncio
+async def test_get_by_key_prefix_returns_none_when_ambiguous(test_db):
+    """Ambiguous prefixes should not resolve to an arbitrary contact."""
+    key1 = "abc1230000000000000000000000000000000000000000000000000000000000"
+    key2 = "abc123ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
+
+    await ContactRepository.upsert({"public_key": key1, "name": "A"})
+    await ContactRepository.upsert({"public_key": key2, "name": "B"})
+
+    contact = await ContactRepository.get_by_key_prefix("abc123")
+    assert contact is None
+
+
+@pytest.mark.asyncio
+async def test_get_by_key_or_prefix_raises_on_ambiguous_prefix(test_db):
+    """Prefix lookup should raise when multiple contacts match."""
+    key1 = "abc1230000000000000000000000000000000000000000000000000000000000"
+    key2 = "abc123ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
+
+    await ContactRepository.upsert({"public_key": key1, "name": "A"})
+    await ContactRepository.upsert({"public_key": key2, "name": "B"})
+
+    with pytest.raises(AmbiguousPublicKeyPrefixError):
+        await ContactRepository.get_by_key_or_prefix("abc123")
+
+
+@pytest.mark.asyncio
+async def test_get_by_key_or_prefix_prefers_exact_full_key(test_db):
+    """Exact key lookup works even when the shorter prefix is ambiguous."""
+    key1 = "abc1230000000000000000000000000000000000000000000000000000000000"
+    key2 = "abc123ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
+
+    await ContactRepository.upsert({"public_key": key1, "name": "A"})
+    await ContactRepository.upsert({"public_key": key2, "name": "B"})
+
+    contact = await ContactRepository.get_by_key_or_prefix(key2.upper())
+    assert contact is not None
+    assert contact.public_key == key2

tests/test_send_messages.py

@@ -4,6 +4,7 @@ import asyncio
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
+from fastapi import HTTPException
 from meshcore import EventType
 
 from app.models import (
@@ -13,6 +14,7 @@ from app.models import (
     SendChannelMessageRequest,
     SendDirectMessageRequest,
 )
+from app.repository import AmbiguousPublicKeyPrefixError
 from app.routers.messages import send_channel_message, send_direct_message
 
 
@@ -123,6 +125,34 @@ class TestOutgoingDMBotTrigger:
             call_kwargs = mock_bot.call_args[1]
             assert call_kwargs["sender_name"] is None
 
+    @pytest.mark.asyncio
+    async def test_send_dm_ambiguous_prefix_returns_409(self):
+        """Ambiguous destination prefix should fail instead of selecting a random contact."""
+        mc = _make_mc()
+
+        with (
+            patch("app.routers.messages.require_connected", return_value=mc),
+            patch(
+                "app.repository.ContactRepository.get_by_key_or_prefix",
+                new=AsyncMock(
+                    side_effect=AmbiguousPublicKeyPrefixError(
+                        "abc123",
+                        [
+                            "abc1230000000000000000000000000000000000000000000000000000000000",
+                            "abc123ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+                        ],
+                    )
+                ),
+            ),
+        ):
+            with pytest.raises(HTTPException) as exc_info:
+                await send_direct_message(
+                    SendDirectMessageRequest(destination="abc123", text="Hello")
+                )
+
+        assert exc_info.value.status_code == 409
+        assert "ambiguous" in exc_info.value.detail.lower()
+
 
 class TestOutgoingChannelBotTrigger:
     """Test that sending a channel message triggers bots with is_outgoing=True."""