From daa8d7a28dc05bb0105c688be7fb96e2c06c950f Mon Sep 17 00:00:00 2001
From: Jason Robinson <jasonr@element.io>
Date: Tue, 23 Jun 2026 12:39:07 +0300
Subject: [PATCH] Ensure lazy loaded profile updates in sync contain full
 profiles for users in timeline

---
 synapse/handlers/sync.py    | 17 +++++++++++++----
 tests/handlers/test_sync.py | 23 +++++++++++++++++++++++
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 19c63945c8..f36a36b3d5 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -2316,10 +2316,19 @@ class SyncHandler:
                     continue
 
                 per_user_updates: dict[str, JsonValue] = {}
-                for field_name in fields:
-                    per_user_updates[field_name] = cast(
-                        JsonValue, profile_data.get(field_name)
-                    )
+                if include_users and other_user_id in include_users:
+                    # Include the full profile as this user has events in
+                    # a lazy loaded sync response
+                    for field_name in profile_data.keys():
+                        per_user_updates[field_name] = cast(
+                            JsonValue, profile_data.get(field_name)
+                        )
+                else:
+                    # Include only the diff
+                    for field_name in fields:
+                        per_user_updates[field_name] = cast(
+                            JsonValue, profile_data.get(field_name)
+                        )
 
                 if per_user_updates:
                     profile_updates[other_user_id] = per_user_updates
diff --git a/tests/handlers/test_sync.py b/tests/handlers/test_sync.py
index fdc9cb4726..2c5fd21599 100644
--- a/tests/handlers/test_sync.py
+++ b/tests/handlers/test_sync.py
@@ -1383,6 +1383,10 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
             incremental_result.profile_updates["@other_user:test"]["m.status"],
             '{"text": "On holiday", "emoji": "\\ud83c\\udfd6"}',
         )
+        # We only send diffs in incremental sync for profile field updates
+        self.assertIsNone(
+            incremental_result.profile_updates["@other_user:test"].get("displayname"),
+        )
 
     @override_config({"experimental_features": {"msc4429_enabled": True}})
     def test_incremental_sync_does_not_filter_profile_updates_when_lazy_loading(
@@ -1468,6 +1472,25 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
                 "@third_user:test",
             ],
         )
+        # This is a field upfate, so should be here
+        self.assertEqual(
+            incremental_result.profile_updates["@other_user:test"]["m.status"],
+            '{"text": "On holiday", "emoji": "\\ud83c\\udfd6"}',
+        )
+        # We don't have events for this user in this response, so their full profile
+        # is not included
+        self.assertIsNone(
+            incremental_result.profile_updates["@other_user:test"].get("displayname"),
+        )
+        # This user has events in the timeline, thus their full profile is included
+        self.assertEqual(
+            incremental_result.profile_updates["@third_user:test"]["m.status"],
+            '{"text": "On fire", "emoji": "\\ud83d\\udd25"}',
+        )
+        self.assertEqual(
+            incremental_result.profile_updates["@third_user:test"]["displayname"],
+            "third_user",
+        )
 
 
 class SyncStateAfterTestCase(tests.unittest.HomeserverTestCase):