diff --git a/migrations/versions/cfaff339d519_add_session_last_message_at.py b/migrations/versions/cfaff339d519_add_session_last_message_at.py
index d4f011ef..9006dc89 100644
--- a/migrations/versions/cfaff339d519_add_session_last_message_at.py
+++ b/migrations/versions/cfaff339d519_add_session_last_message_at.py
@@ -48,7 +48,11 @@ def upgrade() -> None:
     op.create_index(
         INDEX_NAME,
         "sessions",
-        ["workspace_name", "last_message_at", "id"],
+        [
+            "workspace_name",
+            sa.text("last_message_at DESC NULLS LAST"),
+            sa.text("id DESC"),
+        ],
         unique=False,
         schema=schema,
         postgresql_where=sa.text("is_active"),
diff --git a/src/models.py b/src/models.py
index b3bf0654..77d218e4 100644
--- a/src/models.py
+++ b/src/models.py
@@ -199,8 +199,8 @@ class Session(Base):
         Index(
             "ix_sessions_workspace_last_message_at",
             "workspace_name",
-            "last_message_at",
-            "id",
+            text("last_message_at DESC NULLS LAST"),
+            text("id DESC"),
             postgresql_where=text("is_active"),
         ),
         CheckConstraint("length(name) <= 512", name="name_length"),
diff --git a/src/schemas/api.py b/src/schemas/api.py
index b0e65ef5..65e94593 100644
--- a/src/schemas/api.py
+++ b/src/schemas/api.py
@@ -339,6 +339,16 @@ class MessageCreate(MessageBase):
     def sanitize_content(cls, v: str) -> str:
         return strip_nul(v)
 
+    @field_validator("created_at", mode="after")
+    @classmethod
+    def normalize_created_at_timezone(
+        cls, value: datetime.datetime | None
+    ) -> datetime.datetime | None:
+        """Treat timezone-naive message timestamps as UTC."""
+        if value is not None and value.tzinfo is None:
+            return value.replace(tzinfo=datetime.UTC)
+        return value
+
     @property
     def encoded_message(self) -> list[int]:
         return self._encoded_message
diff --git a/tests/alembic/revisions/test_cfaff339d519_add_session_last_message_at.py b/tests/alembic/revisions/test_cfaff339d519_add_session_last_message_at.py
index bfac4af8..9f9f4f95 100644
--- a/tests/alembic/revisions/test_cfaff339d519_add_session_last_message_at.py
+++ b/tests/alembic/revisions/test_cfaff339d519_add_session_last_message_at.py
@@ -102,6 +102,24 @@ def verify_add_session_last_message_at(verifier: MigrationVerifier) -> None:
     verifier.assert_column_type("sessions", "last_message_at", sa.TIMESTAMP)
     verifier.assert_indexes_exist([("sessions", INDEX_NAME)])
 
+    index_definition = verifier.conn.execute(
+        text(
+            """
+            SELECT indexdef
+            FROM pg_indexes
+            WHERE schemaname = :schema
+              AND tablename = 'sessions'
+              AND indexname = :index_name
+            """
+        ),
+        {"schema": verifier.schema, "index_name": INDEX_NAME},
+    ).scalar_one()
+    normalized_index_definition = " ".join(index_definition.replace('"', "").split())
+    assert "(workspace_name, last_message_at DESC NULLS LAST, id DESC)" in (
+        normalized_index_definition
+    )
+    assert "WHERE is_active" in normalized_index_definition
+
     rows = verifier.conn.execute(
         text(
             f"""
diff --git a/tests/routes/test_sessions.py b/tests/routes/test_sessions.py
index 9efe1e78..2d48bd00 100644
--- a/tests/routes/test_sessions.py
+++ b/tests/routes/test_sessions.py
@@ -4,6 +4,7 @@ from typing import Any
 import pytest
 from fastapi.testclient import TestClient
 from nanoid import generate as generate_nanoid
+from sqlalchemy import text
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from src import models
@@ -327,6 +328,48 @@ def test_session_last_message_at_does_not_move_backwards_for_backdated_message(
     ) == datetime.datetime(2026, 1, 3, 12, 0, tzinfo=datetime.UTC)
 
 
+def test_message_batch_normalizes_naive_activity_timestamps(
+    client: TestClient, sample_data: tuple[Workspace, Peer]
+):
+    """Mixed explicit and server-default timestamps remain comparable."""
+    test_workspace, test_peer = sample_data
+    session_id = f"last-activity-mixed-timezone-{generate_nanoid()}"
+
+    response = client.post(
+        f"/v3/workspaces/{test_workspace.name}/sessions/{session_id}/messages",
+        json={
+            "messages": [
+                {
+                    "content": "naive activity timestamp",
+                    "peer_id": test_peer.name,
+                    "created_at": "2024-01-01T10:00:00",
+                },
+                {
+                    "content": "server timestamp",
+                    "peer_id": test_peer.name,
+                },
+            ]
+        },
+    )
+
+    assert response.status_code == 201
+    message_timestamps = [
+        datetime.datetime.fromisoformat(item["created_at"].replace("Z", "+00:00"))
+        for item in response.json()
+    ]
+    assert all(timestamp.utcoffset() is not None for timestamp in message_timestamps)
+
+    session_response = client.post(
+        f"/v3/workspaces/{test_workspace.name}/sessions",
+        json={"id": session_id},
+    )
+    assert session_response.status_code == 200
+    session_activity = datetime.datetime.fromisoformat(
+        session_response.json()["last_message_at"].replace("Z", "+00:00")
+    )
+    assert session_activity == max(message_timestamps)
+
+
 def test_get_sessions_with_empty_filter(
     client: TestClient, sample_data: tuple[Workspace, Peer]
 ):
@@ -462,6 +505,31 @@ async def test_get_sessions_sort_by_last_message_at_reverses_activity_and_keeps_
     assert response.json()["items"][-1]["last_message_at"] is None
 
 
+@pytest.mark.asyncio
+async def test_session_activity_index_supports_reverse_ordering(
+    db_session: AsyncSession,
+):
+    """The model index matches the primary DESC NULLS LAST query shape."""
+    result = await db_session.execute(
+        text(
+            """
+            SELECT indexdef
+            FROM pg_indexes
+            WHERE schemaname = current_schema()
+              AND tablename = 'sessions'
+              AND indexname = 'ix_sessions_workspace_last_message_at'
+            """
+        )
+    )
+    index_definition = result.scalar_one()
+    normalized_index_definition = " ".join(index_definition.replace('"', "").split())
+
+    assert "(workspace_name, last_message_at DESC NULLS LAST, id DESC)" in (
+        normalized_index_definition
+    )
+    assert "WHERE is_active" in normalized_index_definition
+
+
 @pytest.mark.asyncio
 async def test_get_sessions_reverse_uses_id_tiebreaker(
     client: TestClient,