diff --git a/src/routers/ingest.py b/src/routers/ingest.py
index b7f8013..1cd2ca3 100644
--- a/src/routers/ingest.py
+++ b/src/routers/ingest.py
@@ -52,6 +52,23 @@ async def ingest(
                     project.repo_url = s.repo_url
 
             # Upsert session (dedup by user_id + session_id + date)
+            update_fields: dict = dict(
+                end_at=s.end_at,
+                message_count=s.message_count,
+                work_summary=s.work_summary,
+                commits=s.commits,
+                tools_used=s.tools_used,
+                files_changed=s.files_changed,
+                raw_stats=s.raw_stats,
+                input_tokens=s.input_tokens,
+                output_tokens=s.output_tokens,
+                cost_usd=s.cost_usd,
+            )
+            # Only overwrite active_hours when the collector sends a real value (>0).
+            # The cost-backfill script sends 0 to avoid clobbering existing hours.
+            if s.active_hours > 0:
+                update_fields["active_hours"] = s.active_hours
+
             stmt = insert(Session).values(
                 user_id=user.id,
                 project_id=project.id,
@@ -71,19 +88,7 @@ async def ingest(
                 cost_usd=s.cost_usd,
             ).on_conflict_do_update(
                 constraint="uq_session_user_date",
-                set_=dict(
-                    end_at=s.end_at,
-                    active_hours=s.active_hours,
-                    message_count=s.message_count,
-                    work_summary=s.work_summary,
-                    commits=s.commits,
-                    tools_used=s.tools_used,
-                    files_changed=s.files_changed,
-                    raw_stats=s.raw_stats,
-                    input_tokens=s.input_tokens,
-                    output_tokens=s.output_tokens,
-                    cost_usd=s.cost_usd,
-                ),
+                set_=update_fields,
             )
             result = await db.execute(stmt)
             if result.rowcount: