fix(webui): overlay real state.db message count for subagent children so they don't vanish from the sidebar (#5308)

Regression seam behind #5308: a delegated subagent child's sidebar row is built
from a stale sidecar that reports message_count==0, and the state.db count
overlay in _apply_sidebar_state_db_override_metadata was gated on
`state_db_source == 'webui'`. A subagent child (state_db_source=='subagent')
therefore never received its true message count, so the front-end visibility
predicate (_sidebarRowHasVisibleMessages) dropped the row and the subagent
session disappeared entirely (not nested, not orphaned) after #5244+#5306.

Fix: widen the count/last-message overlay to `state_db_source in ('webui','subagent')`,
keeping the same conservative anti-resurrection guard. The source-tag/title
reassignment stays WebUI-only so a subagent child keeps its subagent
classification. Same state.db-blind-metadata root as the #5307 transcript
recovery, fixed server-side rather than by loosening the front-end predicate
(which would fight the #5306 active-parent scoping).

Tests: subagent child gets its count overlaid + classification preserved; a
non-webui/non-subagent foreign source (cron) still gets NO overlay.

Fixes #5308.

api/models.py

@@ -4028,6 +4028,20 @@ def _apply_sidebar_state_db_override_metadata(sessions: list[dict], metadata: di
             session['session_source'] = state_db_session_source
             session['source_label'] = state_db_source_label
             session['is_cli_session'] = False
+        # Overlay the real state.db message count for WebUI-owned rows AND for
+        # delegated subagent children (#5308). A subagent child
+        # (state_db_source == 'subagent') is backed by the delegate runner's
+        # state.db session, but its sidebar row is built from a stale sidecar
+        # that often reports message_count == 0. Without overlaying the true
+        # count, the front-end visibility predicate
+        # (_sidebarRowHasVisibleMessages) drops the row and the subagent
+        # session vanishes from the sidebar entirely (regression seam behind
+        # #5308, same state.db-blind-metadata root as the #5307 transcript
+        # recovery). The count overlay keeps the same conservative
+        # anti-resurrection guard used for WebUI rows. The source-tag / title
+        # reassignment above stays WebUI-only — a subagent child keeps its
+        # subagent classification.
+        if state_db_source in ('webui', 'subagent'):
             try:
                 current_count = max(0, int(session.get('message_count') or 0))
                 state_count = max(0, int(state_db_message_count or 0))

tests/test_webui_state_db_reconciliation.py

@@ -155,6 +155,88 @@ def test_sidebar_state_db_overlay_preserves_numeric_actual_count():
     assert sessions[0]["actual_message_count"] == 5
 
 
+def test_sidebar_state_db_overlay_counts_subagent_child_5308():
+    """#5308: a delegated subagent child whose stale sidecar reports
+    message_count == 0 must receive its real state.db message count so the
+    front-end sidebar visibility predicate does not drop the row (the subagent
+    session vanishing regression). The overlay applies to source == 'subagent'
+    just like 'webui', while the subagent source classification is preserved.
+    """
+    import api.models as models
+
+    sid = "subagent_child_5308"
+    sessions = [
+        {
+            "session_id": sid,
+            "source_tag": "subagent",
+            "raw_source": "subagent",
+            "session_source": "other",
+            "relationship_type": "child_session",
+            "parent_session_id": "parent_abc",
+            "message_count": 0,
+            "actual_message_count": 0,
+            "last_message_at": 0,
+            "updated_at": 0,
+        }
+    ]
+
+    models._apply_sidebar_state_db_override_metadata(
+        sessions,
+        {
+            sid: {
+                "_state_db_source": "subagent",
+                "_state_db_message_count": 6,
+                "_state_db_last_message_at": 2002.0,
+            }
+        },
+    )
+
+    # Real state.db count is now overlaid (was 0) -> row survives the sidebar
+    # visibility predicate instead of vanishing.
+    assert sessions[0]["message_count"] == 6
+    assert sessions[0]["actual_message_count"] == 6
+    assert sessions[0]["last_message_at"] == 2002.0
+    # Subagent classification is preserved (source-tag reassignment stays
+    # WebUI-only) — the child does not get re-tagged as a webui session.
+    assert sessions[0]["source_tag"] == "subagent"
+    assert sessions[0].get("is_cli_session") is not False
+
+
+def test_sidebar_state_db_overlay_does_not_count_foreign_cli_source_5308():
+    """Guard the #5308 overlay scope: a non-webui, non-subagent foreign source
+    (e.g. a messaging/cron/tui CLI row) must NOT get the count overlay — only
+    WebUI-owned rows and delegated subagent children do.
+    """
+    import api.models as models
+
+    sid = "cron_row_5308"
+    sessions = [
+        {
+            "session_id": sid,
+            "source_tag": "cron",
+            "message_count": 0,
+            "actual_message_count": 0,
+            "last_message_at": 0,
+            "updated_at": 0,
+        }
+    ]
+
+    models._apply_sidebar_state_db_override_metadata(
+        sessions,
+        {
+            sid: {
+                "_state_db_source": "cron",
+                "_state_db_message_count": 9,
+                "_state_db_last_message_at": 3003.0,
+            }
+        },
+    )
+
+    # cron is neither webui nor subagent -> count overlay must NOT apply.
+    assert sessions[0]["message_count"] == 0
+    assert sessions[0]["actual_message_count"] == 0
+
+
 def test_tail_cancelled_partial_blocks_state_db_replay():
     from api.models import merge_session_messages_append_only