diff --git a/api/models.py b/api/models.py
index a92c2840..1ab6e537 100644
--- a/api/models.py
+++ b/api/models.py
@@ -2195,6 +2195,70 @@ def _has_live_sidebar_state(session: dict) -> bool:
     )
 
 
+def _is_intentionally_background_sidebar_session(session: dict) -> bool:
+    sid = str(session.get('session_id') or '')
+    source = session.get('source_tag') or session.get('source')
+    return source == 'cron' or sid.startswith('cron_')
+
+
+def _preserve_messageful_sidebar_discoverability(
+    candidates: list[dict],
+    visible: list[dict],
+) -> list[dict]:
+    """Keep at least one messageful row per non-background conversation visible.
+
+    The normal sidebar filters intentionally hide empty drafts, cron/background
+    rows, and duplicate pre-compression snapshots. They must not make the only
+    messageful representative of a conversation disappear. If every visible row
+    for a lineage was filtered out, rescue the best hidden messageful row and
+    mark it so callers can surface or audit the degraded state.
+    """
+    sessions_by_id = {
+        str(session.get('session_id')): session
+        for session in candidates
+        if session.get('session_id')
+    }
+    covered_roots = {
+        _sidebar_lineage_root_id(session, sessions_by_id)
+        for session in visible
+        if _sidebar_message_count(session) > 0
+    }
+    visible_ids = {
+        str(session.get('session_id'))
+        for session in visible
+        if session.get('session_id')
+    }
+    rescue_by_root: dict[str, dict] = {}
+    for session in candidates:
+        sid = str(session.get('session_id') or '')
+        if not sid or sid in visible_ids:
+            continue
+        if _sidebar_message_count(session) <= 0:
+            continue
+        if _is_intentionally_background_sidebar_session(session):
+            continue
+        root = _sidebar_lineage_root_id(session, sessions_by_id)
+        if root in covered_roots:
+            continue
+        current = rescue_by_root.get(root)
+        if current is None or (
+            _sidebar_message_count(session), _session_sort_timestamp(session)
+        ) > (
+            _sidebar_message_count(current), _session_sort_timestamp(current)
+        ):
+            rescued = dict(session)
+            rescued['discoverability_warning'] = 'rescued_messageful_hidden_session'
+            rescue_by_root[root] = rescued
+    if not rescue_by_root:
+        return visible
+    rescued_rows = sorted(
+        rescue_by_root.values(),
+        key=lambda session: (session.get('pinned', False), _session_sort_timestamp(session)),
+        reverse=True,
+    )
+    return visible + rescued_rows
+
+
 def _prefer_fuller_snapshots_for_sidebar(sessions: list[dict]) -> list[dict]:
     """Expose a hidden snapshot when it is the fuller transcript for a lineage.
 
@@ -2412,7 +2476,8 @@ def all_sessions(diag=None):
                 and not s.get('worktree_path')
             )]
             result = _prefer_fuller_snapshots_for_sidebar(result)
-            result = [s for s in result if not _hide_from_default_sidebar(s)]
+            visible_result = [s for s in result if not _hide_from_default_sidebar(s)]
+            result = _preserve_messageful_sidebar_discoverability(result, visible_result)
             _strip_sidebar_internal_flags(result)
             # Backfill: sessions created before Sprint 22 have no profile tag.
             # Attribute them to 'default' so the client profile filter works correctly.
@@ -2450,7 +2515,8 @@ def all_sessions(diag=None):
         and not getattr(s, 'worktree_path', None)
     )]
     result = _prefer_fuller_snapshots_for_sidebar(result)
-    result = [s for s in result if not _hide_from_default_sidebar(s)]
+    visible_result = [s for s in result if not _hide_from_default_sidebar(s)]
+    result = _preserve_messageful_sidebar_discoverability(result, visible_result)
     _strip_sidebar_internal_flags(result)
     for s in result:
         if not s.get('profile'):
diff --git a/tests/test_session_discoverability_invariants.py b/tests/test_session_discoverability_invariants.py
new file mode 100644
index 00000000..4c553bee
--- /dev/null
+++ b/tests/test_session_discoverability_invariants.py
@@ -0,0 +1,62 @@
+"""Regression coverage for session sidebar discoverability invariants."""
+
+
+def test_messageful_hidden_snapshot_is_preserved_when_no_visible_representative():
+    from api.models import _preserve_messageful_sidebar_discoverability
+
+    hidden_snapshot = {
+        "session_id": "root_snapshot",
+        "title": "Long conversation snapshot",
+        "message_count": 42,
+        "pre_compression_snapshot": True,
+    }
+
+    result = _preserve_messageful_sidebar_discoverability(
+        candidates=[hidden_snapshot],
+        visible=[],
+    )
+
+    assert [row["session_id"] for row in result] == ["root_snapshot"]
+    assert result[0]["discoverability_warning"] == "rescued_messageful_hidden_session"
+
+
+def test_messageful_hidden_snapshot_stays_hidden_when_continuation_is_visible():
+    from api.models import _preserve_messageful_sidebar_discoverability
+
+    hidden_snapshot = {
+        "session_id": "root_snapshot",
+        "title": "Archived snapshot",
+        "message_count": 42,
+        "pre_compression_snapshot": True,
+    }
+    visible_tip = {
+        "session_id": "tip_session",
+        "parent_session_id": "root_snapshot",
+        "title": "Visible continuation",
+        "message_count": 50,
+    }
+
+    result = _preserve_messageful_sidebar_discoverability(
+        candidates=[hidden_snapshot, visible_tip],
+        visible=[visible_tip],
+    )
+
+    assert [row["session_id"] for row in result] == ["tip_session"]
+
+
+def test_intentional_background_sessions_are_not_rescued_into_sidebar():
+    from api.models import _preserve_messageful_sidebar_discoverability
+
+    cron_row = {
+        "session_id": "cron_digest_001",
+        "title": "Digest",
+        "source_tag": "cron",
+        "message_count": 12,
+    }
+
+    result = _preserve_messageful_sidebar_discoverability(
+        candidates=[cron_row],
+        visible=[],
+    )
+
+    assert result == []