diff --git a/CHANGELOG.md b/CHANGELOG.md
index 185f15f1..7bcc308f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Rebuild the WebUI session index during startup recovery when `_index.json` is missing, even when no `.bak` session restore occurs, so large state directories avoid repeated full-scan `/api/sessions` fallback after an index loss.
+
 
 ## [v0.51.95] — 2026-05-20 — Release BS (stage-388 — 5-PR batch — live tool callback event dedup + browser-only dashboard links + messaging transcript merge alignment + Geist Contrast skin + SSE runtime diagnostics)
 
diff --git a/api/session_recovery.py b/api/session_recovery.py
index d00c6d68..7dd756da 100644
--- a/api/session_recovery.py
+++ b/api/session_recovery.py
@@ -619,12 +619,13 @@ def recover_all_sessions_on_startup(
             "If you weren't expecting this, check the session list for missing "
             "messages — see #1558.", restored, scanned,
         )
-        if rebuild_index:
-            try:
-                from api.models import _write_session_index
+    if rebuild_index:
+        try:
+            from api.models import SESSION_INDEX_FILE, _write_session_index
+            if restored or not SESSION_INDEX_FILE.exists():
                 _write_session_index(updates=None)
-            except Exception as exc:
-                logger.warning("recover_all_sessions_on_startup: index rebuild failed: %s", exc)
+        except Exception as exc:
+            logger.warning("recover_all_sessions_on_startup: index rebuild failed: %s", exc)
     return {
         "scanned": scanned,
         "restored": restored,
diff --git a/tests/test_metadata_save_wipe_1558.py b/tests/test_metadata_save_wipe_1558.py
index 6ee6e1e7..a4f828e8 100644
--- a/tests/test_metadata_save_wipe_1558.py
+++ b/tests/test_metadata_save_wipe_1558.py
@@ -270,6 +270,24 @@ def test_recover_all_sessions_on_startup_restores_orphan_bak(temp_session_dir):
     assert len(restored["messages"]) == 293
 
 
+def test_recover_all_sessions_on_startup_rebuilds_missing_index_without_restores(temp_session_dir, monkeypatch):
+    """Startup recovery must rebuild a missing index even when no .bak restore runs."""
+    import api.models as _m
+
+    sid = _make_session_on_disk(temp_session_dir, n_msgs=42)
+    missing_index = temp_session_dir / "_index.json"
+    monkeypatch.setattr(_m, "SESSION_INDEX_FILE", missing_index)
+    assert not missing_index.exists()
+
+    from api.session_recovery import recover_all_sessions_on_startup
+    result = recover_all_sessions_on_startup(temp_session_dir, rebuild_index=True)
+
+    assert result["restored"] == 0
+    index = json.loads(missing_index.read_text(encoding="utf-8"))
+    assert [entry["session_id"] for entry in index] == [sid]
+    assert index[0]["message_count"] == 42
+
+
 def test_recover_all_sessions_on_startup_rebuilds_index_after_orphan_restore(temp_session_dir, monkeypatch):
     """A restored orphan must be visible through the WebUI session index immediately."""
     import api.models as _m