diff --git a/CHANGELOG.md b/CHANGELOG.md
index 42919130..137a8a95 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- When the session index is missing, WebUI now starts a background rebuild while preserving the first sidebar full-scan result, so the index is primed for later requests without temporarily hiding existing sessions.
+
 ## [v0.51.134] — 2026-05-25 — Release DF (stage-batch16 — single-PR Windows path defaults)
 
 ### Fixed
diff --git a/api/models.py b/api/models.py
index fcd12ff5..3cd6cb24 100644
--- a/api/models.py
+++ b/api/models.py
@@ -51,6 +51,8 @@ _STALE_TMP_AGE_SECONDS = 3600  # 1 hour
 # Serializes index writers so concurrent Session.save() calls cannot race on
 # stale baselines while still allowing LOCK to be released before disk I/O.
 _INDEX_WRITE_LOCK = threading.RLock()
+_SESSION_INDEX_REBUILD_LOCK = threading.Lock()
+_SESSION_INDEX_REBUILD_THREAD = None
 
 
 def _cleanup_stale_tmp_files() -> None:
@@ -74,6 +76,32 @@ def _cleanup_stale_tmp_files() -> None:
         pass  # SESSION_DIR may not exist yet; that's fine
 
 
+def _rebuild_session_index_background() -> None:
+    try:
+        _write_session_index(updates=None)
+    except Exception:
+        logger.debug("Background session-index rebuild failed", exc_info=True)
+
+
+def _start_session_index_rebuild_thread() -> None:
+    """Start one background full-index rebuild if the index is missing."""
+    global _SESSION_INDEX_REBUILD_THREAD
+    with _SESSION_INDEX_REBUILD_LOCK:
+        if SESSION_INDEX_FILE.exists():
+            return
+        if (
+            _SESSION_INDEX_REBUILD_THREAD is not None
+            and _SESSION_INDEX_REBUILD_THREAD.is_alive()
+        ):
+            return
+        _SESSION_INDEX_REBUILD_THREAD = threading.Thread(
+            target=_rebuild_session_index_background,
+            name="session-index-rebuild",
+            daemon=True,
+        )
+        _SESSION_INDEX_REBUILD_THREAD.start()
+
+
 def _index_entry_exists(session_id: str, in_memory_ids=None) -> bool:
     """Return True if an index entry still has backing state.
 
@@ -2236,6 +2264,9 @@ def all_sessions(diag=None):
     active_stream_ids = _active_stream_ids()
     # Phase C: try index first for O(1) read; fall back to full scan
     _diag_stage(diag, "all_sessions.index_exists")
+    if not SESSION_INDEX_FILE.exists():
+        _diag_stage(diag, "all_sessions.start_index_rebuild")
+        _start_session_index_rebuild_thread()
     if SESSION_INDEX_FILE.exists():
         try:
             _diag_stage(diag, "all_sessions.read_index")
diff --git a/tests/test_issue2863_session_index_prime.py b/tests/test_issue2863_session_index_prime.py
new file mode 100644
index 00000000..62d42c76
--- /dev/null
+++ b/tests/test_issue2863_session_index_prime.py
@@ -0,0 +1,41 @@
+"""Regression tests for #2863 missing session-index background rebuild."""
+from __future__ import annotations
+
+import json
+import time
+
+
+def test_missing_index_starts_background_rebuild_while_preserving_first_scan(monkeypatch, tmp_path):
+    import api.models as models
+    from api.models import all_sessions
+
+    session_dir = tmp_path / "sessions"
+    session_dir.mkdir(parents=True)
+    monkeypatch.setattr(models, "SESSION_DIR", session_dir)
+    monkeypatch.setattr(models, "SESSION_INDEX_FILE", session_dir / "_index.json")
+    models.SESSIONS.clear()
+
+    for idx in range(3):
+        payload = {
+            "session_id": f"issue2863{idx}",
+            "title": f"Session {idx}",
+            "workspace": str(tmp_path),
+            "model": "test-model",
+            "messages": [{"role": "user", "content": f"hello {idx}", "timestamp": time.time() + idx}],
+            "created_at": time.time() + idx,
+            "updated_at": time.time() + idx,
+        }
+        (session_dir / f"issue2863{idx}.json").write_text(json.dumps(payload), encoding="utf-8")
+
+    rows = all_sessions()
+
+    assert {row["session_id"] for row in rows} == {"issue28630", "issue28631", "issue28632"}
+
+    thread = models._SESSION_INDEX_REBUILD_THREAD
+    assert thread is not None
+    thread.join(timeout=5)
+    assert not thread.is_alive()
+
+    index = json.loads(models.SESSION_INDEX_FILE.read_text(encoding="utf-8"))
+    assert {row["session_id"] for row in index} == {"issue28630", "issue28631", "issue28632"}
+    assert {row["session_id"] for row in all_sessions()} == {"issue28630", "issue28631", "issue28632"}