diff --git a/chat/web/snapshots.py b/chat/web/snapshots.py
index ae3cc30..c169e4f 100644
--- a/chat/web/snapshots.py
+++ b/chat/web/snapshots.py
@@ -8,20 +8,27 @@ Routes:
 
 * ``GET  /snapshots``                    list all snapshots (both kinds)
 * ``POST /snapshots/take``               take a periodic snapshot now
-* ``POST /snapshots/restore/{id}``       restore (requires matching ``confirm_id``)
+* ``POST /snapshots/restore/{id}``       restore (requires matching ``confirm_id`` and ``kind``)
 * ``GET  /snapshots/{id}/preview``       show metadata + delta vs current
 
 The ``snapshot_id`` is the filename stem (the UTC timestamp written by
 :func:`chat.services.snapshot.take_snapshot`) — there's no separate UUID,
 and the timestamp filename is already unique per snapshot kind. Both
 periodic and rewind snapshots share the same id space lookup-wise, so
-the restore + preview routes accept ``kind`` as a form/query param to
-disambiguate.
+the restore + preview routes require ``kind`` as a form/query param to
+disambiguate (a missing/empty ``kind`` is a 400, not a silent default).
+
+Note on ``created_at`` mtime drift: the listing's ``created_at`` comes
+from the file's mtime, not the encoded filename timestamp. ``cp -p``
+preserves mtime, but plain ``cp`` resets it to "now" — so a copied
+snapshot can show a misleading ``created_at`` while its filename still
+reflects the original UTC capture time.
 """
 
 from __future__ import annotations
 
 import json
+from datetime import datetime, timezone
 from pathlib import Path
 
 from fastapi import APIRouter, Depends, Form, HTTPException, Request
@@ -52,8 +59,6 @@ def _list_all_snapshots(data_dir: Path) -> list[dict]:
     ``last_event_id`` (parsed from the JSON body — small enough that
     listing isn't a performance concern for the handful of files we keep).
     """
-    from datetime import datetime, timezone
-
     rows: list[dict] = []
     for kind in SNAPSHOT_KINDS:
         snap_dir = data_dir / "snapshots" / kind
@@ -85,12 +90,26 @@ def _list_all_snapshots(data_dir: Path) -> list[dict]:
     return rows
 
 
+def _require_kind(kind: str) -> str:
+    """Reject missing/empty/unknown ``kind`` with 400.
+
+    Defaulting silently to ``"periodic"`` made rewind-snapshot lookups
+    appear as 404s, which is confusing — make the client always state
+    the kind explicitly.
+    """
+    if not kind or kind not in SNAPSHOT_KINDS:
+        raise HTTPException(
+            status_code=400,
+            detail=f"kind must be one of {SNAPSHOT_KINDS}",
+        )
+    return kind
+
+
 def _resolve_snapshot_path(
     data_dir: Path, snapshot_id: str, kind: str
 ) -> Path:
     """Map an ``(id, kind)`` pair to the on-disk file, or 404."""
-    if kind not in SNAPSHOT_KINDS:
-        raise HTTPException(status_code=400, detail=f"unknown kind: {kind}")
+    _require_kind(kind)
     path = data_dir / "snapshots" / kind / f"{snapshot_id}.json"
     if not path.exists():
         raise HTTPException(status_code=404, detail="snapshot not found")
@@ -127,7 +146,7 @@ async def snapshots_restore(
     snapshot_id: str,
     request: Request,
     confirm_id: str = Form(""),
-    kind: str = Form("periodic"),
+    kind: str = Form(""),
     conn=Depends(get_conn),
 ):
     """Hard-confirm restore: ``confirm_id`` must equal the path id.
@@ -135,7 +154,11 @@ async def snapshots_restore(
     Mismatched confirm → 400 (without touching the DB). On match, the
     existing :func:`restore_from_snapshot` clears projected tables and
     re-loads them from the dump.
+
+    ``kind`` is required (must be ``"periodic"`` or ``"rewind"``) — a
+    missing or empty value 400s rather than silently defaulting.
     """
+    _require_kind(kind)
     if confirm_id != snapshot_id:
         raise HTTPException(
             status_code=400,
@@ -151,7 +174,7 @@ async def snapshots_restore(
 async def snapshots_preview(
     snapshot_id: str,
     request: Request,
-    kind: str = "periodic",
+    kind: str = "",
     conn=Depends(get_conn),
 ):
     """Show snapshot metadata + a basic delta against the current event log.
@@ -159,7 +182,10 @@ async def snapshots_preview(
     Phase 4 keeps this simple: the snapshot's ``last_event_id`` plus the
     current ``MAX(event_log.id)`` is enough to tell the user how far the
     log has moved on. A richer per-table diff is a Phase 4.5+ concern.
+
+    ``kind`` is required — see :func:`snapshots_restore`.
     """
+    _require_kind(kind)
     settings = request.app.state.settings
     path = _resolve_snapshot_path(settings.data_dir, snapshot_id, kind)
     dump = json.loads(path.read_text())
diff --git a/tests/test_snapshot_ux.py b/tests/test_snapshot_ux.py
index 347f9ce..3db7cbd 100644
--- a/tests/test_snapshot_ux.py
+++ b/tests/test_snapshot_ux.py
@@ -156,6 +156,28 @@ def test_restore_snapshot_wrong_confirm_400(client, tmp_path):
     assert response.status_code == 400
 
 
+def test_restore_without_kind_returns_400(client, tmp_path):
+    """T105: Missing or empty ``kind`` must be rejected with 400.
+
+    Previously ``kind`` defaulted to ``"periodic"``, which silently 404'd
+    when the caller meant a rewind snapshot. Tighten the contract so the
+    client must always pass an explicit, valid ``kind``.
+    """
+    db_path = tmp_path / "test.db"
+    _seed_bot(db_path, "bot_a", "BotA")
+    snapshot_path = _take_snapshot_via_service(
+        db_path, tmp_path, kind="periodic"
+    )
+    snapshot_id = snapshot_path.stem
+
+    response = client.post(
+        f"/snapshots/restore/{snapshot_id}",
+        data={"confirm_id": snapshot_id},  # no `kind`
+        follow_redirects=False,
+    )
+    assert response.status_code == 400
+
+
 def test_preview_renders_metadata(client, tmp_path):
     db_path = tmp_path / "test.db"
     _seed_bot(db_path, "bot_a", "BotA")