diff --git a/scripts/dossiers-ngi/README.md b/scripts/dossiers-ngi/README.md
index a288176..d4f2efa 100644
--- a/scripts/dossiers-ngi/README.md
+++ b/scripts/dossiers-ngi/README.md
@@ -63,7 +63,7 @@ Per branch in JSON / sticky bar:
 | `vs_parent_plus` / `vs_parent_minus` | `git diff --numstat` tip vs **parent** tip (sticky bar +grün/−rot; omitted when `parent` is empty or not in the tree, e.g. `main` and `feature/nl-net-injestion-injestor-update`) |
 | `loc` | text-file line count of the tip vs the empty tree (gray `N LOC` in the plus/minus columns; only when there is no parent) |
 
-Rebuild runs **only when** the current origin tip is not already a successful freeze (`freeze_ok`; timer never forces).
+Rebuild runs **only when** the current origin tip is not already a successful freeze (`freeze_ok`; timer never forces). Cookie alias `editions/flask` copies from `DEFAULT_BRANCH` unless that alias is already `freeze_ok` for the same tip **and** `.edition-built-at` matches the source freeze.
 Each run also: `git fetch --prune`, `worktree prune`, drop stale
 `editions/<slug>/` (gone remotes), purge leftover `tdpd-*` tmp dirs.
 
diff --git a/scripts/dossiers-ngi/branch-status-hourly.py b/scripts/dossiers-ngi/branch-status-hourly.py
index 7b32e78..8cf6433 100755
--- a/scripts/dossiers-ngi/branch-status-hourly.py
+++ b/scripts/dossiers-ngi/branch-status-hourly.py
@@ -534,15 +534,26 @@ def rebuild_edition(repo: Path, branch: str, tip: str, editions: Path) -> dict:
             shutil.rmtree(wt, ignore_errors=True)
 
 
+def _edition_built_at(editions: Path, slug: str) -> str:
+    p = editions / slug / ".edition-built-at"
+    if not p.is_file():
+        return ""
+    return p.read_text(encoding="utf-8").strip()
+
+
 def sync_flask_alias(editions: Path, tips: dict[str, str]) -> None:
     """Keep editions/flask in sync with DEFAULT_BRANCH freeze when present."""
     src_branch = default_branch()
     if src_branch not in tips:
         return
-    src = editions / edition_slug(src_branch)
+    src_slug = edition_slug(src_branch)
+    src = editions / src_slug
     dst = editions / "flask"
     tip = tips[src_branch]
-    if tip_recorded(editions, "flask") == tip and (dst / "index.html").is_file():
+    flask_ok, _ = freeze_ok_for(editions, "flask", tip)
+    src_built = _edition_built_at(editions, src_slug)
+    flask_built = _edition_built_at(editions, "flask")
+    if flask_ok and src_built and src_built == flask_built:
         return
     if not (src / "index.html").is_file():
         ensure_placeholder(editions, "flask", tip)
diff --git a/scripts/dossiers-ngi/smoke-branch-tree.sh b/scripts/dossiers-ngi/smoke-branch-tree.sh
index 7703693..e180854 100755
--- a/scripts/dossiers-ngi/smoke-branch-tree.sh
+++ b/scripts/dossiers-ngi/smoke-branch-tree.sh
@@ -100,6 +100,24 @@ PY
 printf '1\n' > "$EDITIONS/feat-a/.edition-freeze-ok"
 rm -f "$EDITIONS/feat-a/.edition-freeze-error"
 
+# flask alias: matching tip + stale HTML must copy from DEFAULT_BRANCH
+MAIN_TIP="$(tr -d '\n' < "$EDITIONS/main/.edition-tip")"
+mkdir -p "$EDITIONS/flask"
+printf '%s\n' "$MAIN_TIP" > "$EDITIONS/flask/.edition-tip"
+printf '%s\n' '<html>stale-flask</html>' > "$EDITIONS/flask/index.html"
+printf '%s\n' '2026-09-30T17:35:26+02:00' > "$EDITIONS/flask/.edition-built-at"
+FETCH=0 TDPD_ROOT="$CLONE" OUT="$OUT" EDITIONS_ROOT="$EDITIONS" \
+  FREEZE_CMD="$FREEZE_CMD" REBUILD=1 DEFAULT_BRANCH=main \
+  python3 "$SCRIPT" --rebuild --no-fetch
+grep -q "main" "$EDITIONS/flask/index.html"
+if grep -q "stale-flask" "$EDITIONS/flask/index.html"; then
+  echo "FAIL flask alias kept stale HTML"
+  exit 1
+fi
+test "$(tr -d '\n' < "$EDITIONS/flask/.edition-freeze-ok")" = "1"
+test "$(tr -d '\n' < "$EDITIONS/flask/.edition-built-at")" = "$(tr -d '\n' < "$EDITIONS/main/.edition-built-at")"
+echo "OK flask alias copied after stale skip"
+
 # delete smoke branch on origin; fetch --prune drops remote-tracking ref
 cd "$CLONE"
 git push origin --delete smoke/temp-sibling >/dev/null 2>&1