diff --git a/scripts/backfill_hashes.py b/scripts/backfill_hashes.py
new file mode 100644
index 0000000..729fc9e
--- /dev/null
+++ b/scripts/backfill_hashes.py
@@ -0,0 +1,119 @@
+"""Rename pre-hash embedder figs to include the embed_args hash suffix.
+
+Walks figs/ for `.html` files matching the old stem shape (no hash tail) that
+represent an embedder run (not Reference), reads the sibling
+`<stem>.metrics.json` to recover `meta.embed_args`, computes the hash, and
+renames the .html + .metrics.json in place.
+
+Default is a dry-run — pass `--apply` to actually rename. Reference files are
+left alone (they have no embed_args). Missing metrics.json → warn and skip.
+Target-name collision → warn and skip.
+
+Usage:
+    .venv/bin/python scripts/backfill_hashes.py [--apply] [--figs-dir PATH]
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import re
+import sys
+from pathlib import Path
+
+# Reach up to the project root so we can reuse the canonical hash helper.
+_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(_ROOT))
+from app.web.main import embed_args_hash  # noqa: E402
+
+_LEGACY_STEM = re.compile(
+    r"^(?P<base>make_[A-Za-z_]+?_[A-Za-z]+_N\d+_T\d+_J[\d.]+_s\d+)$"
+)
+
+
+def plan_renames(figs_dir: Path):
+    for html in sorted(figs_dir.glob("*.html")):
+        stem = html.stem
+        m = _LEGACY_STEM.match(stem)
+        if not m:
+            # Either already hashed or doesn't match our scheme at all.
+            continue
+        # Skip Reference runs — they have no embed_args.
+        if "_Reference_" in stem:
+            continue
+        metrics = figs_dir / f"{stem}.metrics.json"
+        if not metrics.is_file():
+            yield (html, None, "missing metrics.json — can't compute hash")
+            continue
+        try:
+            ea = json.loads(metrics.read_text(encoding="utf-8"))["meta"]["embed_args"]
+        except (KeyError, json.JSONDecodeError) as e:
+            yield (html, None, f"bad metrics.json: {e}")
+            continue
+        new_stem = f"{stem}_{embed_args_hash(ea)}"
+        new_html = figs_dir / f"{new_stem}.html"
+        if new_html.exists():
+            yield (html, None, f"target exists: {new_html.name}")
+            continue
+        yield (html, new_stem, None)
+
+
+def apply_rename(figs_dir: Path, old_stem: str, new_stem: str) -> list[str]:
+    """Rename every sidecar sharing the old stem. Returns the renamed files."""
+    renamed = []
+    for suffix in (".html", ".metrics.json", ".frames.json"):
+        src = figs_dir / f"{old_stem}{suffix}"
+        if not src.exists():
+            continue
+        dst = figs_dir / f"{new_stem}{suffix}"
+        src.rename(dst)
+        renamed.append(f"{src.name} -> {dst.name}")
+    return renamed
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument("--apply", action="store_true", help="actually rename (default: dry-run)")
+    ap.add_argument("--figs-dir", default=str(_ROOT / "figs"), help="path to figs/ directory")
+    args = ap.parse_args()
+
+    figs_dir = Path(args.figs_dir).resolve()
+    if not figs_dir.is_dir():
+        print(f"no such directory: {figs_dir}", file=sys.stderr)
+        return 2
+
+    planned, skipped = [], []
+    for html, new_stem, reason in plan_renames(figs_dir):
+        if new_stem is None:
+            skipped.append((html.name, reason))
+        else:
+            planned.append((html.stem, new_stem))
+
+    print(f"scanning {figs_dir}")
+    print(f"  {len(planned)} to rename, {len(skipped)} skipped\n")
+
+    for old, new in planned:
+        print(f"  rename  {old}  ->  {new}")
+    if skipped:
+        print("\n  skipped:")
+        for name, reason in skipped:
+            print(f"    {name}  ({reason})")
+
+    if not planned:
+        return 0
+
+    if not args.apply:
+        print("\n(dry run — pass --apply to rename)")
+        return 0
+
+    print("\napplying...")
+    for old, new in planned:
+        moved = apply_rename(figs_dir, old, new)
+        for line in moved:
+            print(f"  {line}")
+    print(f"done — renamed {len(planned)} run(s)")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())