diff --git a/SPEC.md b/SPEC.md
index 7d145aa..d86e0a8 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -5160,8 +5160,14 @@ never used for routing or lookup. New entries are never assigned one.
 > `POST …/rfcs/{slug}/meta` editor (contributor+, validated at the write
 > boundary), the schema-driven detail panel, all entry **write paths made
 > sidecar-aware** (a migrated body-only `.md` never re-grows frontmatter), and
-> the Owner-gated `POST …/collections/{cid}/migrate` endpoint. Bulk edit
-> (SLICE-5) follows. See §9.5 for the edit-metadata write-through this reuses.
+> the Owner-gated `POST …/collections/{cid}/migrate` endpoint. **Bulk
+> tag/untag (SLICE-5, v0.51.0)** completes the design: the
+> `POST …/collections/{cid}/meta/bulk` endpoint (`{slugs, op: set|add|remove,
+> field, value}`) applies one field change to many entries' sidecars in a
+> **single commit** (D7), validated per entry at the write boundary with
+> partial-rejection reporting (`{applied, rejected}`), plus the catalog's
+> row multi-select + sticky bulk action bar. See §9.5 for the edit-metadata
+> write-through this reuses.
 
 Every collection declares a `type` in its `.collection.yaml` manifest
 (§22.2), chosen at creation and **immutable**: one of `document`,