diff --git a/backend/app/cache.py b/backend/app/cache.py
index 3186005..538689a 100644
--- a/backend/app/cache.py
+++ b/backend/app/cache.py
@@ -170,6 +170,10 @@ def _upsert_cached_rfc(
     # §6.7: funder_login mirrors the optional `funder:` frontmatter
     # field. NULL means absent — operator credentials are used.
     funder_login = entry.funder or None
+    # §22.4a SLICE-3: persist the full per-entry metadata mapping (known keys +
+    # extra, never the body) so facet/filter can read any declared field. Stored
+    # via metadata_dict so the sidecar's forward-compat keys (INV-7) ride along.
+    meta_json = json.dumps(metadata_mod.metadata_dict(entry))
     db.conn().execute(
         """
         INSERT INTO cached_rfcs
@@ -177,8 +181,8 @@ def _upsert_cached_rfc(
              graduated_at, graduated_by, owners_json, arbiters_json, tags_json,
              models_json, funder_login, body, body_sha,
              unreviewed, reviewed_at, reviewed_by, collection_id,
-             metadata_malformed, last_entry_commit_at, updated_at)
-        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, datetime('now'), datetime('now'))
+             metadata_malformed, meta_json, last_entry_commit_at, updated_at)
+        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, datetime('now'), datetime('now'))
         ON CONFLICT(collection_id, slug) DO UPDATE SET
             title = excluded.title,
             state = excluded.state,
@@ -199,6 +203,7 @@ def _upsert_cached_rfc(
             reviewed_at = excluded.reviewed_at,
             reviewed_by = excluded.reviewed_by,
             metadata_malformed = excluded.metadata_malformed,
+            meta_json = excluded.meta_json,
             last_entry_commit_at = datetime('now'),
             updated_at = datetime('now')
         """,
@@ -224,6 +229,7 @@ def _upsert_cached_rfc(
             entry.reviewed_by,
             collection_id,
             1 if metadata_malformed else 0,
+            meta_json,
         ),
     )