diff --git a/internal/check-upg-zone-joins.mjs b/internal/check-upg-zone-joins.mjs
new file mode 100644
index 000000000..62329351f
--- /dev/null
+++ b/internal/check-upg-zone-joins.mjs
@@ -0,0 +1,84 @@
+#!/usr/bin/env node
+/**
+ * check-upg-zone-joins.mjs — the two UPG-100 zone joins, each pinned WITH ITS COST.
+ *
+ * WHY THIS IS A REGISTERED CHECK AND NOT A SCRIPT I RAN ONCE. Both joins below were
+ * verified by hand while they were written, and that verification lived nowhere: not
+ * in a file, not in a gate, only in a terminal. A probe proves something once. The two
+ * defects it guards are both SILENT — neither throws, neither renders an error, and
+ * both return a well-formed answer that is simply smaller than the truth:
+ *
+ *   1. ADMIN NAMES NEED THEIR DIACRITICS FOLDED. admin-to-list.json writes `gujarat`;
+ *      gospel-by-province-groups writes `Gujarāt` (also Karnātaka, Tamil Nādu,
+ *      Uttarākhand, Jammu and Kashmīr, Haryāna). Lowercase alone joins 514 of 1009
+ *      rows across 9 zones; folding joins 1009 across 12. The 50.9% is the dangerous
+ *      number — every surviving zone still looks populated, just smaller.
+ *
+ *   2. THE SELECTOR EVALUATOR WANTS FLAT RECORDS. It reads `r.country_code === want`,
+ *      a plain string, while the source shape is `{ value, label }` before normalize.js
+ *      flattens it. Passing the object form matches nothing and returns an empty Set
+ *      for EVERY zone, which reads as "these zones have no groups".
+ *
+ * EACH ARM IS CHECKED IN BOTH DIRECTIONS. The negative arms are the point: they record
+ * what the fold and the flattening are BUYING, so a later reader asking "is this still
+ * needed?" gets a number instead of a green tick. A check that can only go green when
+ * the guard is present, and never demonstrates the guard's absence, is not evidence.
+ */
+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+import { dirname, join } from 'node:path'
+import evaluate from '../plugin-designs/my-upg-100-list/data/selector-evaluator.js'
+import { zonesByGroup, zonesBySelector, toSelectorRecord } from '../plugin-designs/gospel-internet/src/upgZones.js'
+
+const ROOT = join(dirname(fileURLToPath(import.meta.url)), '..')
+const read = (p) => JSON.parse(readFileSync(join(ROOT, p), 'utf8'))
+const D = 'plugin-designs/my-upg-100-list/data/'
+
+const fail = []
+const ok = (label, got, want) => {
+  if (got === want) console.log(`  ✓ ${label}: ${got}`)
+  else { console.log(`  ✗ ${label}: got ${got}, expected ${want}`); fail.push(label) }
+}
+
+console.log('UPG-ZONE-JOINS')
+
+// ── 1. the province join, with and without the fold ─────────────────────────
+const a2l = read(D + 'admin-to-list.json')
+const prov = read('data/tables/gospel-by-province-groups.json').rows
+const folded = zonesByGroup(a2l, prov)
+ok('province join, folded — rows', folded.size, prov.length)
+ok('province join, folded — zones', new Set(folded.values()).size, 12)
+
+// the negative arm: what lowercase-only would cost, so the fold's value is recorded
+const naive = new Map(Object.entries(a2l).map(([k, v]) => [k.trim().toLowerCase(), v]))
+const naiveHits = prov.filter((r) => naive.get(String(r.province).trim().toLowerCase())).length
+ok('province join, UNFOLDED — rows (the fold buys the difference)', naiveHits, 514)
+if (naiveHits >= prov.length) {
+  console.log('  ✗ the unfolded path no longer loses rows — this check can no longer fail; re-derive it')
+  fail.push('fold arm is inert')
+}
+
+// ── 2. the selector join, flat records vs the object shape ──────────────────
+const clusters = read(D + 'clusters.json')
+const zoneLists = [...(clusters.lists || []), ...(clusters.deaf_lists || []), ...(clusters.asia_lists || [])]
+const nn = read('data/tables/near-neighbors.json')
+const ix = Object.fromEntries(nn.cols.map((c, i) => [c, i]))
+const dec = (r, c) => (nn.dict[c] ? nn.dict[c][r[ix[c]]] : r[ix[c]])
+const records = nn.rows.map((r, i) => toSelectorRecord(
+  { iso3: dec(r, 'iso3'), name: dec(r, 'name'), langName: dec(r, 'langName'), bloc: dec(r, 'bloc') }, `nn-${i}`))
+
+const flat = zonesBySelector(zoneLists, records, evaluate)
+ok('selector join, FLAT — zones with members', [...flat.members.values()].filter((s) => s.size).length, 8)
+ok('selector join — zones the evaluator rejects by design (pg_slugs)', flat.unanswerable.size, 12)
+
+// the negative arm: the object shape must collapse to nothing, loudly recorded
+const objShaped = nn.rows.map((r, i) => ({ slug: `nn-${i}`, country_code: { value: dec(r, 'iso3') } }))
+const obj = zonesBySelector(zoneLists, objShaped, evaluate)
+ok('selector join, OBJECT-shaped — zones with members (must be 0)',
+  [...obj.members.values()].filter((s) => s.size).length, 0)
+
+if (fail.length) {
+  console.log(`\nUPG-ZONE-JOINS: FAIL — ${fail.length} assertion(s): ${fail.join(', ')}`)
+  process.exit(1)
+}
+console.log('\nUPG-ZONE-JOINS: PASS — both joins pinned in both directions')
diff --git a/package.json b/package.json
index b5b22adcb..8436a9d4c 100644
--- a/package.json
+++ b/package.json
@@ -70,6 +70,7 @@
     "check:experimental-shelf": "node internal/check-experimental-shelf.mjs",
     "check:export-poster-prints": "node plugin-designs/doxa-research-map/scripts/check-export-poster-prints.mjs",
     "check:export-poster-prints:maplibre": "node plugin-designs/doxa-research-map/scripts/check-export-poster-prints.mjs --provider maplibre",
+    "check:upg-zone-joins": "node internal/check-upg-zone-joins.mjs",
     "check:facet-cascade": "node internal/check-facet-cascade.mjs",
     "check:facet-cascade:offline": "node internal/check-facet-cascade.mjs --offline",
     "check:facet-cascade:self-test": "node internal/check-facet-cascade.mjs --self-test",
@@ -267,7 +268,7 @@
     "fix:sri": "node internal/fix-sri.mjs",
     "gallery": "node internal/generate-gallery.mjs",
     "gates:post": "node internal/run-parallel.mjs --label=post -- check:capability-derived check:drop check:parse verify check:sri check:public-hygiene check:label-joins check:facet-cascade check:facet-one-field check:heartbeat check:kpi-unit check:composite-reg check:descriptions check:no-abs-paths check:no-codename check:dep-names check:composite-srcs check:embed-targets check:nested-token check:i18n-parity check:tabs-localized check:packets-all check:packet-strings-parity check:zone-census check:zoning-declared check:share-policy check:library-index check:csv-shape check:capability-citations check:catalog-identity check:no-orphan-gates check:quoted-scripts check:done-evidence check:script-flags check:backlog-status check:capability-exemplars check:packet-fixture check:packet-browser check:provider-switch check:locale-reachable check:url-resolver check:manifest-covers check:one-palette check:pin-dim-slider check:legend-source check:backlog-refs check:clustering-modes check:composite-embed-knobs check:composite-locale-leaks check:linked-vocabulary check:wp-embed check:dashboard-geocoder-provider check:leaf-engine-per-provider check:map-props-unit check:composites-mount check:composite-interop check:no-free-identifiers check:map-fly-reduced-motion check:mst-clustering check:list-prop-unit check:profiles-mount check:cdn-pins check:no-eval check:popup-escaping check:i18n-treeshake check:pinned-umd check:one-palette:self-test check:no-cross-plugin-imports check:no-cross-plugin-imports:self-test check:map-attribution check:studio-rail check:approx-pin-note check:embedder-keys check:provider-for check:legend-logical-insets check:legend-logical-insets:self-test check:optional-t-equivalence check:text-highlight check:religion-label-not-value check:i18n-stopped-scope check:no-silent-engine check:regions-uses-library check:shadow-css-parses check:square-map-controls check:preset-config-merge check:frozen-labels",
-    "gates:pre": "node internal/run-parallel.mjs --label=pre -- check:template-literal check:mjs-parse check:sfc-parse check:tab-radius check:synced-css check:doc-scripts check:setup-order check:package-json-unique-keys check:no-repo-pb-hooks check:no-credentials-omit check:no-credentials-omit:self-test check:shadow-reduced-motion check:poster-action-print check:targets check:targets:self-test check:browser-gates-guarded check:pin-selection check:facet-shape check:hardcoded-english:self-test check:list-selection check:selector-fields",
+    "gates:pre": "node internal/run-parallel.mjs --label=pre -- check:template-literal check:mjs-parse check:sfc-parse check:tab-radius check:synced-css check:doc-scripts check:setup-order check:package-json-unique-keys check:no-repo-pb-hooks check:no-credentials-omit check:no-credentials-omit:self-test check:shadow-reduced-motion check:poster-action-print check:targets check:targets:self-test check:browser-gates-guarded check:pin-selection check:facet-shape check:hardcoded-english:self-test check:list-selection check:selector-fields check:upg-zone-joins",
     "gen:iso3-to-alpha2": "node library/scripts/generate-iso3-to-alpha2.mjs",
     "internet:adm1": "node plugin-designs/cities-internet/scripts/build-adm1-internet.mjs",
     "library-index": "node internal/generate-library-index.mjs",