diff --git a/plugin-designs/research-map-plus-grid/scripts/check-linked-vocabulary.mjs b/plugin-designs/research-map-plus-grid/scripts/check-linked-vocabulary.mjs
index 1d6143c42..a39c2ad4d 100644
--- a/plugin-designs/research-map-plus-grid/scripts/check-linked-vocabulary.mjs
+++ b/plugin-designs/research-map-plus-grid/scripts/check-linked-vocabulary.mjs
@@ -82,9 +82,18 @@ await requireThisTree(TREE, ROOT, TAG)
 const browser = await chromium.launch({ args: ['--enable-unsafe-swiftshader', '--use-gl=angle', '--use-angle=swiftshader'] })
 const fails = []
 const lines = []
+// ★ THE PROVIDER MATRIX. The same assertions, run once per engine, because interoperability that
+// holds on one provider and not the other is not interoperability
+// (BUILD-INTEROP-CONTRACT-GRID-UNDER-MAP-BOTH-PROVIDERS). It is not a formality: the map half of
+// this link resolves a religion family through its own legend, and the legend is drawn by the map
+// engine — a MapLibre-only paint or layout mistake drops a whole layer silently, which would show
+// up here as a link that delivers nothing while the Mapbox run stays green.
+const PROVIDERS = ['mapbox', 'maplibre']
+const results = {}
 try {
+  for (const engine of PROVIDERS) {
   const page = await browser.newPage({ viewport: { width: 1440, height: 1000 } })
-  await page.goto(LEAF, { waitUntil: 'domcontentloaded', timeout: 60000 })
+  await page.goto(`${LEAF}?provider=${engine}`, { waitUntil: 'domcontentloaded', timeout: 60000 })
 
   // Everything below runs in ONE evaluate: the totals have to be read from the same live page, and
   // a `groups-filtered` listener installed from Node would miss events fired between round trips.
@@ -186,19 +195,38 @@ try {
     return { unfiltered, family, letter, direct, linked, mutant, mapVocab, gridVocab: gridVocab.slice(0, 12) }
   })
 
-  if (out.skip) { await browser.close(); cleanup(); skip(out.skip) }
-  if (out.fail) fails.push(out.fail)
-  else {
-    lines.push(`  · vocabulary — the map names ${out.mapVocab.length} religion families, the grid names ${out.gridVocab.length}+; '${out.family}' is shared`)
-    lines.push(`  · direct     — religion-family="${out.family}" set on the grid itself: ${out.direct} of ${out.unfiltered}`)
-    lines.push(`  · linked     — the same family over the composite's wire: ${out.linked}`)
-    lines.push(`  · mutant     — the legend KEY '${out.letter}' over the same wire: ${out.mutant}`)
+  await page.close()
+  if (out.skip) { await browser.close(); cleanup(); skip(`${out.skip} (under ${engine})`) }
+  results[engine] = out
+  if (out.fail) { fails.push(`[${engine}] ${out.fail}`); continue }
 
-    if (out.mutant === out.direct) {
-      fails.push(`NO TEETH — sending the legend key '${out.letter}' narrowed the grid to the same ${out.direct} rows as the family name '${out.family}'. This gate cannot tell a right mapping from a wrong one, so its pass would mean nothing.`)
+  lines.push(`  ── ${engine}`)
+  lines.push(`     vocabulary — the map names ${out.mapVocab.length} religion families, the grid names ${out.gridVocab.length}+; '${out.family}' is shared`)
+  lines.push(`     direct     — religion-family="${out.family}" set on the grid itself: ${out.direct} of ${out.unfiltered}`)
+  lines.push(`     linked     — the same family over the composite's wire: ${out.linked}`)
+  lines.push(`     mutant     — the legend KEY '${out.letter}' over the same wire: ${out.mutant}`)
+
+  if (out.mutant === out.direct) {
+    fails.push(`[${engine}] NO TEETH — sending the legend key '${out.letter}' narrowed the grid to the same ${out.direct} rows as the family name '${out.family}'. This gate cannot tell a right mapping from a wrong one, so its pass would mean nothing.`)
+  }
+  if (out.linked !== out.direct) {
+    fails.push(`[${engine}] the link does not deliver the facet — '${out.family}' set on the grid gives ${out.direct} rows, the same family over the composite's wire gives ${out.linked}.`)
+  }
+  }
+
+  // ★ AND THE TWO ENGINES MUST AGREE. Each run above only proves the link works within one engine;
+  // identical COUNTS is what proves the contract is the same product on both. A per-engine pass with
+  // different numbers would mean the two builds narrow to different peoples for the same choice.
+  const [a, b] = PROVIDERS
+  if (results[a] && results[b] && !results[a].fail && !results[b].fail) {
+    if (results[a].unfiltered !== results[b].unfiltered) {
+      fails.push(`the two engines start from different data — ${a} lists ${results[a].unfiltered} groups, ${b} lists ${results[b].unfiltered}.`)
     }
-    if (out.linked !== out.direct) {
-      fails.push(`the link does not deliver the facet — '${out.family}' set on the grid gives ${out.direct} rows, the same family over the composite's wire gives ${out.linked}.`)
+    if (results[a].family === results[b].family && results[a].direct !== results[b].direct) {
+      fails.push(`'${results[a].family}' narrows to ${results[a].direct} rows under ${a} and ${results[b].direct} under ${b} — the same choice gives different peoples on the two engines.`)
+    }
+    if (results[a].linked !== results[b].linked) {
+      fails.push(`the link delivers ${results[a].linked} rows under ${a} and ${results[b].linked} under ${b} — interoperability that holds on one provider and not the other is not interoperability.`)
     }
   }
 } catch (err) {
@@ -217,4 +245,4 @@ if (fails.length) {
   console.log('  NAME per row. Both halves must speak the NAME on the wire.')
   process.exit(1)
 }
-console.log(`${TAG}: PASS — the composite's religion link delivers exactly the facet the grid delivers on its own, and a wrong mapping does not.`)
+console.log(`${TAG}: PASS — on BOTH providers the composite's religion link delivers exactly the facet the grid delivers on its own, a wrong mapping does not, and the two engines agree to the row.`)