diff --git a/internal/check-upg-zone-joins.mjs b/internal/check-upg-zone-joins.mjs
index 62329351f..e58828b31 100644
--- a/internal/check-upg-zone-joins.mjs
+++ b/internal/check-upg-zone-joins.mjs
@@ -18,6 +18,13 @@
  *      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".
+ *
+   3. THE ZONE AND REGION ID SPACES OVERLAP. The legend emits ONE BARE STRING on one
+ *      channel — a leaf emits its zone id, a region row emits the region's — and of 33 zone
+ *      ids and 7 region ids exactly one name is shared, whose region is itself. Un-namespaced
+ *      that string cannot say which row was clicked. It is harmless today ONLY because that
+ *      region currently holds one zone, which is a fact about the data rather than a property
+ *      of the design.
  *
  * 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
@@ -28,6 +35,8 @@ 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 { buildZoneTree, zonesForSelection, isRegionId, regionOf, humanize, REGION_PREFIX }
+  from '../plugin-designs/gospel-internet/src/zoneTree.js'
 import { zonesByGroup, zonesBySelector, toSelectorRecord } from '../plugin-designs/gospel-internet/src/upgZones.js'
 
 const ROOT = join(dirname(fileURLToPath(import.meta.url)), '..')
@@ -77,8 +86,45 @@ 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)
 
+// ── 3. the zone/region id namespaces ────────────────────────────────────────
+// The fixture gives the colliding region TWO zones ON PURPOSE. Today's data gives it one,
+// so both readings of the bare id return the same set and a check built on the real table
+// would pass without ever exercising the difference — it would be measuring the data, not
+// the code. regions' check watches the shipped tables and fails if the colliding set moves;
+// this pins the behaviour that makes a collision harmless when it arrives.
+const ZR = {
+  europe: 'europe',                 // the colliding name: a zone AND a region
+  'europe-balkans': 'europe',       // the second zone today's data does not yet have
+  'asia-india-gujarat': 'asia',
+  brazil: 'latam',
+}
+// ?. and ?? -1 below are NOT defensive noise. Poisoning the namespace makes `region:asia`
+// stop existing, and an unguarded `.count` THREW — the file died at the third assertion and
+// the seven after it, including the one this section exists for, never ran. The crash still
+// exited 1, so it read like the check catching the bug. A check that dies reports less than
+// one that fails by name: guard the lookups so every arm runs and says what it saw.
+const tree = buildZoneTree(ZR, { counts: { 'asia-india-gujarat': 7 } })
+ok('tree: every region row is namespaced', tree.every((n) => n.id.startsWith(REGION_PREFIX)) ? 1 : 0, 1)
+ok('tree: leaves stay bare', tree.flatMap((n) => n.children).every((c) => !isRegionId(c.id)) ? 1 : 0, 1)
+ok('tree: a region rolls up its children counts', tree.find((n) => n.id === 'region:asia')?.count ?? -1, 7)
+const asLeaf = zonesForSelection('europe', ZR)
+const asRegion = zonesForSelection('region:europe', ZR)
+ok('tree: the bare id means that ZONE only', asLeaf?.length ?? -1, 1)
+ok('tree: the namespaced id expands to the REGION\'s zones', asRegion?.length ?? -1, 2)
+// A SUMMARY ARM, AND THE WEAKEST ONE — kept for what it says, not for what it catches.
+// Under a poisoned namespace the two readings still DIFFER (2 vs 0), both wrong, so this
+// arm stays green while the two above it go red. The specific lengths are the guards; this
+// line only states the property they add up to. Labelled honestly so nobody trusts it alone.
+ok('tree: the two readings of the colliding name are not the same set (summary)',
+  JSON.stringify(asLeaf) !== JSON.stringify(asRegion) ? 1 : 0, 1)
+ok('tree: a bare id never reads as a region', (!isRegionId('europe') && regionOf('europe') === null) ? 1 : 0, 1)
+const sweep = new Set(tree.flatMap((n) => zonesForSelection(n.id, ZR)))
+ok('tree: sweeping every region reaches every FILED zone and no more', sweep.size, Object.keys(ZR).length)
+ok('tree: there is no region-based "all" to misuse', zonesForSelection(null, ZR) === null ? 1 : 0, 1)
+ok('tree: humanize is only a fallback label', humanize('asia-india-gujarat') === 'Asia India Gujarat' ? 1 : 0, 1)
+
 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')
+console.log('\nUPG-ZONE-JOINS: PASS — both joins pinned in both directions, and the id namespaces cannot collapse')