diff --git a/internal/check-facet-shape.mjs b/internal/check-facet-shape.mjs
index 8df2d2999..ed3aa2b1f 100644
--- a/internal/check-facet-shape.mjs
+++ b/internal/check-facet-shape.mjs
@@ -6,6 +6,7 @@
  * EXIT 0 pass · 1 fail
  */
 import { WALK } from './lib/facet-walk.mjs'
+import { readSourceFeatures } from '../library/maps/pin-selection.js'
 
 // ── SHAPE TEST (headless, no browser) ────────────────────────────────────────────────────────────
 // ★ NEGATIVE-TESTS __srcFeatures AGAINST BOTH ENGINE SHAPES, in sr's check-pin-selection style.
@@ -25,8 +26,14 @@ const cases = [
   // drew nothing", which is the finding this gate exists to report.
   ['remote  (_data = { url }) -> null, not 0',   () => srcFeatures(mapWith({ _data: { url: 'https://x.invalid/a.json' } }), 'rd-bubbles'), null],
   ['absent source -> null',                      () => srcFeatures(mapWith(null), 'rd-bubbles'), null],
+  // ★ maplibre's THIRD shape. updateData() stores _data = { updateable } (maplibre-gl :49001). Nothing
+  // calls updateData today, so this fixture is here to make ADOPTING it trip a test instead of blanking
+  // the map: neither reader understands the shape, and the one thing both must not do is answer 0.
+  ['maplibre updateData (_data = { updateable }) -> null, not 0',
+    () => srcFeatures(mapWith({ _data: { updateable: new Map() } }), 'rd-bubbles'), null],
 ]
 let ok = 0
+let total = cases.length
 for (const [name, run, want] of cases) {
   const got = run()
   const n = got === null || got === undefined ? null : got.length
@@ -34,7 +41,68 @@ for (const [name, run, want] of cases) {
   console.log(`  ${pass ? '✓' : '✗'} ${name}  -> ${n === null ? 'null' : n + ' feature(s)'}`)
   if (pass) ok++
 }
-console.log(ok === cases.length
-  ? `FACET-SHAPE-TEST: PASS — ${ok}/${cases.length}, both engine shapes measured`
-  : `FACET-SHAPE-TEST: FAIL — ${ok}/${cases.length}`)
-process.exit(ok === cases.length ? 0 : 1)
+// ── THE TWO READERS MUST NOT DRIFT ───────────────────────────────────────────────────────────────
+// This gate's helper (__srcFeatures, injected into the page) and the PRODUCT's reader
+// (library/maps/pin-selection.js readSourceFeatures, used by the packet and zoomToLegendRow) answer the
+// same question about the same private field, and they are two separate copies because one must be a
+// string evaluated in a browser and the other an ES module. Nothing but this block holds them together.
+//
+// ★ ONE DIFFERENCE IS DELIBERATE AND IS ASSERTED AS SUCH, not waived. On a url-backed source the gate
+// returns null ("cannot measure") while the product returns [] and warns. Both are right for their
+// caller: a gate whose whole job is to distinguish "the map drew nothing" from "I could not look" must
+// have a third answer, while a selection returning null would break the Set arithmetic every caller
+// does on it. Pinning the fork here means the next person to touch either reader is told that it is a
+// decision, and any OTHER disagreement is drift and fails.
+const agree = [
+  ['mapbox   (_data = FeatureCollection)', { _data: FC }, 2],
+  ['maplibre (_data = { geojson })',       { _data: { geojson: FC } }, 2],
+  ['serialize() fallback',                 { serialize: () => ({ data: FC }) }, 2],
+  ['single Feature, not a collection',     { _data: { type: 'Feature', properties: { slug: 'a' } } }, 1],
+  ['unrecognised shape ({ updateable })',  { _data: { updateable: new Map() } }, 0],
+]
+for (const [name, src, wantCount] of agree) {
+  const a = srcFeatures(mapWith(src), 'rd-bubbles')
+  const b = readSourceFeatures(mapWith(src), 'rd-bubbles')
+  const na = a === null || a === undefined ? 0 : a.length
+  const nb = b === null || b === undefined ? 0 : b.length
+  const pass = na === nb && na === wantCount
+  console.log(`  ${pass ? '✓' : '✗'} both readers agree: ${name}  -> gate ${na}, product ${nb}`)
+  if (pass) ok++
+  total++
+}
+{
+  // The documented fork, asserted in BOTH directions so neither side can quietly adopt the other's.
+  const src = { _data: { url: 'https://x.invalid/a.json' } }
+  const a = srcFeatures(mapWith(src), 'rd-bubbles')
+  const b = readSourceFeatures(mapWith(src), 'rd-bubbles')
+  const pass = a === null && Array.isArray(b) && b.length === 0
+  console.log(`  ${pass ? '✓' : '✗'} documented fork on a url source: gate null ("cannot measure"), product [] + warn`)
+  if (pass) ok++
+  total++
+}
+// ── AND NEITHER MAY BE SILENT ABOUT A SHAPE IT DOES NOT KNOW ────────────────────────────────────
+// "Returns 0" and "returns 0 while saying it did not understand the payload" are different products.
+// Only the second lets someone adopting updateData() find out from a console instead of from a map.
+for (const [label, run] of [
+  ['gate helper', () => srcFeatures(mapWith({ _data: { updateable: new Map() } }), 'rd-bubbles')],
+  // A map whose getSource answers to ANY id: mapWith() only knows 'rd-bubbles', so asking it for a
+  // fresh id returns no source at all and the reader exits before it can warn — the assertion would
+  // have passed for the wrong reason on a reader that never warns. The fresh id is still needed,
+  // because the warning is once-per-source by design.
+  ['product reader', () => readSourceFeatures(
+    { getSource: () => ({ _data: { updateable: new Map() } }) }, 'a-source-not-yet-warned-about')],
+]) {
+  const said = []
+  const real = console.warn
+  console.warn = (...a) => said.push(a.join(' '))
+  try { run() } finally { console.warn = real }
+  const pass = said.some((m) => /updateable/.test(m))
+  console.log(`  ${pass ? '✓' : '✗'} ${label} names the unrecognised shape rather than failing silently`)
+  if (pass) ok++
+  total++
+}
+
+console.log(ok === total
+  ? `FACET-SHAPE-TEST: PASS — ${ok}/${total}, both engine shapes measured, the gate's reader and the product's reader agree everywhere except their one documented fork, and neither is silent about a shape it does not know`
+  : `FACET-SHAPE-TEST: FAIL — ${ok}/${total}`)
+process.exit(ok === total ? 0 : 1)
diff --git a/internal/lib/facet-walk.mjs b/internal/lib/facet-walk.mjs
index 4e6cdac03..899c92db3 100644
--- a/internal/lib/facet-walk.mjs
+++ b/internal/lib/facet-walk.mjs
@@ -57,7 +57,22 @@ function __srcFeatures(m, id) {
     // catch. null means "could not measure", and the caller must not treat it as zero.
     else if (data.url !== undefined) return null;
   }
-  return (data && data.features) || null;
+  if (data && data.features) return data.features;
+  // A SOURCE MAY HOLD ONE FEATURE, NOT A COLLECTION — both engines accept it, and the product's reader
+  // already did. This helper did not, so it answered "cannot measure" for a payload it could read
+  // perfectly well. Found by the cross-check in check-facet-shape, not by anyone reading either file.
+  if (data && data.type === 'Feature') return [data];
+  // ★ AN UNRECOGNISED SHAPE MUST SAY SO BY NAME, not read as "the map drew nothing". maplibre has a
+  // THIRD _data shape this reader does not handle: updateData() stores { updateable } (maplibre-gl
+  // :49001), and nothing in the product calls updateData today — so the day someone adopts partial
+  // updates for performance, this would have gone quietly back to reporting an empty map about a map
+  // full of dots, which is the exact defect this gate exists to report. Naming the keys makes the
+  // next reader's first question ("what shape IS it?") answered by the message itself.
+  try {
+    console.warn('[facet-walk] source "' + id + '": unrecognised _data shape {'
+      + Object.keys(data || {}).join(',') + '} — cannot measure, NOT zero features');
+  } catch (e) {}
+  return null;
 }
 function __live() {
   const m = window.__rdashMap;
diff --git a/library/maps/pin-selection.js b/library/maps/pin-selection.js
index bbc853514..19ba453b0 100644
--- a/library/maps/pin-selection.js
+++ b/library/maps/pin-selection.js
@@ -163,6 +163,18 @@ export function readSourceFeatures(map, sourceId) {
     if (!data) return [];
     if (Array.isArray(data?.features)) return data.features;
     if (data?.type === 'Feature') return [data];
+    // ★ THE SAME RULE AS THE url BRANCH, FOR THE SHAPES WE HAVE NOT MET YET. Falling off the end used
+    // to return [] in silence, which is "the map drew nothing" stated as fact about a payload nobody
+    // recognised — the precise failure the url warning above exists to prevent, left in place one line
+    // below it. maplibre already HAS a third shape: updateData() stores _data = { updateable }
+    // (maplibre-gl :49001). No product code calls updateData today, so this is a landmine rather than
+    // a bug, and it is armed the moment someone adopts partial updates. Warned once per source, with
+    // the keys named, so the message identifies the shape instead of merely reporting a miss.
+    if (data && typeof data === 'object' && !_warnedUrlSources.has(sourceId)) {
+        _warnedUrlSources.add(sourceId);
+        // eslint-disable-next-line no-console
+        console.warn(`[pin-selection] source "${sourceId}": unrecognised data shape {${Object.keys(data).join(',')}} — nothing can be selected from it; this is NOT a confirmed empty source.`);
+    }
     return [];
 }