diff --git a/internal/check-poster-raster.mjs b/internal/check-poster-raster.mjs
new file mode 100644
index 000000000..f588d80cb
--- /dev/null
+++ b/internal/check-poster-raster.mjs
@@ -0,0 +1,86 @@
+// check-poster-raster — a poster with NO pre-rendered image must still download.
+//
+// ★ THE POINT: 378 posters ship as .webp — 41 MB in every clone, delivered to every
+// reader whether or not they open one, and a publish was ALREADY rejected once at a
+// 74 MB asset tree. Driver: "it would be a big win to not have to have 80mb of
+// images in a public folder and let the posters get generated by the website."
+//
+// The fallback rasterises posters/<id>.html in the browser. This gate proves the
+// generated image is REAL, because the failure mode here is silent and total:
+//
+// ⚠ AN SVG LOADED THROUGH <img> IS AN ISOLATED DOCUMENT. It cannot see the host
+// page's CSS, so every var() falls back to its initial value — and `fill`'s initial
+// value is BLACK. A poster that lost its stylesheet downloads as a black rectangle
+// with no error, no exception and a perfectly normal file size. So this gate
+// MEASURES PIXELS: mean luminance and colour variety, not "did it produce bytes".
+//
+// Usage: node internal/check-poster-raster.mjs [zoneId]
+//   PACKET_ORIGIN (default http://127.0.0.1:4898)
+import { chromium } from 'playwright'
+
+const ORIGIN = process.env.PACKET_ORIGIN || 'http://127.0.0.1:4898'
+const ZONE = process.argv[2] || 'laos'
+const HTML = `${ORIGIN}/library/scripts/posters/out/posters/${ZONE}.html`
+
+const b = await chromium.launch()
+const pg = await b.newPage({ viewport: { width: 1200, height: 900 } })
+const errs = []; pg.on('pageerror', e => errs.push(String(e).slice(0, 140)))
+await pg.goto(ORIGIN + '/')
+
+const r = await pg.evaluate(async ({ ORIGIN, HTML }) => {
+  const { rasterPoster, looksRendered, inlineSvg } =
+    await import(`${ORIGIN}/library/packet/poster-raster.js`)
+  const res = await fetch(HTML)
+  if (!res.ok) return { error: `poster HTML ${res.status}` }
+  const html = await res.text()
+
+  // The stylesheet must actually travel with the svg.
+  const svg = inlineSvg(html)
+  const carriesCss = /<style>/.test(svg)
+
+  const blob = await rasterPoster(html, { scale: 2 })
+  const ok = await looksRendered(blob)
+
+  // Independent pixel read: sample a grid and count distinct coarse colours. A
+  // black rectangle has ~1; a real poster has many.
+  const bmp = await createImageBitmap(blob)
+  const c = document.createElement('canvas')
+  c.width = 200; c.height = 260
+  const x = c.getContext('2d')
+  x.drawImage(bmp, 0, 0, 200, 260)
+  const d = x.getImageData(0, 0, 200, 260).data
+  const seen = new Set()
+  let sum = 0
+  for (let i = 0; i < d.length; i += 4) {
+    sum += (d[i] + d[i + 1] + d[i + 2]) / 3
+    seen.add(`${d[i] >> 5},${d[i + 1] >> 5},${d[i + 2] >> 5}`)
+  }
+  return { bytes: blob.size, w: bmp.width, h: bmp.height, ok, carriesCss,
+           meanLuma: Math.round(sum / (d.length / 4)), colours: seen.size }
+}, { ORIGIN, HTML })
+await b.close()
+
+if (r.error) { console.error(`POSTER-RASTER: FAIL — ${r.error}`); process.exit(1) }
+
+const fails = []
+if (!r.carriesCss) fails.push('the inlined SVG carries no <style> — every fill will render BLACK')
+if (!r.bytes) fails.push('no image produced')
+if (r.w < 1000 || r.h < 1000) fails.push(`rasterised at ${r.w}x${r.h} — too small to print`)
+if (r.meanLuma < 60) fails.push(`mean luminance ${r.meanLuma} — the poster rendered dark/black`)
+if (r.colours < 12) fails.push(`only ${r.colours} distinct colours — this is not a painted poster`)
+if (!r.ok) fails.push('looksRendered() rejected the output')
+if (errs.length) fails.push(`${errs.length} page error(s): ${errs[0]}`)
+
+console.log(`  zone            : ${ZONE}`)
+console.log(`  rasterised      : ${r.w} x ${r.h}`)
+console.log(`  image bytes     : ${(r.bytes / 1024).toFixed(0)} KB`)
+console.log(`  stylesheet inline: ${r.carriesCss}`)
+console.log(`  mean luminance  : ${r.meanLuma}   (near 0 = black = CSS lost)`)
+console.log(`  distinct colours: ${r.colours}`)
+
+if (fails.length) {
+  console.error(`\nPOSTER-RASTER: FAIL — ${fails.length} problem(s)`)
+  for (const f of fails) console.error('  - ' + f)
+  process.exit(1)
+}
+console.log(`\nPOSTER-RASTER: PASS — ${ZONE} generates a real ${r.w}x${r.h} poster with no pre-rendered image.`)
diff --git a/package.json b/package.json
index 640731f35..492bf8414 100644
--- a/package.json
+++ b/package.json
@@ -56,6 +56,7 @@
     "check:heartbeat:self-test": "node internal/check-heartbeat-threshold.mjs --self-test   # THREE regression cases, each required to fire ITS OWN assertion: [literal] resolver replaced by the old hard-coded array -> B; [shipped-default] dashboard.config.js heartbeatThreshold changed -> A2; [shipped-mode] heartbeatMode flipped to quantile -> A2. Poisons IN MEMORY, never the file. Testing that the FEATURE works is not testing that the REGRESSION is caught — this gate shipped with only the first and missed the second.",
     "build:unlocked": "bun run gates:pre && bun run library-index && bun run check:library-index && bun run clean && bun run token && bun run build:bundles && bun run tree && bun run gates:post && bun run gallery",
     "check:card-deck": "node internal/check-card-deck.mjs",
+    "check:poster-raster": "node internal/check-poster-raster.mjs",
     "check:packet-browser": "node internal/check-packet-browser.mjs",
     "check:packet-render": "node internal/check-packet-render.mjs",
     "check:template-literal": "node internal/template-literal-check.mjs",