diff --git a/package.json b/package.json
index 1791ad656..289c64a97 100644
--- a/package.json
+++ b/package.json
@@ -50,6 +50,7 @@
     "qa:street-level": "node scripts/qa-street-level.mjs",
     "qa:street-level:fixtures": "node scripts/qa-street-level.mjs --fixtures",
     "check:boundaries": "node scripts/check-import-directions.mjs && node scripts/check-package-boundaries.mjs",
+    "check:main-chunk": "node scripts/check-main-chunk.mjs",
     "qa:transit": "node scripts/qa-transit.mjs",
     "mcp": "node server/mcp/stdio.js"
   },
diff --git a/scripts/check-main-chunk.mjs b/scripts/check-main-chunk.mjs
new file mode 100644
index 000000000..7de3fa55b
--- /dev/null
+++ b/scripts/check-main-chunk.mjs
@@ -0,0 +1,94 @@
+#!/usr/bin/env node
+/**
+ * The build gate for the Cesium-mode bundle: the main chunk (the module
+ * script dist/index.html loads, or the Vite manifest's entry when the build
+ * writes one) and every chunk it imports statically must carry no three or
+ * Navara code; those load only through navaraMode.js's import(). Prints the
+ * sizes (raw and gzip) and fails when a marker is found.
+ *
+ *   npm run build && node scripts/check-main-chunk.mjs [--dist=dist] [--json]
+ */
+import fs from 'node:fs';
+import path from 'node:path';
+import { gzipSync } from 'node:zlib';
+import { fileURLToPath } from 'node:url';
+import {
+  entryFromHtml,
+  entryFromManifest,
+  forbiddenMarkersIn,
+  initialChunks,
+} from '../src/tooling/mainChunk.js';
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const option = (name) =>
+  process.argv.find((arg) => arg.startsWith(`--${name}=`))?.split('=')[1];
+const dist = path.resolve(root, option('dist') ?? 'dist');
+const asJson = process.argv.includes('--json');
+
+function findEntry() {
+  const manifestPath = path.join(dist, '.vite', 'manifest.json');
+  if (fs.existsSync(manifestPath)) {
+    const entry = entryFromManifest(
+      JSON.parse(fs.readFileSync(manifestPath, 'utf8')),
+    );
+    if (entry) return { entry, via: 'manifest' };
+  }
+  const htmlPath = path.join(dist, 'index.html');
+  if (!fs.existsSync(htmlPath))
+    throw new Error(`No build at ${dist}: run npm run build first`);
+  const entry = entryFromHtml(fs.readFileSync(htmlPath, 'utf8'));
+  if (!entry) throw new Error(`${htmlPath} loads no module script`);
+  return { entry, via: 'index.html' };
+}
+
+const read = (file) => {
+  const absolute = path.join(dist, file);
+  return fs.existsSync(absolute) ? fs.readFileSync(absolute, 'utf8') : null;
+};
+
+const { entry, via } = findEntry();
+const chunks = initialChunks(entry, read).map((file) => {
+  const code = read(file);
+  if (code == null)
+    return { file, missing: true, bytes: 0, gzipBytes: 0, markers: [] };
+  return {
+    file,
+    bytes: Buffer.byteLength(code),
+    gzipBytes: gzipSync(code).length,
+    markers: forbiddenMarkersIn(code),
+  };
+});
+const main = chunks[0];
+const failures = chunks.filter(
+  (chunk) => chunk.missing || chunk.markers.length,
+);
+const result = {
+  entry,
+  via,
+  mainBytes: main.bytes,
+  mainGzipBytes: main.gzipBytes,
+  initialBytes: chunks.reduce((sum, chunk) => sum + chunk.bytes, 0),
+  chunks,
+  ok: failures.length === 0,
+};
+
+if (asJson) console.log(JSON.stringify(result, null, 2));
+else {
+  const kb = (bytes) => `${(bytes / 1000).toFixed(2)} kB`;
+  console.log(`[main-chunk] entry ${entry} (via ${via})`);
+  for (const chunk of chunks)
+    console.log(
+      `[main-chunk]   ${chunk.file}: ${kb(chunk.bytes)} (gzip ${kb(chunk.gzipBytes)})` +
+        (chunk.missing ? ' MISSING' : '') +
+        (chunk.markers.length ? ` FORBIDDEN: ${chunk.markers.join(', ')}` : ''),
+    );
+  console.log(
+    `[main-chunk] main ${main.bytes} bytes (gzip ${main.gzipBytes}); initial chunks ${result.initialBytes} bytes`,
+  );
+  console.log(
+    result.ok
+      ? '[main-chunk] ok: no three or Navara in the main chunk'
+      : '[main-chunk] FAIL',
+  );
+}
+process.exitCode = result.ok ? 0 : 1;
diff --git a/scripts/format-scope.json b/scripts/format-scope.json
index 8e0aa8922..46d7469e6 100644
--- a/scripts/format-scope.json
+++ b/scripts/format-scope.json
@@ -1063,5 +1063,8 @@
   "src/services/groundSnap.test.mjs",
   "src/sharelinkPose.test.mjs",
   "src/tooling/navaraAssets.test.mjs",
-  "src/tooling/tileCache.test.mjs"
+  "src/tooling/tileCache.test.mjs",
+  "scripts/check-main-chunk.mjs",
+  "src/tooling/mainChunk.test.mjs",
+  "scripts/qa-w1-baseline.mjs"
 ]
diff --git a/src/tooling/mainChunk.js b/src/tooling/mainChunk.js
new file mode 100644
index 000000000..dd2bfe29b
--- /dev/null
+++ b/src/tooling/mainChunk.js
@@ -0,0 +1,80 @@
+// The main chunk of a Vite build and what it must not carry: the Cesium-mode
+// page loads the entry chunk and every chunk it imports statically; Navara and
+// three belong in the lazy chunk navaraMode.js loads with import(). Pure
+// helpers for scripts/check-main-chunk.mjs.
+
+/**
+ * Markers that only three or Navara code leaves in a bundle (minification
+ * keeps string literals): three's own warnings name their classes, Navara's
+ * package names, wasm modules and attribution card classes. Plain engine
+ * names ('navara' as an engine kind) are not markers.
+ */
+export const FORBIDDEN_MARKERS = Object.freeze([
+  Object.freeze({
+    name: 'three',
+    pattern: /THREE\.(?:WebGLRenderer|GLTFLoader|EXRLoader|Vector\d)/,
+  }),
+  Object.freeze({
+    name: 'navara',
+    pattern: /@navaramap\/|navara_wasm|navara-attr-/,
+  }),
+]);
+
+/** The module script a built index.html loads, relative to dist ('assets/…'), or null. */
+export function entryFromHtml(html) {
+  const scripts = String(html).match(/<script\b[^>]*>/g) ?? [];
+  for (const tag of scripts) {
+    if (!/\btype\s*=\s*["']module["']/.test(tag)) continue;
+    const src = tag.match(/\bsrc\s*=\s*["']([^"']+)["']/);
+    if (src) return src[1].replace(/^\.?\//, '');
+  }
+  return null;
+}
+
+/** The file of the manifest chunk marked isEntry for index.html (or any entry), or null. */
+export function entryFromManifest(manifest) {
+  const chunks = Object.values(manifest ?? {});
+  const entry =
+    chunks.find((chunk) => chunk.isEntry && chunk.src === 'index.html') ??
+    chunks.find((chunk) => chunk.isEntry);
+  return entry?.file ?? null;
+}
+
+/** Sibling chunks a chunk imports statically (`from"./x.js"`, `import"./x.js"`). */
+export function staticImportsOf(code) {
+  const found = [];
+  const pattern = /(?:\bfrom|\bimport)\s*(["'])\.\/([^"']+\.js)\1/g;
+  for (const match of String(code).matchAll(pattern)) {
+    if (!found.includes(match[2])) found.push(match[2]);
+  }
+  return found;
+}
+
+/**
+ * The entry and every chunk reachable from it by static imports, in
+ * discovery order. `read(file)` returns a chunk's code (file relative to
+ * dist, in the entry's directory).
+ */
+export function initialChunks(entry, read) {
+  const directory = entry.includes('/')
+    ? entry.slice(0, entry.lastIndexOf('/') + 1)
+    : '';
+  const seen = [entry];
+  for (let index = 0; index < seen.length; index++) {
+    const code = read(seen[index]);
+    if (code == null) continue;
+    for (const name of staticImportsOf(code)) {
+      const file = directory + name;
+      if (!seen.includes(file)) seen.push(file);
+    }
+  }
+  return seen;
+}
+
+/** The names of FORBIDDEN_MARKERS found in code. */
+export function forbiddenMarkersIn(code) {
+  const text = String(code);
+  return FORBIDDEN_MARKERS.filter(({ pattern }) => pattern.test(text)).map(
+    ({ name }) => name,
+  );
+}
diff --git a/src/tooling/mainChunk.test.mjs b/src/tooling/mainChunk.test.mjs
new file mode 100644
index 000000000..dd1ee2418
--- /dev/null
+++ b/src/tooling/mainChunk.test.mjs
@@ -0,0 +1,86 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  FORBIDDEN_MARKERS,
+  entryFromHtml,
+  entryFromManifest,
+  forbiddenMarkersIn,
+  initialChunks,
+  staticImportsOf,
+} from './mainChunk.js';
+
+test('the entry is the module script the built index.html loads', () => {
+  const html = `<!doctype html><head>
+    <script src="/cesium/Cesium.js"></script>
+    <script type="module" crossorigin src="/assets/index--RGa-KXE.js"></script>
+    <link rel="modulepreload" crossorigin href="/assets/vendor-Ab.js">
+  </head>`;
+  assert.equal(entryFromHtml(html), 'assets/index--RGa-KXE.js');
+});
+
+test('an index.html without a module script has no entry', () => {
+  assert.equal(
+    entryFromHtml('<script src="/cesium/Cesium.js"></script>'),
+    null,
+  );
+});
+
+test('the entry of a Vite manifest is the chunk marked isEntry for index.html', () => {
+  const manifest = {
+    'src/app/navaraMode.js': {
+      file: 'assets/navaraMode-B.js',
+      isDynamicEntry: true,
+    },
+    'index.html': {
+      file: 'assets/index-A.js',
+      isEntry: true,
+      src: 'index.html',
+      imports: ['_vendor-C.js'],
+    },
+    '_vendor-C.js': { file: 'assets/vendor-C.js' },
+  };
+  assert.equal(entryFromManifest(manifest), 'assets/index-A.js');
+});
+
+test('static imports are found, dynamic imports are not', () => {
+  const code =
+    'import{a as b}from"./vendor-C.js";import"./side-D.js";' +
+    'const x=()=>import("./navaraMode-B.js");' +
+    "import { y } from './spaced-E.js';";
+  assert.deepEqual(staticImportsOf(code), [
+    'vendor-C.js',
+    'side-D.js',
+    'spaced-E.js',
+  ]);
+});
+
+test('the initial chunks are the entry and everything it imports statically', () => {
+  const files = {
+    'assets/index-A.js': 'import"./vendor-C.js";import("./lazy-L.js")',
+    'assets/vendor-C.js': 'import{z}from"./deep-D.js"',
+    'assets/deep-D.js': 'import"./vendor-C.js"',
+    'assets/lazy-L.js': 'THREE.WebGLRenderer',
+  };
+  assert.deepEqual(
+    initialChunks('assets/index-A.js', (file) => files[file]),
+    ['assets/index-A.js', 'assets/vendor-C.js', 'assets/deep-D.js'],
+  );
+});
+
+test('three and Navara markers are reported, ordinary engine names are not', () => {
+  assert.deepEqual(
+    forbiddenMarkersIn(
+      'B4=Object.freeze(["cesium","navara"]);FP={cesium:49,navara:80}',
+    ),
+    [],
+  );
+  assert.deepEqual(
+    forbiddenMarkersIn('console.warn("THREE.WebGLRenderer: Context Lost.")'),
+    ['three'],
+  );
+  assert.deepEqual(forbiddenMarkersIn('import "@navaramap/three"'), ['navara']);
+  assert.deepEqual(forbiddenMarkersIn('new URL("navara_wasm_bg.wasm")'), [
+    'navara',
+  ]);
+  assert.ok(FORBIDDEN_MARKERS.length >= 2);
+});