diff --git a/internal/generate-tree.mjs b/internal/generate-tree.mjs
index 543d1153c..3ed471f7f 100644
--- a/internal/generate-tree.mjs
+++ b/internal/generate-tree.mjs
@@ -161,6 +161,19 @@ const DEFINE_RE = /customElements\.define\(\s*['"]([^'"]+)['"]/g  // registered
 // manifest lists exactly what the dev manifest lists (a blank template-map-a/b in the built studio was
 // the only difference, found 2026-09-25). Not gatedProfiles: that list is governance (private data).
 const SCAFFOLD_BUNDLES = ['template-bundle']
+// ★ THE MAPBOX ENGINE URL, read from the ONE place that loads it (useMapInstance.js ENGINES.mapbox.js),
+// so the embed snippet's preload can never point at a different version than the one the bundle
+// requests — a mismatched preload is not used, and the engine would download twice.
+const MAPBOX_GL_JS = (() => {
+  try {
+    const src = readFileSync(resolve(__dirname, '..', 'library', 'composables', 'useMapInstance.js'), 'utf8')
+    const m = src.match(/mapbox:\s*\{[^}]*?js:\s*'([^']+mapbox-gl\.js)'/)
+    if (m) return m[1]
+  } catch { /* fall through */ }
+  console.warn('[generate-tree] could not read the Mapbox engine URL from useMapInstance.js; embed snippets get no preload')
+  return ''
+})()
+
 const isBundleDir = (name) => !name.startsWith('_') && !name.startsWith('.') && !DEST_CONFIG.gatedProfiles.includes(name) && !SCAFFOLD_BUNDLES.includes(name)
 
 // ★ CUSTOM-ELEMENT NAME RULE (HTML spec): lowercase, must contain a hyphen, no spaces.
@@ -1204,7 +1217,20 @@ function scanBundles() {
         isMap,
         iframeUrl,
         scriptUrl,
-        embed: { iframe: iframeEmbed, script: scriptEmbed },
+        // ★ ENGINE PRELOAD, map snippets only (measured 2026-09-25, cold, 10 Mbps). A partner page that
+        // pastes this snippet has no leaf page to fetch mapbox-gl.js, so the bundle requested it only
+        // after it had downloaded, run and mounted: doxa-simple-map's engine started at 1.06 s, right
+        // after its bundle (0.99 s); regions-mfe's at 2.67 s. A preload lets the ~480 KB engine download
+        // alongside the bundle. NO crossorigin attribute, on purpose: ensureEngineScript injects a plain
+        // <script src>, and a preload is only reused when its request mode matches — with crossorigin it
+        // would download twice. Mapbox only: a non-map profile gets none, and a partner switching to
+        // provider:"maplibre" is told to drop the line, since that engine must make zero Mapbox requests.
+        embed: {
+          iframe: iframeEmbed,
+          script: isMap && MAPBOX_GL_JS
+            ? `<!-- Mapbox engine, fetched early; remove this line if you set "provider":"maplibre" -->\n<link rel="preload" as="script" href="${MAPBOX_GL_JS}">\n` + scriptEmbed
+            : scriptEmbed,
+        },
         ...(instances.length ? { instances } : {}),
         // Extra profile-config keys from <profile>.meta.json `config` (e.g. pinHint:false).
         // GENERATOR-INTERNAL: stripped before the manifest is written (see writeManifestAndHtml)
diff --git a/internal/measure-map-load.mjs b/internal/measure-map-load.mjs
index dce13dce3..f11adba6d 100644
--- a/internal/measure-map-load.mjs
+++ b/internal/measure-map-load.mjs
@@ -87,7 +87,10 @@ for (const ref of ONLY) {
     const first = (k) => all.filter((x) => x.kind === k).sort((a, c) => a.start - c.start)[0]
     const bundleReq = all.find((x) => x.kind === 'bundle' && x.url.includes(`/${bundle}/${bundle}.js`)) || first('bundle')
     const eng = first('engine'), sty = first('style'), dat = first('data')
+    // A preload the browser cannot reuse (request mode mismatch) shows up as a SECOND engine request.
+    const engineRequests = all.filter((x) => x.kind === 'engine').length
     runs.push({
+      engineRequests,
       loaded, styled, bundleEnd: bundleReq && bundleReq.end, engineStart: eng && eng.start, engineEnd: eng && eng.end,
       styleStart: sty && sty.start, dataStart: dat && dat.start,
       bytes: { bundle: 0 },
@@ -95,7 +98,7 @@ for (const ref of ONLY) {
     await ctx.close()
   }
   const med = (k) => { const v = runs.map((x) => x[k]).filter((x) => x != null).sort((a, c) => a - c); return v.length ? v[Math.floor(v.length / 2)] : null }
-  console.log('  measured ' + ref + ': ' + JSON.stringify(runs.map((x) => x.loaded)))
+  console.log('  measured ' + ref + ': ' + JSON.stringify(runs.map((x) => x.loaded)) + ' · engine requests per run ' + JSON.stringify(runs.map((x) => x.engineRequests)))
   rows.push({ ref, styled: med('styled'), loaded: med('loaded'), bundleEnd: med('bundleEnd'), engineStart: med('engineStart'), engineEnd: med('engineEnd'), styleStart: med('styleStart'), dataStart: med('dataStart'), runs: runs.map((x) => x.loaded) })
 }
 await browser.close(); srv.kill(); host.close()