diff --git a/packages/react-icons-font-subsetting-webpack-plugin/test/make-configs.js b/packages/react-icons-font-subsetting-webpack-plugin/test/make-configs.js
index def7b1de4e..3b966888fd 100644
--- a/packages/react-icons-font-subsetting-webpack-plugin/test/make-configs.js
+++ b/packages/react-icons-font-subsetting-webpack-plugin/test/make-configs.js
@@ -5,7 +5,8 @@
  * The same entries, thresholds and assertions run against both webpack and rspack; only the
  * CSS-extraction and HTML plugins differ, so they are injected by the bundler-specific configs.
  */
-const { resolve } = require('path');
+const { resolve, join } = require('path');
+const { readFileSync } = require('fs');
 
 const { default: FluentUIReactIconsFontSubsettingPlugin } = require('../lib/');
 
@@ -29,6 +30,22 @@ const entries = {
     useAtomicLoader: true,
     assertNoGriffel: true,
   },
+  // Async chunks: one icon is eager, the other reachable only through `import()`, and they live in
+  // different font families. The size ceiling cannot police this on its own — losing the async
+  // glyph makes the font *smaller* — so glyph counts are asserted too (`.notdef` is always glyph 0).
+  lazyAtoms: {
+    src: './src/lazy-atoms.js',
+    threshold: 2 * 1_024, // 2 KB
+    fontGlyphCounts: { 'FluentSystemIcons-Resizable': 2, 'FluentSystemIcons-Filled': 2 },
+  },
+  // The harder variant: both icons are sized+Filled, so a *single* emitted font must carry glyphs
+  // contributed by two different chunks. Fonts are subset per family across the whole build, not
+  // per chunk, so the eager half must not subset the async half's glyph away.
+  lazySharedFontFamily: {
+    src: './src/lazy-shared-family.js',
+    threshold: 2 * 1_024, // 2 KB
+    fontGlyphCounts: { 'FluentSystemIcons-Filled': 3 },
+  },
 };
 
 /**
@@ -138,7 +155,7 @@ function createConfig(name, entry, adapter, isDevServer) {
  * Fails the build when a font asset was not subset, or when a headless entry leaked Griffel.
  *
  * @param {string} name
- * @param {{ threshold: number, assertNoGriffel?: boolean }} entry
+ * @param {{ threshold: number, assertNoGriffel?: boolean, fontGlyphCounts?: Record<string, number> }} entry
  * @param {BundlerAdapter} adapter
  */
 function createAssertionPlugin(name, entry, adapter) {
@@ -160,6 +177,26 @@ function createAssertionPlugin(name, entry, adapter) {
           }
         }
 
+        for (const [fontBaseName, expectedGlyphs] of Object.entries(entry.fontGlyphCounts ?? {})) {
+          // Only .ttf is inspected; .woff/.woff2 wrap the same glyphs in a compressed container.
+          const asset = fontAssets.find(({ name: assetName }) =>
+            new RegExp(`^${fontBaseName}[.-][^/]*\\.ttf$`).test(assetName),
+          );
+
+          if (!asset) {
+            throw new Error(`[${adapter.name}/${name}] No emitted .ttf asset for "${fontBaseName}".`);
+          }
+
+          // `afterEmit` downgrades sources to size-only, so the bytes come back off disk.
+          const glyphCount = readGlyphCount(readFileSync(join(compiler.outputPath, asset.name)));
+          if (glyphCount < expectedGlyphs) {
+            throw new Error(
+              `[${adapter.name}/${name}] Asset "${asset.name}" has ${glyphCount} glyphs, expected at least ` +
+                `${expectedGlyphs} (including .notdef) — an icon that should have been kept was subset away.`,
+            );
+          }
+        }
+
         // Headless builds must not pull in Griffel.
         if (entry.assertNoGriffel) {
           for (const m of compilation.modules) {
@@ -177,4 +214,27 @@ function createAssertionPlugin(name, entry, adapter) {
   };
 }
 
+/**
+ * Reads `numGlyphs` out of a TrueType font's `maxp` table.
+ *
+ * Byte sizes make a poor correctness signal here: a font that wrongly dropped a glyph is *smaller*,
+ * so it slips under any ceiling. The glyph count says outright whether an icon survived.
+ *
+ * @param {Buffer} ttf
+ * @returns {number}
+ */
+function readGlyphCount(ttf) {
+  const tableCount = ttf.readUInt16BE(4);
+
+  for (let i = 0; i < tableCount; i++) {
+    // Table directory: 12-byte header, then 16 bytes per record (tag, checksum, offset, length).
+    const record = 12 + i * 16;
+    if (ttf.toString('ascii', record, record + 4) === 'maxp') {
+      return ttf.readUInt16BE(ttf.readUInt32BE(record + 8) + 4);
+    }
+  }
+
+  throw new Error('Font has no `maxp` table — not a TrueType font?');
+}
+
 module.exports = { makeConfigs, entries };
diff --git a/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.async.js b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.async.js
new file mode 100644
index 0000000000..41f1a3589e
--- /dev/null
+++ b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.async.js
@@ -0,0 +1,5 @@
+// @ts-check
+// Reached only via `import()`, so its icon exists solely in an async chunk.
+import { XboxConsole24Filled } from '@fluentui/react-icons/fonts/xbox-console';
+
+export { XboxConsole24Filled };
diff --git a/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.js b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.js
new file mode 100644
index 0000000000..60dcbcabd2
--- /dev/null
+++ b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-atoms.js
@@ -0,0 +1,8 @@
+// @ts-check
+// The eager half. Its sibling is reachable only through a dynamic import, so the two icons land in
+// different chunks — and, deliberately, in different font families.
+import { GamesFilled } from '@fluentui/react-icons/fonts/games';
+
+console.dir({ GamesFilled });
+
+import('./lazy-atoms.async').then((m) => console.dir(m));
diff --git a/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.async.js b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.async.js
new file mode 100644
index 0000000000..faebff9945
--- /dev/null
+++ b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.async.js
@@ -0,0 +1,5 @@
+// @ts-check
+// Shares the Filled family with the eager half, but is reachable only through `import()`.
+import { XboxConsole24Filled } from '@fluentui/react-icons/fonts/xbox-console';
+
+export { XboxConsole24Filled };
diff --git a/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.js b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.js
new file mode 100644
index 0000000000..861a39389d
--- /dev/null
+++ b/packages/react-icons-font-subsetting-webpack-plugin/test/src/lazy-shared-family.js
@@ -0,0 +1,8 @@
+// @ts-check
+// Same font family as its async sibling: both icons are sized+Filled, so one emitted font file has
+// to carry glyphs contributed from two different chunks.
+import { Games24Filled } from '@fluentui/react-icons/fonts/games';
+
+console.dir({ Games24Filled });
+
+import('./lazy-shared-family.async').then((m) => console.dir(m));