diff --git a/next.config.mjs b/next.config.mjs
index a9e0087f92..2049739ab5 100644
--- a/next.config.mjs
+++ b/next.config.mjs
@@ -329,11 +329,12 @@ const nextConfig = {
     // TODO: Re-enable after fixing all sub-component useTranslations scope issues
     ignoreBuildErrors: true,
   },
-  webpack(config, { webpack }) {
+  webpack(config, { dev, webpack }) {
     config.ignoreWarnings = [
       ...(config.ignoreWarnings || []),
       isNextIntlExtractorDynamicImportWarning,
     ];
+    const nextDefaultSplitChunks = config.optimization?.splitChunks;
     config.optimization = config.optimization || {};
     config.optimization.splitChunks = {
       ...config.optimization.splitChunks,
@@ -392,6 +393,9 @@ const nextConfig = {
         },
       },
     };
+    // Next's development defaults are tuned for incremental route compilation.
+    // Retain the custom vendor topology for production without imposing it on dev.
+    if (dev) config.optimization.splitChunks = nextDefaultSplitChunks;
 
     if (isMinimalBuild) {
       // Mirror the turbopack.resolveAlias entries for webpack-built artifacts.
diff --git a/src/app/globals.css b/src/app/globals.css
index 939fc9ada7..5a2b558236 100644
--- a/src/app/globals.css
+++ b/src/app/globals.css
@@ -1,4 +1,4 @@
-@import "tailwindcss";
+@import "tailwindcss" source(none);
 @import "fumadocs-ui/css/neutral.css";
 @import "fumadocs-ui/css/preset.css";
 /* Self-hosted Material Symbols icon font (#3695): the Google Fonts CDN
@@ -10,14 +10,12 @@
 @import "material-symbols/outlined.css";
 @source "../../node_modules/fumadocs-ui/css/generated/*.css";
 
-/* Tailwind v4 auto-detection cannot scan directories with parentheses
-   (e.g. Next.js route groups like "(dashboard)"). Explicit @source
-   directives ensure all utility classes in route groups are included. */
-@source "../app/(dashboard)";
+/* Keep Tailwind v4 from scanning the entire repository. The UI lives in the
+   App Router and shared component trees; third-party Fumadocs sources remain
+   explicit because node_modules is ignored by automatic detection. */
+@source "../app";
+@source "../shared";
 @source "../../node_modules/fumadocs-ui/dist/**/*.js";
-@source not "../../*.sqlite*";
-@source not "../../.claude*";
-@source not "../../.claude-memory";
 
 @custom-variant dark (&:where(.dark, .dark *));
 
diff --git a/tests/unit/dev-bundler-boundaries-12074.test.ts b/tests/unit/dev-bundler-boundaries-12074.test.ts
new file mode 100644
index 0000000000..9e8c20065f
--- /dev/null
+++ b/tests/unit/dev-bundler-boundaries-12074.test.ts
@@ -0,0 +1,77 @@
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import test from "node:test";
+import path from "node:path";
+import { pathToFileURL } from "node:url";
+
+const repoRoot = process.cwd();
+const nextConfigPath = path.join(repoRoot, "next.config.mjs");
+
+async function loadNextConfig(label: string) {
+  return import(`${pathToFileURL(nextConfigPath).href}?case=${label}-${Date.now()}`);
+}
+
+test("Tailwind scans only the UI source roots declared by OmniRoute (#12074 phase 1)", () => {
+  const css = readFileSync(path.join(repoRoot, "src/app/globals.css"), "utf8");
+
+  assert.match(
+    css,
+    /@import\s+"tailwindcss"\s+source\(none\);/,
+    "automatic repository-wide Tailwind source detection must stay disabled"
+  );
+  assert.match(css, /@source\s+"\.\.\/app";/, "App Router components must stay scanned");
+  assert.match(css, /@source\s+"\.\.\/shared";/, "shared UI components must stay scanned");
+  assert.match(
+    css,
+    /@source\s+"\.\.\/\.\.\/node_modules\/fumadocs-ui\/dist\/\*\*\/\*\.js";/,
+    "Fumadocs runtime classes must stay scanned"
+  );
+});
+
+test("webpack dev keeps Next defaults instead of production vendor cache groups (#12074 phase 1)", async () => {
+  const { default: nextConfig } = await loadNextConfig("dev-split-chunks");
+  const originalSplitChunks = {
+    chunks: "async",
+    cacheGroups: {
+      framework: { name: "framework" },
+    },
+  };
+  const config = {
+    context: repoRoot,
+    ignoreWarnings: [],
+    optimization: { splitChunks: originalSplitChunks },
+    plugins: [],
+  };
+
+  nextConfig.webpack(config, {
+    dev: true,
+    isServer: false,
+    defaultLoaders: { babel: {} },
+    webpack: {},
+  });
+
+  assert.equal(config.optimization.splitChunks, originalSplitChunks);
+  const cacheGroups = config.optimization.splitChunks.cacheGroups as Record<string, unknown>;
+  assert.equal(cacheGroups.recharts, undefined);
+  assert.equal(cacheGroups.fumadocs, undefined);
+});
+
+test("webpack production retains OmniRoute vendor cache groups (#12074 phase 1)", async () => {
+  const { default: nextConfig } = await loadNextConfig("production-split-chunks");
+  const config = {
+    context: repoRoot,
+    ignoreWarnings: [],
+    optimization: { splitChunks: { cacheGroups: {} as Record<string, unknown> } },
+    plugins: [],
+  };
+
+  nextConfig.webpack(config, {
+    dev: false,
+    isServer: false,
+    defaultLoaders: { babel: {} },
+    webpack: {},
+  });
+
+  assert.ok(config.optimization.splitChunks.cacheGroups.recharts);
+  assert.ok(config.optimization.splitChunks.cacheGroups.fumadocs);
+});