diff --git a/packages/vinext/src/cli.ts b/packages/vinext/src/cli.ts
index 0b4f253e..05ddfcf0 100644
--- a/packages/vinext/src/cli.ts
+++ b/packages/vinext/src/cli.ts
@@ -24,6 +24,7 @@ import { deploy as runDeploy, parseDeployArgs } from "./deploy.js";
 import { runCheck, formatReport } from "./check.js";
 import { init as runInit, getReactUpgradeDeps } from "./init.js";
 import { loadDotenv } from "./config/dotenv.js";
+import { ensureESModule, renameCJSConfigs, hasViteConfig as hasViteConfigInRoot } from "./utils/project.js";
 
 // ─── Resolve Vite from the project root ────────────────────────────────────────
 //
@@ -178,6 +179,61 @@ function buildViteConfig(overrides: Record<string, unknown> = {}) {
   return config;
 }
 
+/**
+ * Ensure the project's package.json has `"type": "module"` before Vite loads
+ * the vite.config.ts. This prevents the esbuild CJS-bundling path that Vite
+ * takes for projects without `"type": "module"`, which produces a `.mjs` temp
+ * file containing `require()` calls — calls that fail on Node 22 when
+ * targeting pure-ESM packages like `@cloudflare/vite-plugin`.
+ *
+ * This mirrors what `vinext init` does, but is applied lazily at dev/build
+ * time for projects that were set up before `vinext init` added the step, or
+ * that were migrated manually.
+ *
+ * Side effects: may rename `.js` config files like `postcss.config.js` to
+ * `.cjs` to avoid CJS/ESM confusion after `"type": "module"` is added (the
+ * same rename logic used by `vinext init`).
+ */
+function ensureViteConfigCompatibility(root: string): void {
+  // Only act when there is a vite.config — auto-config mode sets
+  // configFile: false and doesn't go through Vite's file-loading path.
+  if (!hasViteConfigInRoot(root)) return;
+
+  const pkgPath = path.join(root, "package.json");
+  if (!fs.existsSync(pkgPath)) return;
+
+  let pkg: Record<string, unknown>;
+  try {
+    pkg = JSON.parse(fs.readFileSync(pkgPath, "utf-8"));
+  } catch {
+    return;
+  }
+
+  // Already correct — nothing to do.
+  if (pkg.type === "module") return;
+
+  // Respect explicit "type": "commonjs" — the user chose this deliberately.
+  if (pkg.type !== undefined) return;
+
+  // Rename any `.js` CJS config files first so they don't break after we
+  // add "type": "module".
+  const renamed = renameCJSConfigs(root);
+  for (const [oldName, newName] of renamed) {
+    console.warn(
+      `  [vinext] Renamed ${oldName} → ${newName} (required for "type": "module")`
+    );
+  }
+
+  // Add "type": "module" so Vite loads vite.config.ts as ESM.
+  const added = ensureESModule(root);
+  if (added) {
+    console.warn(
+      `  [vinext] Added "type": "module" to package.json (required for Vite ESM config loading).\n` +
+        `  Run \`vinext init\` to review all project configuration.`
+    );
+  }
+}
+
 // ─── Commands ─────────────────────────────────────────────────────────────────
 
 async function dev() {
@@ -189,6 +245,11 @@ async function dev() {
     mode: "development",
   });
 
+  // Ensure "type": "module" in package.json before Vite loads vite.config.ts.
+  // Without this, Vite bundles the config as CJS and tries require() on pure-ESM
+  // packages like @cloudflare/vite-plugin, which fails on Node 22.
+  ensureViteConfigCompatibility(process.cwd());
+
   const vite = await loadVite();
 
   const port = parsed.port ?? 3000;
@@ -214,6 +275,11 @@ async function buildApp() {
     mode: "production",
   });
 
+  // Ensure "type": "module" in package.json before Vite loads vite.config.ts.
+  // Without this, Vite bundles the config as CJS and tries require() on pure-ESM
+  // packages like @cloudflare/vite-plugin, which fails on Node 22.
+  ensureViteConfigCompatibility(process.cwd());
+
   const vite = await loadVite();
 
   console.log(`\n  vinext build  (Vite ${getViteVersion()})\n`);
diff --git a/tests/deploy.test.ts b/tests/deploy.test.ts
index a9f939ca..8f1f8c38 100644
--- a/tests/deploy.test.ts
+++ b/tests/deploy.test.ts
@@ -1988,3 +1988,118 @@ describe("findInNodeModules", () => {
     expect(result).toBe(path.join(appDir, "node_modules", "@cloudflare", "vite-plugin"));
   });
 });
+
+// ─── ESM config compatibility (issue #184) ──────────────────────────────────
+//
+// Regression tests for ensureViteConfigCompatibility() — the wrapper in cli.ts
+// that calls ensureESModule + renameCJSConfigs before Vite loads the config.
+//
+// Scenario from issue #184:
+//   Project has vite.config.ts importing @cloudflare/vite-plugin.
+//   package.json lacks "type":"module".
+//   Vite bundles the config with esbuild → outputs CJS require() for the plugin.
+//   Node 22 throws:  Error: Dynamic require of "…index.mjs" is not supported
+//
+// The underlying ensureESModule() and renameCJSConfigs() are already tested
+// above. These tests cover the unique scenarios from the issue and the
+// integration of both functions together.
+
+describe("ESM config compatibility — issue #184", () => {
+  it("full fix scenario: renames CJS postcss config + adds type:module", () => {
+    // Yarn 1 monorepo scenario: no "type":"module", CJS postcss config
+    writeFile(
+      tmpDir,
+      "package.json",
+      JSON.stringify({ name: "web", version: "1.0.0" }),
+    );
+    writeFile(
+      tmpDir,
+      "vite.config.ts",
+      'import { cloudflare } from "@cloudflare/vite-plugin";\nexport default { plugins: [cloudflare()] };',
+    );
+    writeFile(
+      tmpDir,
+      "postcss.config.js",
+      "module.exports = { plugins: { tailwindcss: {}, autoprefixer: {} } };",
+    );
+
+    // Simulate what ensureViteConfigCompatibility does
+    const renamed = renameCJSConfigs(tmpDir);
+    const added = ensureESModule(tmpDir);
+
+    expect(added).toBe(true);
+    const pkg = JSON.parse(fs.readFileSync(path.join(tmpDir, "package.json"), "utf-8"));
+    expect(pkg.type).toBe("module");
+
+    expect(renamed).toEqual([["postcss.config.js", "postcss.config.cjs"]]);
+    expect(fs.existsSync(path.join(tmpDir, "postcss.config.cjs"))).toBe(true);
+    expect(fs.existsSync(path.join(tmpDir, "postcss.config.js"))).toBe(false);
+  });
+
+  it("handles a workspaces monorepo: only updates the leaf package.json", () => {
+    const webDir = path.join(tmpDir, "apps", "web");
+    fs.mkdirSync(webDir, { recursive: true });
+
+    // Root package.json (CJS)
+    writeFile(tmpDir, "package.json", JSON.stringify({ name: "monorepo", workspaces: ["apps/*"] }));
+    // Workspace package.json (no type:module)
+    writeFile(webDir, "package.json", JSON.stringify({ name: "web", version: "1.0.0" }));
+    writeFile(webDir, "vite.config.ts", "export default {};");
+
+    ensureESModule(webDir);
+
+    const rootPkg = JSON.parse(fs.readFileSync(path.join(tmpDir, "package.json"), "utf-8"));
+    const webPkg = JSON.parse(fs.readFileSync(path.join(webDir, "package.json"), "utf-8"));
+
+    // Only the workspace package should be modified
+    expect(webPkg.type).toBe("module");
+    expect(rootPkg.type).toBeUndefined();
+  });
+
+  it("preserves all existing package.json fields when adding type:module", () => {
+    writeFile(
+      tmpDir,
+      "package.json",
+      JSON.stringify({
+        name: "web",
+        version: "0.1.0",
+        private: true,
+        scripts: { dev: "vinext dev", build: "vinext build" },
+        dependencies: { react: "^19.0.0", "react-dom": "^19.0.0" },
+        devDependencies: { vinext: "^0.0.13", "@cloudflare/vite-plugin": "^1.0.0" },
+      }),
+    );
+
+    ensureESModule(tmpDir);
+
+    const pkg = JSON.parse(fs.readFileSync(path.join(tmpDir, "package.json"), "utf-8"));
+    expect(pkg.type).toBe("module");
+    expect(pkg.name).toBe("web");
+    expect(pkg.version).toBe("0.1.0");
+    expect(pkg.private).toBe(true);
+    expect(pkg.scripts.build).toBe("vinext build");
+    expect(pkg.dependencies.react).toBe("^19.0.0");
+    expect(pkg.devDependencies.vinext).toBe("^0.0.13");
+  });
+
+  it("handles multiple CJS config files simultaneously", () => {
+    writeFile(
+      tmpDir,
+      "package.json",
+      JSON.stringify({ name: "my-app", version: "1.0.0" }),
+    );
+    writeFile(tmpDir, "postcss.config.js", "module.exports = {};");
+    writeFile(tmpDir, "tailwind.config.js", "module.exports = {};");
+    writeFile(tmpDir, ".eslintrc.js", "module.exports = {};");
+
+    const renamed = renameCJSConfigs(tmpDir);
+    ensureESModule(tmpDir);
+
+    expect(renamed).toHaveLength(3);
+    expect(fs.existsSync(path.join(tmpDir, "postcss.config.cjs"))).toBe(true);
+    expect(fs.existsSync(path.join(tmpDir, "tailwind.config.cjs"))).toBe(true);
+    expect(fs.existsSync(path.join(tmpDir, ".eslintrc.cjs"))).toBe(true);
+    const pkg = JSON.parse(fs.readFileSync(path.join(tmpDir, "package.json"), "utf-8"));
+    expect(pkg.type).toBe("module");
+  });
+});