diff --git a/.agents/skills/migrate-to-vinext/references/troubleshooting.md b/.agents/skills/migrate-to-vinext/references/troubleshooting.md
index d25705eb..0e1b14b4 100644
--- a/.agents/skills/migrate-to-vinext/references/troubleshooting.md
+++ b/.agents/skills/migrate-to-vinext/references/troubleshooting.md
@@ -24,6 +24,22 @@ When adding `"type": "module"`, any `.js` file using `module.exports` or `requir
 
 Alternatively, convert these files to ESM (`export default` syntax) and keep the `.js` extension.
 
+## Third-Party Package ESM Resolution Errors
+
+**Symptom:** `Cannot find module '...'` errors in dev server when using certain npm packages.
+
+**Example Error:**
+```
+Cannot find module '\node_modules.pnpm\validator@13.15.26\node_modules\validator\es\lib\util\assertString' 
+imported from \node_modules.pnpm\validator@13.15.26\node_modules\validator\es\lib\isEmail.js
+```
+
+**Cause:** Some ESM packages have complex internal import structures that Node.js module resolution can't handle when externalized.
+
+**vinext Fix:** vinext sets `noExternal: true` in all server environments (RSC and SSR), which forces all dependencies through Vite's transform pipeline. This resolves extensionless import issues automatically.
+
+**No configuration needed** — this is the default behavior.
+
 ## App Router vs Pages Router Issues
 
 **Symptom:** RSC-related errors, "client/server component" boundary violations.
diff --git a/packages/vinext/src/index.ts b/packages/vinext/src/index.ts
index 710c0d13..b6aec81c 100644
--- a/packages/vinext/src/index.ts
+++ b/packages/vinext/src/index.ts
@@ -2059,6 +2059,8 @@ hydrate();
           //   Any user-provided `ssr.noExternal` is intentionally superseded
           //   by this setting; only `ssr.external` entries escape Vite's transform.
           // Skip when targeting bundled runtimes (Cloudflare/Nitro bundle everything).
+          // This also resolves extensionless-import issues in packages like
+          // `validator` (see #189) by routing them through Vite's resolver.
           ...(hasCloudflarePlugin || hasNitroPlugin ? {} : {
             ssr: {
               external: ["react", "react-dom", "react-dom/server"],
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c6985916..367f1e1b 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -654,6 +654,25 @@ importers:
         specifier: ^7.3.1
         version: 7.3.1(@types/node@25.2.3)(jiti@2.6.1)(lightningcss@1.31.1)
 
+  tests/fixtures/ecosystem/validator:
+    dependencies:
+      react:
+        specifier: ^19.0.0
+        version: 19.2.4
+      react-dom:
+        specifier: ^19.0.0
+        version: 19.2.4(react@19.2.4)
+      validator:
+        specifier: ^13.15.26
+        version: 13.15.26
+    devDependencies:
+      vinext:
+        specifier: workspace:*
+        version: link:../../../../packages/vinext
+      vite:
+        specifier: ^7.3.1
+        version: 7.3.1(@types/node@25.2.3)(jiti@2.6.1)(lightningcss@1.31.1)
+
   tests/fixtures/pages-basic:
     dependencies:
       react:
@@ -4050,6 +4069,10 @@ packages:
   util-deprecate@1.0.2:
     resolution: {integrity: sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw==}
 
+  validator@13.15.26:
+    resolution: {integrity: sha512-spH26xU080ydGggxRyR1Yhcbgx+j3y5jbNXk/8L+iRvdIEQ4uTRH2Sgf2dokud6Q4oAtsbNvJ1Ft+9xmm6IZcA==}
+    engines: {node: '>= 0.10'}
+
   vfile-message@4.0.3:
     resolution: {integrity: sha512-QTHzsGd1EhbZs4AsQ20JX1rC3cOlt/IWJruk893DfLRr57lcnOeMaWG4K0JrRta4mIJZKth2Au3mM3u03/JWKw==}
 
@@ -7369,6 +7392,8 @@ snapshots:
 
   util-deprecate@1.0.2: {}
 
+  validator@13.15.26: {}
+
   vfile-message@4.0.3:
     dependencies:
       '@types/unist': 3.0.3
diff --git a/tests/ecosystem.test.ts b/tests/ecosystem.test.ts
index e0c445a9..7f18bcfc 100644
--- a/tests/ecosystem.test.ts
+++ b/tests/ecosystem.test.ts
@@ -406,3 +406,27 @@ describe("shadcn", () => {
     expect(html).toContain("Open Menu");
   });
 });
+
+// ─── validator ──────────────────────────────────────────────────────────────
+
+describe("validator", () => {
+  let proc: ChildProcess | null = null;
+  let fetchPage: (pathname: string) => Promise<{ html: string; status: number }>;
+
+  beforeAll(async () => {
+    const fixture = await startFixture("validator", 4405);
+    proc = fixture.process;
+    fetchPage = fixture.fetchPage;
+  }, 30000);
+
+  afterAll(() => killProcess(proc));
+
+  it("can import and use validator/es/lib/isEmail.js in SSR", async () => {
+    const { html, status } = await fetchPage("/");
+    expect(status).toBe(200);
+    expect(html).toContain("<h1>Validator Test</h1>");
+    // React adds HTML comments for hydration markers, so check without whitespace sensitivity
+    expect(html).toMatch(/Email:.*test@example\.com/);
+    expect(html).toMatch(/Valid:.*true/);
+  });
+});
diff --git a/tests/fixtures/ecosystem/validator/package.json b/tests/fixtures/ecosystem/validator/package.json
new file mode 100644
index 00000000..06bbf201
--- /dev/null
+++ b/tests/fixtures/ecosystem/validator/package.json
@@ -0,0 +1,14 @@
+{
+  "name": "test-validator",
+  "private": true,
+  "type": "module",
+  "dependencies": {
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
+    "validator": "^13.15.26"
+  },
+  "devDependencies": {
+    "vinext": "workspace:*",
+    "vite": "^7.3.1"
+  }
+}
diff --git a/tests/fixtures/ecosystem/validator/pages/index.tsx b/tests/fixtures/ecosystem/validator/pages/index.tsx
new file mode 100644
index 00000000..fea3d111
--- /dev/null
+++ b/tests/fixtures/ecosystem/validator/pages/index.tsx
@@ -0,0 +1,14 @@
+import isEmail from "validator/es/lib/isEmail.js";
+
+export default function ValidatorPage() {
+  const email = "test@example.com";
+  const valid = isEmail(email);
+
+  return (
+    <div>
+      <h1>Validator Test</h1>
+      <p>Email: {email}</p>
+      <p>Valid: {String(valid)}</p>
+    </div>
+  );
+}
diff --git a/tests/fixtures/ecosystem/validator/vite.config.ts b/tests/fixtures/ecosystem/validator/vite.config.ts
new file mode 100644
index 00000000..a6529598
--- /dev/null
+++ b/tests/fixtures/ecosystem/validator/vite.config.ts
@@ -0,0 +1,6 @@
+import { defineConfig } from "vite";
+import vinext from "vinext";
+
+export default defineConfig({
+  plugins: [vinext()],
+});