diff --git a/dist/index.js b/dist/index.js
index d884a9e..260a1bf 100644
Binary files a/dist/index.js and b/dist/index.js differ
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 9020108..16befdb 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -7,7 +7,7 @@ export default tseslint.config(
   // Ignore non-project files
   {
     name: 'ignore',
-    ignores: ['.github', 'dist', 'coverage', '**/*.json', 'jest.setup.js', 'eslint.config.mjs']
+    ignores: ['.github', 'dist', 'coverage', '**/*.json', 'jest.setup.js', 'eslint.config.mjs', 'scripts']
   },
   // Use recommended rules from ESLint, TypeScript, and other plugins
   eslint.configs.recommended,
diff --git a/package.json b/package.json
index 1b8af5f..224a5c9 100644
--- a/package.json
+++ b/package.json
@@ -24,6 +24,7 @@
     "node": ">=24"
   },
   "scripts": {
+    "postinstall": "node scripts/postinstall.js",
     "bundle": "npm run format:write && npm run package",
     "ci-test": "jest",
     "format:write": "prettier --write **/*.ts",
@@ -57,8 +58,12 @@
       "/dist/"
     ],
     "transform": {
-      "^.+\\.ts$": "ts-jest"
+      "^.+\\.ts$": "ts-jest",
+      "node_modules/@actions/.+\\.js$": "ts-jest"
     },
+    "transformIgnorePatterns": [
+      "node_modules/(?!@actions/)"
+    ],
     "coverageReporters": [
       "json-summary",
       "text",
diff --git a/scripts/postinstall.js b/scripts/postinstall.js
new file mode 100644
index 0000000..bea47e2
--- /dev/null
+++ b/scripts/postinstall.js
@@ -0,0 +1,60 @@
+/**
+ * Postinstall script to patch ESM-only dependencies for CJS compatibility.
+ *
+ * @actions/core v3+ and its transitive dependencies are ESM-only
+ * (exports only "import" condition). This script adds a "default"
+ * export condition so that CJS bundlers (like @vercel/ncc) and test
+ * runners (like Jest/ts-jest) can resolve the packages.
+ */
+const fs = require('fs')
+const path = require('path')
+
+function patchPackage(pkgPath) {
+  try {
+    const pkg = JSON.parse(fs.readFileSync(pkgPath, 'utf8'))
+    if (!pkg.exports) return
+
+    let patched = false
+    for (const key of Object.keys(pkg.exports)) {
+      const entry = pkg.exports[key]
+      if (typeof entry === 'object' && entry['import'] && !entry['default']) {
+        entry['default'] = entry['import']
+        patched = true
+      }
+    }
+
+    if (patched) {
+      fs.writeFileSync(pkgPath, JSON.stringify(pkg, null, 2) + '\n')
+      console.log(
+        `Patched ${pkg.name}@${pkg.version} exports with "default" condition`
+      )
+    }
+  } catch (e) {
+    // Ignore errors - the package might not be installed yet
+  }
+}
+
+function findPackageJsonFiles(dir) {
+  const results = []
+  try {
+    const entries = fs.readdirSync(dir, { withFileTypes: true })
+    for (const entry of entries) {
+      const fullPath = path.join(dir, entry.name)
+      if (entry.isDirectory() && entry.name !== '.cache') {
+        results.push(...findPackageJsonFiles(fullPath))
+      } else if (entry.name === 'package.json') {
+        results.push(fullPath)
+      }
+    }
+  } catch (e) {
+    // Ignore permission or missing directory errors
+  }
+  return results
+}
+
+const nodeModulesDir = path.join(__dirname, '..', 'node_modules', '@actions')
+const packageFiles = findPackageJsonFiles(nodeModulesDir)
+for (const pkgPath of packageFiles) {
+  patchPackage(pkgPath)
+}
+