diff --git a/scripts/postbuild-esm.cjs b/scripts/postbuild-esm.cjs
index cfc246a..aa984ca 100644
--- a/scripts/postbuild-esm.cjs
+++ b/scripts/postbuild-esm.cjs
@@ -1,17 +1,102 @@
 const fs = require('node:fs/promises');
+const fsSync = require('node:fs');
 const path = require('node:path');
 
-async function ensureEsmPackageJson(rootDir = process.cwd()) {
-  const targetDir = path.join(rootDir, 'esm');
+async function ensureEsmPackageJson(targetDir) {
   const manifestPath = path.join(targetDir, 'package.json');
   const content = JSON.stringify({ type: 'module' }, null, 2) + '\n';
 
-  await fs.mkdir(targetDir, { recursive: true });
   await fs.writeFile(manifestPath, content, 'utf8');
 }
 
-ensureEsmPackageJson().catch(err => {
-  console.error('[postbuild-esm] Failed to write esm/package.json');
+function tryAppendExtension(filePath, specifier) {
+  if (!specifier.startsWith('.')) {
+    return specifier;
+  }
+
+  const [base, suffix] = (() => {
+    const queryIndex = specifier.indexOf('?');
+    const hashIndex = specifier.indexOf('#');
+    let cutIndex = specifier.length;
+
+    if (queryIndex !== -1) cutIndex = Math.min(cutIndex, queryIndex);
+    if (hashIndex !== -1) cutIndex = Math.min(cutIndex, hashIndex);
+
+    return [specifier.slice(0, cutIndex), specifier.slice(cutIndex)];
+  })();
+
+  if (base.endsWith('/')) {
+    return specifier;
+  }
+
+  const knownSuffixes = new Set(['.js', '.cjs', '.mjs', '.json', '.node']);
+  if (knownSuffixes.has(path.extname(base))) {
+    return specifier;
+  }
+
+  const candidate = `${base}.js`;
+  const resolvedCandidate = path.resolve(path.dirname(filePath), candidate);
+
+  if (fsSync.existsSync(resolvedCandidate)) {
+    return `${candidate}${suffix}`;
+  }
+
+  return specifier;
+}
+
+function rewriteRelativeSpecifiers(filePath, source) {
+  const fromPattern = /(from\s+['"])(\.{1,2}\/[^'"]+)(['"])/g;
+  const importPattern = /(import\(\s*['"])(\.{1,2}\/[^'"]+)(['"]\s*\))/g;
+
+  const replaceWithExtension = (_, prefix, specifier, suffix) => {
+    const nextSpecifier = tryAppendExtension(filePath, specifier);
+    return `${prefix}${nextSpecifier}${suffix}`;
+  };
+
+  let transformed = source.replace(fromPattern, replaceWithExtension);
+  transformed = transformed.replace(importPattern, replaceWithExtension);
+
+  return transformed;
+}
+
+async function fixEsmImports(targetDir) {
+  const stack = [targetDir];
+
+  while (stack.length > 0) {
+    const current = stack.pop();
+    const entries = await fs.readdir(current, { withFileTypes: true });
+
+    for (const entry of entries) {
+      const fullPath = path.join(current, entry.name);
+
+      if (entry.isDirectory()) {
+        stack.push(fullPath);
+        continue;
+      }
+
+      if (!entry.isFile() || !entry.name.endsWith('.js')) {
+        continue;
+      }
+
+      const original = await fs.readFile(fullPath, 'utf8');
+      const updated = rewriteRelativeSpecifiers(fullPath, original);
+
+      if (updated !== original) {
+        await fs.writeFile(fullPath, updated, 'utf8');
+      }
+    }
+  }
+}
+
+async function postbuild(rootDir = process.cwd()) {
+  const esmDir = path.join(rootDir, 'esm');
+  await fs.mkdir(esmDir, { recursive: true });
+  await ensureEsmPackageJson(esmDir);
+  await fixEsmImports(esmDir);
+}
+
+postbuild().catch(err => {
+  console.error('[postbuild-esm] Failed to finalize ESM output');
   console.error(err);
   process.exitCode = 1;
 });