diff --git a/docs/esm-migration-plan.md b/docs/esm-migration-plan.md
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+# ESM Migration Plan: Add File Extensions to Imports
+
+## Overview
+
+This document outlines the plan to migrate from TypeScript's deprecated `"moduleResolution": "node"` (node10) to `"moduleResolution": "node16"` or `"nodenext"`. This change is necessary because the published ESM packages have extensionless imports that don't work correctly in modern ESM environments.
+
+## Issues Fixed
+
+This migration will resolve the following issues:
+
+- **#154** - Upgrade `moduleResolution` from `node` to `node16` or `nodenext` in tsconfig
+- **#110** - Published ESM code has imports without file extensions
+- **#64** - expressions: ERR_MODULE_NOT_FOUND attempting to run example demo script  
+- **#146** - Can not import `@actions/workflow-parser`
+
+## Problem Statement
+
+### Current State
+
+All packages use `"moduleResolution": "node"`:
+
+| Package | moduleResolution | TypeScript |
+|---------|------------------|------------|
+| expressions | `"node"` | ^4.7.4 |
+| workflow-parser | `"node"` | ^4.8.4 |
+| languageservice | `"node"` | ^4.8.4 |
+| languageserver | `"node"` | ^4.8.4 |
+| browser-playground | `"Node16"` ✅ | ^4.9.4 |
+
+This causes TypeScript to emit code like:
+```javascript
+// Published to npm - INVALID ESM
+export { Expr } from "./ast";  // Missing .js extension!
+```
+
+### Why This Fails
+
+ESM in Node.js 12+ **requires** explicit file extensions. When users try to import these packages:
+
+```javascript
+// User's code
+import { Expr } from "@actions/expressions";
+```
+
+Node.js fails with:
+```
+Error [ERR_MODULE_NOT_FOUND]: Cannot find module '.../node_modules/@actions/expressions/dist/ast'
+```
+
+## Migration Strategy
+
+### Option A: TypeScript 5.7+ with `rewriteRelativeImportExtensions` (Recommended)
+
+TypeScript 5.7 introduced a new compiler option that automatically rewrites `.ts` extensions to `.js` in output:
+
+```jsonc
+{
+  "compilerOptions": {
+    "moduleResolution": "node16",  // or "nodenext"
+    "rewriteRelativeImportExtensions": true
+  }
+}
+```
+
+**Source code:**
+```typescript
+import { Expr } from "./ast.ts";
+```
+
+**Compiled output:**
+```javascript
+export { Expr } from "./ast.js";
+```
+
+**Pros:**
+- Source uses `.ts` extensions (matches actual files)
+- Works with Deno (which requires `.ts` extensions)
+- TypeScript automatically transforms to `.js`
+- Modern, forward-looking approach
+
+**Cons:**
+- Requires TypeScript 5.7+
+- Relatively new feature
+- **BUG:** See "Known Issues" section below
+
+### Option B: Manual `.js` Extensions
+
+Use `.js` extensions in source TypeScript files:
+
+```typescript
+import { Expr } from "./ast.js";  // Points to .ts file, but use .js extension
+```
+
+**Pros:**
+- Works with TypeScript 4.7+ (with node16 moduleResolution)
+- Well-established pattern
+- No post-processing needed
+
+**Cons:**
+- Confusing - `.js` files don't exist at write time
+- Doesn't work with Deno out of the box
+
+### Recommendation
+
+**Use Option A** with workarounds for known issues (see below).
+
+---
+
+## Known Issues and Workarounds (December 2025)
+
+### 1. TypeScript Version Conflicts in Monorepo
+
+**Problem:** The root `node_modules/typescript` was version 4.9.5 (pulled in by `ts-node` and `tsutils` dependencies), while workspace packages specified `^5.8.3`.
+
+**Symptoms:** 
+- `npx tsc --version` showed 4.9.5
+- `require('typescript').version` in ts-jest showed 5.8.3
+- Confusing build failures
+
+**Solution:** Add npm overrides in root `package.json`:
+```json
+{
+  "overrides": {
+    "typescript": "5.8.3"
+  }
+}
+```
+
+### 2. ts-jest Compatibility with TypeScript 5.9+
+
+**Problem:** ts-jest 29.4.6 uses `typescript.JSDocParsingMode.ParseAll` which doesn't exist in TypeScript's ES module exports.
+
+**Error:**
+```
+TypeError: Cannot read properties of undefined (reading 'ParseAll')
+at Object.<anonymous> (node_modules/ts-jest/dist/compiler/ts-compiler.js:43:123)
+```
+
+**Root Cause:** ts-jest accesses `typescript_1.default.JSDocParsingMode.ParseAll` but TypeScript has no default export in ESM.
+
+**Solution:** 
+- Use ts-jest 29.0.3 (older version that doesn't use this API)
+- OR wait for ts-jest fix
+- **Stay on TypeScript 5.8.3, not 5.9+**
+
+### 3. TypeScript `rewriteRelativeImportExtensions` Bug with .d.ts Files
+
+**Problem:** TypeScript's `rewriteRelativeImportExtensions: true` correctly rewrites `.ts` → `.js` in `.js` output files, but **incorrectly keeps `.ts` extensions in `.d.ts` declaration files**.
+
+**Example:**
+- Source: `export { Expr } from "./ast.ts";`
+- Output `index.js`: `export { Expr } from "./ast.js";` ✅ Correct
+- Output `index.d.ts`: `export { Expr } from "./ast.ts";` ❌ Wrong (should be `.js`)
+
+**Upstream Issue:** https://github.com/microsoft/TypeScript/issues/61037 (marked "Help Wanted", in Backlog, NOT FIXED as of Dec 2025)
+
+**Workaround:** Post-process `.d.ts` files with a script. Create `script/fix-dts-extensions.cjs`:
+
+```javascript
+#!/usr/bin/env node
+/**
+ * Post-build script to fix TypeScript's rewriteRelativeImportExtensions bug
+ * where .d.ts files get .ts extensions instead of .js extensions.
+ * See: https://github.com/microsoft/TypeScript/issues/61037
+ */
+
+const fs = require('fs');
+const path = require('path');
+
+function fixDtsFile(filePath) {
+    let content = fs.readFileSync(filePath, 'utf8');
+    const original = content;
+    
+    // Replace .ts extensions in import/export statements with .js
+    content = content.replace(/(from\s+["'])([^"']+)\.ts(["'])/g, '$1$2.js$3');
+    content = content.replace(/(import\s*\(\s*["'])([^"']+)\.ts(["']\s*\))/g, '$1$2.js$3');
+    content = content.replace(/(export\s+\*\s+from\s+["'])([^"']+)\.ts(["'])/g, '$1$2.js$3');
+    
+    if (content !== original) {
+        fs.writeFileSync(filePath, content, 'utf8');
+        console.log(`Fixed: ${filePath}`);
+        return true;
+    }
+    return false;
+}
+
+function walkDir(dir, callback) {
+    const files = fs.readdirSync(dir);
+    for (const file of files) {
+        const filePath = path.join(dir, file);
+        const stat = fs.statSync(filePath);
+        if (stat.isDirectory()) {
+            walkDir(filePath, callback);
+        } else if (file.endsWith('.d.ts')) {
+            callback(filePath);
+        }
+    }
+}
+
+function main() {
+    const args = process.argv.slice(2);
+    if (args.length === 0) {
+        console.error('Usage: fix-dts-extensions.cjs <dist-dir> [<dist-dir2> ...]');
+        process.exit(1);
+    }
+    
+    let fixedCount = 0;
+    for (const dir of args) {
+        if (!fs.existsSync(dir)) {
+            console.warn(`Directory not found: ${dir}`);
+            continue;
+        }
+        walkDir(dir, (filePath) => {
+            if (fixDtsFile(filePath)) {
+                fixedCount++;
+            }
+        });
+    }
+    console.log(`Fixed ${fixedCount} .d.ts file(s)`);
+}
+
+main();
+```
+
+**Note:** Use `.cjs` extension since root `package.json` has `"type": "module"`.
+
+**Usage in package.json build scripts:**
+```json
+{
+  "scripts": {
+    "build": "tsc --build tsconfig.build.json && node ../script/fix-dts-extensions.cjs dist"
+  }
+}
+```
+
+### 4. languageserver Tests Hang
+
+**Problem:** The languageserver tests hang indefinitely when running with the ESM configuration.
+
+**Status:** Not fully diagnosed. Tests pass on main branch but hang on ESM branch.
+
+**Possible causes:**
+- Jest ESM module resolution issues
+- Cross-package source mappings in jest.config.js
+- vscode-languageserver ESM compatibility issues
+- Specific test file causing hang (needs isolation testing)
+
+**Investigation needed:**
+- Run individual test files to isolate the hanging test
+- Check if vscode-languageserver has ESM compatibility issues
+- Review jest configuration for problematic mappings
+- Try running with `--detectOpenHandles` flag
+
+---
+
+## Required Configuration Changes
+
+### tsconfig.json (each package)
+
+```json
+{
+  "compilerOptions": {
+    "module": "node16",
+    "moduleResolution": "node16",
+    "rewriteRelativeImportExtensions": true,
+    "lib": ["ES2022"],
+    "target": "ES2022"
+  }
+}
+```
+
+### jest.config.js (each package)
+
+```javascript
+/** @type {import('ts-jest').JestConfigWithTsJest} */
+export default {
+  preset: "ts-jest/presets/default-esm",
+  moduleNameMapper: {
+    "^(\\.{1,2}/.*)\\.js$": "$1",
+    "^(\\.{1,2}/.*)\\.ts$": "$1",
+  },
+  transform: {
+    "^.+\\.tsx?$": [
+      "ts-jest",
+      {
+        useESM: true,
+        isolatedModules: true,
+      },
+    ],
+  },
+  moduleFileExtensions: ["ts", "js"],
+};
+```
+
+### Root package.json
+
+```json
+{
+  "overrides": {
+    "typescript": "5.8.3"
+  }
+}
+```
+
+### Each workspace package.json
+
+```json
+{
+  "devDependencies": {
+    "typescript": "^5.8.3",
+    "ts-jest": "^29.0.3"
+  }
+}
+```
+
+---
+
+## Test Results (December 2025 Attempt)
+
+| Package | Tests | Status |
+|---------|-------|--------|
+| expressions | 1068 | ✅ Pass |
+| workflow-parser | 292 | ✅ Pass |
+| languageservice | 452 | ✅ Pass |
+| languageserver | 6 files | ❌ Hangs (needs investigation) |
+
+---
+
+## Implementation Steps
+
+### Phase 1: Prerequisites
+- [ ] Merge all pending PRs to avoid conflicts
+- [ ] Ensure clean main branch state
+
+### Phase 2: TypeScript & Dependencies
+- [ ] Add `"overrides": { "typescript": "5.8.3" }` to root package.json
+- [ ] Update all workspace packages to TypeScript ^5.8.3
+- [ ] Downgrade ts-jest to ^29.0.3 in all packages
+- [ ] Run `npm install` to apply changes
+
+### Phase 3: tsconfig Updates
+- [ ] Update tsconfig.json in each package with node16 settings
+- [ ] Add `rewriteRelativeImportExtensions: true`
+
+### Phase 4: Add .ts Extensions to Imports
+- [ ] Update all relative imports in source files to use `.ts` extensions
+- [ ] This was already done in PR #243
+
+### Phase 5: Create Fix Script
+- [ ] Add `script/fix-dts-extensions.cjs` to repository
+- [ ] Update build scripts to run fix after tsc
+
+### Phase 6: Fix languageserver Tests
+- [ ] Investigate why tests hang
+- [ ] Isolate problematic test file
+- [ ] Apply fix or workaround
+
+### Phase 7: Verification
+- [ ] Run `npm run build` in all packages
+- [ ] Run `npm test` in all packages
+- [ ] Test importing published packages in Node.js ESM mode
+- [ ] Verify browser-playground still works
+
+### Phase 8: CI Updates
+- [ ] Update GitHub Actions workflows if needed
+- [ ] Ensure fix-dts-extensions.cjs runs in CI
+
+---
+
+## Summary of Required Changes
+
+1. **Root package.json**: Add TypeScript override
+2. **All packages**: TypeScript 5.8.3, ts-jest 29.0.3
+3. **All tsconfigs**: node16 moduleResolution, rewriteRelativeImportExtensions
+4. **All imports**: Add .ts extensions (already done)
+5. **Build scripts**: Add post-build .d.ts fix
+6. **languageserver**: Debug test hang issue
+
+---
+
+## References
+
+- [TypeScript moduleResolution reference](https://www.typescriptlang.org/docs/handbook/modules/reference.html)
+- [TypeScript 5.7 rewriteRelativeImportExtensions](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-7.html#path-rewriting-for-relative-paths)
+- [TypeScript .d.ts extension bug #61037](https://github.com/microsoft/TypeScript/issues/61037)
+- [Node.js ESM mandatory extensions](https://nodejs.org/api/esm.html#mandatory-file-extensions)
+- [ts-jest ESM support](https://kulshekhar.github.io/ts-jest/docs/guides/esm-support)
+- [Community fork that works](https://github.com/boxbuild-io/actions-languageservices/commit/077fb2b58dfd2cca3d6e3df1fdf9e26e75db24ae)
diff --git a/script/fix-dts-extensions.cjs b/script/fix-dts-extensions.cjs
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index 00000000..a47200a4
--- /dev/null
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+#!/usr/bin/env node
+
+/**
+ * Post-build script to fix TypeScript's rewriteRelativeImportExtensions bug
+ * where .d.ts files get .ts extensions instead of .js extensions.
+ * See: https://github.com/microsoft/TypeScript/issues/61037
+ */
+
+const fs = require('fs');
+const path = require('path');
+
+function fixDtsFile(filePath) {
+    let content = fs.readFileSync(filePath, 'utf8');
+    const original = content;
+    
+    // Replace .ts extensions in import/export statements with .js
+    // Matches: from "./foo.ts" or from './foo.ts'
+    content = content.replace(/(from\s+["'])([^"']+)\.ts(["'])/g, '$1$2.js$3');
+    
+    // Matches: import("./foo.ts") or import('./foo.ts')
+    content = content.replace(/(import\s*\(\s*["'])([^"']+)\.ts(["']\s*\))/g, '$1$2.js$3');
+    
+    // Matches: export * from "./foo.ts"
+    content = content.replace(/(export\s+\*\s+from\s+["'])([^"']+)\.ts(["'])/g, '$1$2.js$3');
+    
+    if (content !== original) {
+        fs.writeFileSync(filePath, content, 'utf8');
+        console.log(`Fixed: ${filePath}`);
+        return true;
+    }
+    return false;
+}
+
+function walkDir(dir, callback) {
+    const files = fs.readdirSync(dir);
+    for (const file of files) {
+        const filePath = path.join(dir, file);
+        const stat = fs.statSync(filePath);
+        if (stat.isDirectory()) {
+            walkDir(filePath, callback);
+        } else if (file.endsWith('.d.ts')) {
+            callback(filePath);
+        }
+    }
+}
+
+function main() {
+    const args = process.argv.slice(2);
+    if (args.length === 0) {
+        console.error('Usage: fix-dts-extensions.js <dist-dir> [<dist-dir2> ...]');
+        process.exit(1);
+    }
+    
+    let fixedCount = 0;
+    for (const dir of args) {
+        if (!fs.existsSync(dir)) {
+            console.warn(`Directory not found: ${dir}`);
+            continue;
+        }
+        walkDir(dir, (filePath) => {
+            if (fixDtsFile(filePath)) {
+                fixedCount++;
+            }
+        });
+    }
+    console.log(`Fixed ${fixedCount} .d.ts file(s)`);
+}
+
+main();