diff --git a/integration-tests/src/reporter/reportGenerator.ts b/integration-tests/src/reporter/reportGenerator.ts
index c46c655b94d7..21f5b0b3f4bb 100644
--- a/integration-tests/src/reporter/reportGenerator.ts
+++ b/integration-tests/src/reporter/reportGenerator.ts
@@ -170,6 +170,8 @@ function tabToSpace(text: string, tabWidth = 4): string {
     return result;
 }
 
-export const __testing__ = {
+export const __testing__: {
+    padLines: typeof padLines;
+} = {
     padLines,
 };
diff --git a/integration-tests/src/repositoryHelper.ts b/integration-tests/src/repositoryHelper.ts
index 05c386a83fb9..2222ec469247 100644
--- a/integration-tests/src/repositoryHelper.ts
+++ b/integration-tests/src/repositoryHelper.ts
@@ -15,7 +15,7 @@ import type { Logger } from './types.js';
 
 const __dirname = fileURLToPath(new URL('.', import.meta.url));
 
-export const repositoryDir = Path.resolve(Path.join(__dirname, '../repositories/temp'));
+export const repositoryDir: string = Path.resolve(Path.join(__dirname, '../repositories/temp'));
 
 function mkdirp(p: string) {
     return fs.promises.mkdir(p, { recursive: true });
diff --git a/integration-tests/src/snapshots.ts b/integration-tests/src/snapshots.ts
index 06f1e1cbb6bf..cca5b86ae6bb 100644
--- a/integration-tests/src/snapshots.ts
+++ b/integration-tests/src/snapshots.ts
@@ -10,7 +10,7 @@ import type { Repository } from './configDef.js';
 
 const __dirname = fileURLToPath(new URL('.', import.meta.url));
 
-export const snapshotDir = Path.resolve(Path.join(__dirname, '..', 'snapshots'));
+export const snapshotDir: string = Path.resolve(Path.join(__dirname, '..', 'snapshots'));
 const snapshotFileName = 'snapshot.txt';
 const reportFileName = 'report.yaml';
 
diff --git a/package.json b/package.json
index d7c08ac1a2d1..390c75b81902 100644
--- a/package.json
+++ b/package.json
@@ -111,7 +111,7 @@
     "@rollup/plugin-terser": "^0.4.4",
     "@rollup/plugin-typescript": "^12.1.2",
     "@swc-node/register": "^1.10.10",
-    "@tsconfig/node18": "^18.2.4",
+    "@tsconfig/node20": "^20.1.5",
     "@types/node": "^20.19.0",
     "@vitest/coverage-istanbul": "~3.2.2",
     "conventional-changelog-conventionalcommits": "^9.0.0",
diff --git a/packages/cspell-bundled-dicts/.gitignore b/packages/cspell-bundled-dicts/.gitignore
index df8442de85db..20a88ec5c2ab 100644
--- a/packages/cspell-bundled-dicts/.gitignore
+++ b/packages/cspell-bundled-dicts/.gitignore
@@ -1 +1,4 @@
 # Things to ignore.
+
+*.d.ts
+*.map
diff --git a/packages/cspell-bundled-dicts/tsconfig.json b/packages/cspell-bundled-dicts/tsconfig.json
index b5ae3e81f707..b4025bc75fac 100644
--- a/packages/cspell-bundled-dicts/tsconfig.json
+++ b/packages/cspell-bundled-dicts/tsconfig.json
@@ -2,6 +2,7 @@
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
         "declaration": false,
+        "isolatedDeclarations": false,
         "declarationMap": false,
         "sourceMap": false
     },
diff --git a/packages/cspell-config-lib/tsconfig.json b/packages/cspell-config-lib/tsconfig.json
index 89f72354670e..1b3bb91f043b 100644
--- a/packages/cspell-config-lib/tsconfig.json
+++ b/packages/cspell-config-lib/tsconfig.json
@@ -1,6 +1,7 @@
 {
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
         "types": ["node"],
         "outDir": "dist"
     },
diff --git a/packages/cspell-dictionary/src/SpellingDictionary/CachingDictionary.ts b/packages/cspell-dictionary/src/SpellingDictionary/CachingDictionary.ts
index 11e481698d1d..efb02ff34ad6 100644
--- a/packages/cspell-dictionary/src/SpellingDictionary/CachingDictionary.ts
+++ b/packages/cspell-dictionary/src/SpellingDictionary/CachingDictionary.ts
@@ -122,7 +122,7 @@ export function createCachingDictionary(
     return cached;
 }
 
-export function enableLogging(enabled = !logRequests): void {
+export function enableLogging(enabled: boolean = !logRequests): void {
     logRequests = enabled;
 }
 
diff --git a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryCollection.ts b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryCollection.ts
index b38841be7272..e1c07f02edd2 100644
--- a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryCollection.ts
+++ b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryCollection.ts
@@ -174,7 +174,10 @@ export function isSpellingDictionaryCollection(dict: SpellingDictionary): dict i
     return dict instanceof SpellingDictionaryCollectionImpl;
 }
 
-export const __testing__ = {
+export const __testing__: {
+    isWordInAnyDictionary: typeof isWordInAnyDictionary;
+    isWordForbiddenInAnyDictionary: typeof isWordForbiddenInAnyDictionary;
+} = {
     isWordInAnyDictionary,
     isWordForbiddenInAnyDictionary,
 };
diff --git a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryFromTrie.ts b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryFromTrie.ts
index 8c4bfa431e2c..262ac4096441 100644
--- a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryFromTrie.ts
+++ b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryFromTrie.ts
@@ -29,8 +29,8 @@ import type { SuggestOptions } from './SuggestOptions.js';
 
 export class SpellingDictionaryFromTrie implements SpellingDictionary {
     private _size = 0;
-    readonly knownWords = new Set<string>();
-    readonly unknownWords = new Set<string>();
+    readonly knownWords: Set<string> = new Set<string>();
+    readonly unknownWords: Set<string> = new Set<string>();
     readonly mapWord: (word: string) => string;
     readonly remapWord: (word: string) => string[];
     readonly type = 'SpellingDictionaryFromTrie';
@@ -254,4 +254,6 @@ function* outerWordForms(word: string, mapWord: (word: string) => string[]): Ite
     return;
 }
 
-export const __testing__ = { outerWordForms };
+export const __testing__: {
+    outerWordForms: typeof outerWordForms;
+} = { outerWordForms };
diff --git a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryMethods.ts b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryMethods.ts
index a939441c6a03..1566277e38cb 100644
--- a/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryMethods.ts
+++ b/packages/cspell-dictionary/src/SpellingDictionary/SpellingDictionaryMethods.ts
@@ -98,7 +98,10 @@ export function createWeightMapFromDictionaryInformation(di: DictionaryInformati
     return di ? mapDictionaryInformationToWeightMap(di) : undefined;
 }
 
-export const __testMethods__ = {
+export const __testMethods__: {
+    wordSearchForms: typeof wordSearchForms;
+    wordSearchFormsArray: typeof wordSearchFormsArray;
+} = {
     wordSearchForms,
     wordSearchFormsArray,
 };
diff --git a/packages/cspell-dictionary/src/index.ts b/packages/cspell-dictionary/src/index.ts
index dcc678c2d9bd..50b6e1e7b0eb 100644
--- a/packages/cspell-dictionary/src/index.ts
+++ b/packages/cspell-dictionary/src/index.ts
@@ -33,7 +33,10 @@ export {
 /**
  * Debugging utilities.
  */
-export const _debug = {
+export const _debug: {
+    cacheDictionaryEnableLogging: typeof cacheDictionaryEnableLogging;
+    cacheDictionaryGetLog: typeof cacheDictionaryGetLog;
+} = {
     cacheDictionaryEnableLogging,
     cacheDictionaryGetLog,
 };
diff --git a/packages/cspell-dictionary/src/util/AutoCache.ts b/packages/cspell-dictionary/src/util/AutoCache.ts
index 090c28954b76..13eee8d2b764 100644
--- a/packages/cspell-dictionary/src/util/AutoCache.ts
+++ b/packages/cspell-dictionary/src/util/AutoCache.ts
@@ -68,7 +68,7 @@ export function createCache01<R>(size: number): Cache01<R> {
     return new Cache01Map(size);
 }
 
-export function autoCache<R>(fn: (p: string) => R, size = CACHE_SIZE): AutoCache<R> {
+export function autoCache<R>(fn: (p: string) => R, size: number = CACHE_SIZE): AutoCache<R> {
     const cache = createCache01<R>(size);
 
     const ac: AutoCache<R> = get as AutoCache<R>;
diff --git a/packages/cspell-dictionary/src/util/AutoResolve.ts b/packages/cspell-dictionary/src/util/AutoResolve.ts
index c23d249c8930..6fe0e5a73e55 100644
--- a/packages/cspell-dictionary/src/util/AutoResolve.ts
+++ b/packages/cspell-dictionary/src/util/AutoResolve.ts
@@ -7,7 +7,7 @@ export function autoResolve<K, V>(map: Map<K, V>, key: K, resolve: (k: K) => V):
 }
 
 export class AutoResolveCache<K, V> {
-    readonly map = new Map<K, V>();
+    readonly map: Map<K, V> = new Map<K, V>();
 
     get(k: K): V | undefined;
     get(k: K, resolve: (k: K) => V): V;
@@ -39,7 +39,7 @@ export function autoResolveWeak<K extends object, V>(map: WeakMap<K, V>, key: K,
 }
 
 export class AutoResolveWeakCache<K extends object, V> {
-    readonly map = new WeakMap<K, V>();
+    readonly map: WeakMap<K, V> = new WeakMap<K, V>();
 
     get(k: K): V | undefined;
     get(k: K, resolve: (k: K) => V): V;
diff --git a/packages/cspell-dictionary/src/util/repMap.ts b/packages/cspell-dictionary/src/util/repMap.ts
index 991af3379790..fa3a2459984c 100644
--- a/packages/cspell-dictionary/src/util/repMap.ts
+++ b/packages/cspell-dictionary/src/util/repMap.ts
@@ -183,7 +183,13 @@ function addToTrie(node: RepTrieNode, match: string, replaceWith: string) {
     node.rep = [...s];
 }
 
-export const __testing__ = {
+export const __testing__: {
+    charsetToRepMap: typeof charsetToRepMapRegEx;
+    createMapperRegExp: typeof createMapperRegExp;
+    createTrie: typeof createTrie;
+    calcAllEdits: typeof calcAllEdits;
+    applyEdits: typeof applyEdits;
+} = {
     charsetToRepMap: charsetToRepMapRegEx,
     createMapperRegExp,
     createTrie,
diff --git a/packages/cspell-dictionary/src/util/simpleCache.ts b/packages/cspell-dictionary/src/util/simpleCache.ts
index 57349e60e1b6..3f256eaf5fb0 100644
--- a/packages/cspell-dictionary/src/util/simpleCache.ts
+++ b/packages/cspell-dictionary/src/util/simpleCache.ts
@@ -27,7 +27,7 @@ export class SimpleWeakCache<K extends object, T> {
         return undefined;
     }
 
-    set(key: K, value: T) {
+    set(key: K, value: T): void {
         this._set(key, { v: value });
     }
 
@@ -110,7 +110,7 @@ export class SimpleCache<K, T> {
         return undefined;
     }
 
-    set(key: K, value: T) {
+    set(key: K, value: T): void {
         this._set(key, { v: value });
     }
 
diff --git a/packages/cspell-eslint-plugin/fixtures/simple/tsconfig.json b/packages/cspell-eslint-plugin/fixtures/simple/tsconfig.json
index 6f81f5afdaed..65fe461958eb 100644
--- a/packages/cspell-eslint-plugin/fixtures/simple/tsconfig.json
+++ b/packages/cspell-eslint-plugin/fixtures/simple/tsconfig.json
@@ -1,7 +1,6 @@
 {
-    "extends": "@tsconfig/node18/tsconfig.json",
+    "extends": "@tsconfig/node20/tsconfig.json",
     "compilerOptions": {
-        "allowJs": true,
         "rootDir": ".",
         "outDir": "dist"
     },
diff --git a/packages/cspell-eslint-plugin/scripts/build-options-schema.mjs b/packages/cspell-eslint-plugin/scripts/build-options-schema.mjs
index 5e64357e6464..2722b88f2d27 100755
--- a/packages/cspell-eslint-plugin/scripts/build-options-schema.mjs
+++ b/packages/cspell-eslint-plugin/scripts/build-options-schema.mjs
@@ -62,7 +62,13 @@ async function run() {
 
     await writeFile(
         new URL(outFileTs, rootUrl),
-        '// NOTICE: This file is autogenerated.\nexport const optionsSchema = ' + schemaString,
+        `\
+// NOTICE: This file is autogenerated.
+
+import type { Rule } from 'eslint';
+
+export const optionsSchema: Rule.RuleMetaData['schema'] = ${schemaString}\
+`,
     );
 }
 
diff --git a/packages/cspell-eslint-plugin/src/common/logger.cts b/packages/cspell-eslint-plugin/src/common/logger.cts
index cf6c599b530f..813fcb30f7b5 100644
--- a/packages/cspell-eslint-plugin/src/common/logger.cts
+++ b/packages/cspell-eslint-plugin/src/common/logger.cts
@@ -14,6 +14,8 @@ export interface LoggerOptions {
     useAsync?: boolean;
 }
 
+type Log = (...p: Parameters<typeof console.log>) => void;
+
 export class Logger {
     readonly logFile: string;
     readonly cwd: string;
@@ -39,7 +41,7 @@ export class Logger {
             : fs.appendFileSync(this.logFile, message);
     }
 
-    log = this._log.bind(this);
+    log: Log = this._log.bind(this);
 }
 
 let logger: Logger | undefined;
diff --git a/packages/cspell-eslint-plugin/src/generated/schema.cts b/packages/cspell-eslint-plugin/src/generated/schema.cts
index 7223495471e9..ef5bbcf25e6f 100644
--- a/packages/cspell-eslint-plugin/src/generated/schema.cts
+++ b/packages/cspell-eslint-plugin/src/generated/schema.cts
@@ -1,5 +1,8 @@
 // NOTICE: This file is autogenerated.
-export const optionsSchema = {
+
+import type { Rule } from 'eslint';
+
+export const optionsSchema: Rule.RuleMetaData['schema'] = {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "additionalProperties": false,
   "definitions": {},
diff --git a/packages/cspell-eslint-plugin/src/plugin/cspell-eslint-plugin.cts b/packages/cspell-eslint-plugin/src/plugin/cspell-eslint-plugin.cts
index 7e8400a72d38..c327afeb8f75 100644
--- a/packages/cspell-eslint-plugin/src/plugin/cspell-eslint-plugin.cts
+++ b/packages/cspell-eslint-plugin/src/plugin/cspell-eslint-plugin.cts
@@ -179,4 +179,4 @@ export const configs: ESlintPlugin['configs'] = {
     'recommended-legacy': recommended,
 };
 
-export const plugin = { rules, configs, meta } satisfies ESlintPlugin;
+export const plugin: ESlintPlugin = { rules, configs, meta } satisfies ESlintPlugin;
diff --git a/packages/cspell-eslint-plugin/src/plugin/recommended.cts b/packages/cspell-eslint-plugin/src/plugin/recommended.cts
index 97b1421aa965..11a992467c10 100644
--- a/packages/cspell-eslint-plugin/src/plugin/recommended.cts
+++ b/packages/cspell-eslint-plugin/src/plugin/recommended.cts
@@ -11,5 +11,5 @@ const config: Linter.Config = {
     },
 };
 
-export const plugins = config.plugins;
-export const rules = config.rules;
+export const plugins: Linter.Config['plugins'] = config.plugins;
+export const rules: Linter.Config['rules'] = config.rules;
diff --git a/packages/cspell-eslint-plugin/src/spellCheckAST/walkTree.cts b/packages/cspell-eslint-plugin/src/spellCheckAST/walkTree.cts
index a086350c194a..9289f5b36b7c 100644
--- a/packages/cspell-eslint-plugin/src/spellCheckAST/walkTree.cts
+++ b/packages/cspell-eslint-plugin/src/spellCheckAST/walkTree.cts
@@ -6,7 +6,7 @@ import type { ASTPath, ASTPathElement, Key } from './ASTPath.js' with { 'resolut
 
 const debugMode = false;
 
-export function walkTree(node: ASTNode, enter: (path: ASTPath) => void) {
+export function walkTree(node: ASTNode, enter: (path: ASTPath) => void): void {
     const visited = new Set<object>();
 
     let pathNode: ASTPath | undefined = undefined;
diff --git a/packages/cspell-filetypes/src/filetypes.ts b/packages/cspell-filetypes/src/filetypes.ts
index 97c199d45726..9d902d7b019a 100644
--- a/packages/cspell-filetypes/src/filetypes.ts
+++ b/packages/cspell-filetypes/src/filetypes.ts
@@ -6,9 +6,9 @@ type ExtensionToFileTypeIdMap = Map<string, string[]>;
 
 const binaryFormatIds = definitions.filter((d) => d.format === 'Binary').map((d) => d.id);
 
-export const binaryLanguages = new Set(['binary', 'image', 'video', 'fonts', ...binaryFormatIds]);
+export const binaryLanguages: Set<string> = new Set(['binary', 'image', 'video', 'fonts', ...binaryFormatIds]);
 
-export const generatedFiles = new Set([
+export const generatedFiles: Set<string> = new Set([
     ...binaryLanguages,
     'map',
     'lock',
diff --git a/packages/cspell-gitignore/src/GitIgnoreFile.ts b/packages/cspell-gitignore/src/GitIgnoreFile.ts
index b88ef0cc6316..d947d3e0472f 100644
--- a/packages/cspell-gitignore/src/GitIgnoreFile.ts
+++ b/packages/cspell-gitignore/src/GitIgnoreFile.ts
@@ -149,6 +149,8 @@ function globToString(glob: GlobPatternWithRoot, relativeToDir: string | URL): s
     return (base ? base + '/' : '') + glob.glob;
 }
 
-export const __testing__ = {
+export const __testing__: {
+    mustBeHierarchical: typeof mustBeHierarchical;
+} = {
     mustBeHierarchical,
 };
diff --git a/packages/cspell-gitignore/src/helpers.ts b/packages/cspell-gitignore/src/helpers.ts
index ff195e522da0..902ee5d66168 100644
--- a/packages/cspell-gitignore/src/helpers.ts
+++ b/packages/cspell-gitignore/src/helpers.ts
@@ -130,7 +130,7 @@ const defaultHelper = factoryPathHelper(path);
  * @returns root directory
  * @deprecated to be removed in the next major version.
  */
-export const directoryRoot = defaultHelper.directoryRoot;
+export const directoryRoot: (directory: string) => string = defaultHelper.directoryRoot;
 
 /**
  * Checks to see if the child directory is nested under the parent directory.
@@ -139,7 +139,7 @@ export const directoryRoot = defaultHelper.directoryRoot;
  * @returns true iff child is a child of parent.
  * @deprecated to be removed in the next major version.
  */
-export const isParentOf = defaultHelper.isParentOf;
+export const isParentOf: (parent: string, child: string) => boolean = defaultHelper.isParentOf;
 
 /**
  * Check to see if a parent directory contains a child directory.
@@ -148,7 +148,7 @@ export const isParentOf = defaultHelper.isParentOf;
  * @returns true iff child is the same as the parent or nested in the parent.
  * @deprecated to be removed in the next major version.
  */
-export const contains = defaultHelper.contains;
+export const contains: (parent: string, child: string) => boolean = defaultHelper.contains;
 
 /**
  * Make a path relative to another if the other is a parent.
@@ -157,7 +157,7 @@ export const contains = defaultHelper.contains;
  * @returns the normalized relative path or undefined if rootPath is not a parent.
  * @deprecated to be removed in the next major version.
  */
-export const makeRelativeTo = defaultHelper.makeRelativeTo;
+export const makeRelativeTo: (path: string, rootPath: string) => string | undefined = defaultHelper.makeRelativeTo;
 
 /**
  * Normalize a path to have only forward slashes.
@@ -165,7 +165,7 @@ export const makeRelativeTo = defaultHelper.makeRelativeTo;
  * @returns a normalized string.
  * @deprecated to be removed in the next major version.
  */
-export const normalizePath = defaultHelper.normalizePath;
+export const normalizePath: (path: string) => string = defaultHelper.normalizePath;
 
 export const DefaultPathHelper: PathHelper = {
     directoryRoot,
diff --git a/packages/cspell-glob/tsconfig.json b/packages/cspell-glob/tsconfig.json
index 30891b89672c..4285e1c25b7e 100644
--- a/packages/cspell-glob/tsconfig.json
+++ b/packages/cspell-glob/tsconfig.json
@@ -1,6 +1,7 @@
 {
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
         "rootDir": "src",
         "outDir": "dist",
         "types": ["node"]
diff --git a/packages/cspell-grammar/package.json b/packages/cspell-grammar/package.json
index b5637b83c49e..a78acb37b822 100644
--- a/packages/cspell-grammar/package.json
+++ b/packages/cspell-grammar/package.json
@@ -66,12 +66,12 @@
   ],
   "scripts": {
     "clean": "shx rm -rf dist temp coverage \"*.tsbuildInfo\"",
-    "build": "tsc -b . -f",
+    "build": "tsc -p .",
     "clean-build": "pnpm run clean && pnpm run build",
     "coverage": "vitest run --coverage",
     "test:watch": "vitest",
     "test": "vitest run",
-    "watch": "tsc -b . -w -f"
+    "watch": "tsc -p . -w"
   },
   "repository": {
     "type": "git",
diff --git a/packages/cspell-grammar/tsconfig.esm.json b/packages/cspell-grammar/tsconfig.esm.json
deleted file mode 100644
index 54835c32c5c8..000000000000
--- a/packages/cspell-grammar/tsconfig.esm.json
+++ /dev/null
@@ -1,11 +0,0 @@
-{
-    "extends": "../../tsconfig.esm.json",
-    "compilerOptions": {
-        "composite": true,
-        "tsBuildInfoFile": "dist/compile.esm.tsbuildInfo",
-        "rootDir": "src",
-        "outDir": "dist",
-        "types": ["node"]
-    },
-    "include": ["src"]
-}
diff --git a/packages/cspell-grammar/tsconfig.json b/packages/cspell-grammar/tsconfig.json
index 635469e894c2..4285e1c25b7e 100644
--- a/packages/cspell-grammar/tsconfig.json
+++ b/packages/cspell-grammar/tsconfig.json
@@ -1,4 +1,10 @@
 {
-    "files": [],
-    "references": [{ "path": "./tsconfig.esm.json" }]
+    "extends": "../../tsconfig.esm.json",
+    "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
+        "rootDir": "src",
+        "outDir": "dist",
+        "types": ["node"]
+    },
+    "include": ["src"]
 }
diff --git a/packages/cspell-lib/tsconfig.json b/packages/cspell-lib/tsconfig.json
index 30891b89672c..4285e1c25b7e 100644
--- a/packages/cspell-lib/tsconfig.json
+++ b/packages/cspell-lib/tsconfig.json
@@ -1,6 +1,7 @@
 {
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
         "rootDir": "src",
         "outDir": "dist",
         "types": ["node"]
diff --git a/packages/cspell-pipe/tsconfig.json b/packages/cspell-pipe/tsconfig.json
index a8f3aeb00246..174baffc7365 100644
--- a/packages/cspell-pipe/tsconfig.json
+++ b/packages/cspell-pipe/tsconfig.json
@@ -2,6 +2,7 @@
     "$schema": "https://json.schemastore.org/tsconfig",
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
         "rootDir": "src",
         "outDir": "dist",
         "types": ["node"]
diff --git a/packages/cspell-tools/tsconfig.json b/packages/cspell-tools/tsconfig.json
index 5d3fa446c62e..a6a1f470c0db 100644
--- a/packages/cspell-tools/tsconfig.json
+++ b/packages/cspell-tools/tsconfig.json
@@ -1,7 +1,8 @@
 {
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
-        "exactOptionalPropertyTypes": false, // make this true
+        "declaration": true,
+        "isolatedDeclarations": false,
         "types": ["node"],
         "outDir": "dist"
     },
diff --git a/packages/cspell/tsconfig.json b/packages/cspell/tsconfig.json
index 5d6f73157382..c42b8964cb12 100644
--- a/packages/cspell/tsconfig.json
+++ b/packages/cspell/tsconfig.json
@@ -1,6 +1,7 @@
 {
     "extends": "../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false, // make this true
         "rootDir": "src/app",
         "outDir": "dist/esm",
         "target": "es2022",
diff --git a/packages/dynamic-import/src/test/helper.ts b/packages/dynamic-import/src/test/helper.ts
index 260db9240e97..d1d40ce88333 100644
--- a/packages/dynamic-import/src/test/helper.ts
+++ b/packages/dynamic-import/src/test/helper.ts
@@ -14,7 +14,7 @@ async function callHello(file: string | URL, paths?: string | URL | (string | UR
     return (await dynamicImport<Hello>(file, paths)).sayHello('Bob.');
 }
 
-export function test() {
+export function test(): Promise<string[]> {
     return Promise.all([
         callHello(pathToFileURL('./fixtures/hello_world.mjs')),
         callHello(path.resolve('./fixtures/hello_world.mjs')),
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 76fbbeae2ba2..187b1b28bdc2 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -47,9 +47,9 @@ importers:
       '@swc-node/register':
         specifier: ^1.10.10
         version: 1.10.10(@swc/core@1.7.26)(@swc/types@0.1.12)(typescript@5.8.3)
-      '@tsconfig/node18':
-        specifier: ^18.2.4
-        version: 18.2.4
+      '@tsconfig/node20':
+        specifier: ^20.1.5
+        version: 20.1.5
       '@types/node':
         specifier: ^20.19.0
         version: 20.19.0
@@ -4176,8 +4176,8 @@ packages:
   '@tsconfig/node16@1.0.4':
     resolution: {integrity: sha512-vxhUy4J8lyeyinH7Azl1pdd43GJhZH/tP2weN8TntQblOY+A0XbT8DJk1/oCPuOOyg/Ja757rG0CgHcWC8OfMA==}
 
-  '@tsconfig/node18@18.2.4':
-    resolution: {integrity: sha512-5xxU8vVs9/FNcvm3gE07fPbn9tl6tqGGWA9tSlwsUEkBxtRnTsNmwrV8gasZ9F/EobaSv9+nu8AxUKccw77JpQ==}
+  '@tsconfig/node20@20.1.5':
+    resolution: {integrity: sha512-Vm8e3WxDTqMGPU4GATF9keQAIy1Drd7bPwlgzKJnZtoOsTm1tduUTbDjg0W5qERvGuxPI2h9RbMufH0YdfBylA==}
 
   '@tufjs/canonical-json@2.0.0':
     resolution: {integrity: sha512-yVtV8zsdo8qFHe+/3kw81dSLyF7D576A5cCFCi4X7B39tWT7SekaEFUnvnWJHz+9qO7qJTah1JbrDjWKqFtdWA==}
@@ -14728,7 +14728,7 @@ snapshots:
 
   '@tsconfig/node16@1.0.4': {}
 
-  '@tsconfig/node18@18.2.4': {}
+  '@tsconfig/node20@20.1.5': {}
 
   '@tufjs/canonical-json@2.0.0': {}
 
diff --git a/rfc/drafts/cspell-extensions/tsconfig.json b/rfc/drafts/cspell-extensions/tsconfig.json
index 080cf2ff110d..9169a91fbfcd 100644
--- a/rfc/drafts/cspell-extensions/tsconfig.json
+++ b/rfc/drafts/cspell-extensions/tsconfig.json
@@ -1,6 +1,6 @@
 {
     "$schema": "https://json.schemastore.org/tsconfig",
-    "extends": "@tsconfig/node18/tsconfig.json",
+    "extends": "@tsconfig/node20/tsconfig.json",
     "compilerOptions": {
         "outDir": "dist"
     },
diff --git a/rfc/rfc-0001 suggestions/src/config-definitions.ts b/rfc/rfc-0001 suggestions/src/config-definitions.ts
index c49872b7a9f2..b784c79a9bcb 100644
--- a/rfc/rfc-0001 suggestions/src/config-definitions.ts	
+++ b/rfc/rfc-0001 suggestions/src/config-definitions.ts	
@@ -69,7 +69,11 @@ const exampleTerms: Terms = {
 
 /*********************/
 
-export const __testing__ = {
+export const __testing__: {
+    exampleDef: DictionaryDefinitionSuggestions;
+    exampleFlagWords: NewBaseSettings;
+    exampleTerms: Terms;
+} = {
     exampleDef,
     exampleFlagWords,
     exampleTerms,
diff --git a/test-packages/cspell-dictionary/test-cspell-dictionary/tsconfig.json b/test-packages/cspell-dictionary/test-cspell-dictionary/tsconfig.json
index 104cfab402ee..31e1107ed780 100644
--- a/test-packages/cspell-dictionary/test-cspell-dictionary/tsconfig.json
+++ b/test-packages/cspell-dictionary/test-cspell-dictionary/tsconfig.json
@@ -5,6 +5,7 @@
         "moduleResolution": "NodeNext",
         "composite": true,
         "tsBuildInfoFile": "dist/compile.esm.tsbuildInfo",
+        "isolatedDeclarations": false,
         "rootDir": "src",
         "outDir": "dist/esm",
         "types": ["node"]
diff --git a/test-packages/cspell-gitignore/test-cspell-gitignore/tsconfig.json b/test-packages/cspell-gitignore/test-cspell-gitignore/tsconfig.json
index 104cfab402ee..31e1107ed780 100644
--- a/test-packages/cspell-gitignore/test-cspell-gitignore/tsconfig.json
+++ b/test-packages/cspell-gitignore/test-cspell-gitignore/tsconfig.json
@@ -5,6 +5,7 @@
         "moduleResolution": "NodeNext",
         "composite": true,
         "tsBuildInfoFile": "dist/compile.esm.tsbuildInfo",
+        "isolatedDeclarations": false,
         "rootDir": "src",
         "outDir": "dist/esm",
         "types": ["node"]
diff --git a/test-packages/cspell-glob/test-cspell-glob/src/index.ts b/test-packages/cspell-glob/test-cspell-glob/src/index.ts
index 0c98a8fb8b28..6730e2911b2f 100644
--- a/test-packages/cspell-glob/test-cspell-glob/src/index.ts
+++ b/test-packages/cspell-glob/test-cspell-glob/src/index.ts
@@ -2,6 +2,6 @@ import { resolve } from 'node:path';
 
 import * as glob from 'cspell-glob';
 
-export function run(filename: string) {
+export function run(filename: string): string {
     return glob.fileOrGlobToGlob(filename, resolve('.')).glob;
 }
diff --git a/test-packages/cspell-io/test-cspell-io/src/index.ts b/test-packages/cspell-io/test-cspell-io/src/index.ts
index 910be247b672..772b5d544f16 100644
--- a/test-packages/cspell-io/test-cspell-io/src/index.ts
+++ b/test-packages/cspell-io/test-cspell-io/src/index.ts
@@ -1,6 +1,6 @@
 import { readFileText } from 'cspell-io';
 
-export function run(file: string) {
+export function run(file: string): Promise<string> {
     return read(file);
 }
 
diff --git a/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/src/index.ts b/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/src/index.ts
index 72927d7d22cb..34f793956ae7 100644
--- a/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/src/index.ts
+++ b/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/src/index.ts
@@ -26,7 +26,7 @@ async function checkFile(filename: string) {
     return result.issues;
 }
 
-export async function run() {
+export async function run(): Promise<void> {
     console.log(`Start: ${new Date().toISOString()}`);
     const r = await checkSpelling('These are my coztom wordz.');
     console.log(`End: ${new Date().toISOString()}`);
diff --git a/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/tsconfig.json b/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/tsconfig.json
index 820a931c2d8e..b2de342a5336 100644
--- a/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/tsconfig.json
+++ b/test-packages/cspell-lib/test-cspell-esbuild-cjs/source/tsconfig.json
@@ -1,8 +1,7 @@
 {
     "extends": "../../../../tsconfig.esm.json",
     "compilerOptions": {
-        "outDir": "dist",
-        "allowJs": true
+        "outDir": "dist"
     },
     "include": ["src"]
 }
diff --git a/test-packages/cspell-lib/test-cspell-lib-esm/src/index.ts b/test-packages/cspell-lib/test-cspell-lib-esm/src/index.ts
index a375aee39790..88a8da310def 100644
--- a/test-packages/cspell-lib/test-cspell-lib-esm/src/index.ts
+++ b/test-packages/cspell-lib/test-cspell-lib-esm/src/index.ts
@@ -14,7 +14,7 @@ async function checkSpelling(phrase: string) {
     return result.issues;
 }
 
-export async function run() {
+export async function run(): Promise<void> {
     console.log(`Start: ${new Date().toISOString()}`);
     const r = await checkSpelling('These are my coztom wordz.');
     console.log(`End: ${new Date().toISOString()}`);
diff --git a/test-packages/cspell-lib/test-cspell-lib-esm/tsconfig.json b/test-packages/cspell-lib/test-cspell-lib-esm/tsconfig.json
index c1be12d6b38f..1e61513b1328 100644
--- a/test-packages/cspell-lib/test-cspell-lib-esm/tsconfig.json
+++ b/test-packages/cspell-lib/test-cspell-lib-esm/tsconfig.json
@@ -1,6 +1,7 @@
 {
     "extends": "../../../tsconfig.esm.json",
     "compilerOptions": {
+        "isolatedDeclarations": false,
         "outDir": "dist"
     },
     "include": ["src"]
diff --git a/test-packages/cspell-lib/test-cspell-lib-rollup/plugin/tsconfig.json b/test-packages/cspell-lib/test-cspell-lib-rollup/plugin/tsconfig.json
index 820a931c2d8e..b2de342a5336 100644
--- a/test-packages/cspell-lib/test-cspell-lib-rollup/plugin/tsconfig.json
+++ b/test-packages/cspell-lib/test-cspell-lib-rollup/plugin/tsconfig.json
@@ -1,8 +1,7 @@
 {
     "extends": "../../../../tsconfig.esm.json",
     "compilerOptions": {
-        "outDir": "dist",
-        "allowJs": true
+        "outDir": "dist"
     },
     "include": ["src"]
 }
diff --git a/test-packages/cspell-lib/test-cspell-lib-rollup/src/index.ts b/test-packages/cspell-lib/test-cspell-lib-rollup/src/index.ts
index a375aee39790..88a8da310def 100644
--- a/test-packages/cspell-lib/test-cspell-lib-rollup/src/index.ts
+++ b/test-packages/cspell-lib/test-cspell-lib-rollup/src/index.ts
@@ -14,7 +14,7 @@ async function checkSpelling(phrase: string) {
     return result.issues;
 }
 
-export async function run() {
+export async function run(): Promise<void> {
     console.log(`Start: ${new Date().toISOString()}`);
     const r = await checkSpelling('These are my coztom wordz.');
     console.log(`End: ${new Date().toISOString()}`);
diff --git a/test-packages/cspell-lib/test-cspell-lib-rollup/tsconfig.json b/test-packages/cspell-lib/test-cspell-lib-rollup/tsconfig.json
index 86731d264737..c1be12d6b38f 100644
--- a/test-packages/cspell-lib/test-cspell-lib-rollup/tsconfig.json
+++ b/test-packages/cspell-lib/test-cspell-lib-rollup/tsconfig.json
@@ -1,8 +1,7 @@
 {
     "extends": "../../../tsconfig.esm.json",
     "compilerOptions": {
-        "outDir": "dist",
-        "allowJs": true
+        "outDir": "dist"
     },
     "include": ["src"]
 }
diff --git a/test-packages/cspell-lib/test-cspell-lib-tsup/tsconfig.json b/test-packages/cspell-lib/test-cspell-lib-tsup/tsconfig.json
index 36b906dba3d7..92cd071c6af5 100644
--- a/test-packages/cspell-lib/test-cspell-lib-tsup/tsconfig.json
+++ b/test-packages/cspell-lib/test-cspell-lib-tsup/tsconfig.json
@@ -2,8 +2,7 @@
     "extends": "../../../tsconfig.esm.json",
     "compilerOptions": {
         "noEmit": true,
-        "outDir": "dist",
-        "allowJs": true
+        "outDir": "dist"
     },
     "include": ["src"]
 }
diff --git a/test-packages/cspell-service-bus/test-cspell-service-bus-esm/src/index.mts b/test-packages/cspell-service-bus/test-cspell-service-bus-esm/src/index.mts
index 77db1e118338..fe7bcabd8cb2 100644
--- a/test-packages/cspell-service-bus/test-cspell-service-bus-esm/src/index.mts
+++ b/test-packages/cspell-service-bus/test-cspell-service-bus-esm/src/index.mts
@@ -4,6 +4,7 @@ import { assert } from 'node:console';
 import type {
     Dispatcher,
     Handler,
+    RequestFactory,
     ServiceBus,
     ServiceRequest,
     ServiceRequestFactoryRequestType,
@@ -34,15 +35,23 @@ function calcFib(request: FibRequest): ServiceResponse<number> {
 }
 
 const TypeRequestFib = 'Computations:calc-fib' as const;
-export const FibRequestFactory = requestFactory<typeof TypeRequestFib, { readonly fib: number }, number>(
-    TypeRequestFib,
-);
+export const FibRequestFactory: RequestFactory<
+    'Computations:calc-fib',
+    {
+        readonly fib: number;
+    },
+    number
+> = requestFactory<typeof TypeRequestFib, { readonly fib: number }, number>(TypeRequestFib);
 type FibRequestFactory = typeof FibRequestFactory;
 type FibRequest = ServiceRequestFactoryRequestType<FibRequestFactory>;
 
-export const StringLengthRequestFactory = requestFactory<'calc-string-length', { readonly str: string }, number>(
+export const StringLengthRequestFactory: RequestFactory<
     'calc-string-length',
-);
+    {
+        readonly str: string;
+    },
+    number
+> = requestFactory<'calc-string-length', { readonly str: string }, number>('calc-string-length');
 
 export class StringToUpperRequest extends ServiceRequestCls<'toUpper', { readonly str: string }, string> {
     constructor(readonly str: string) {
diff --git a/test-packages/cspell-trie-lib/test-cspell-trie-lib/src/index.ts b/test-packages/cspell-trie-lib/test-cspell-trie-lib/src/index.ts
index 8e29e6df6ee3..82879cc74213 100644
--- a/test-packages/cspell-trie-lib/test-cspell-trie-lib/src/index.ts
+++ b/test-packages/cspell-trie-lib/test-cspell-trie-lib/src/index.ts
@@ -10,7 +10,7 @@ three
     return parseDictionary(words);
 }
 
-export function run() {
+export function run(): boolean {
     const dict = createDictionary();
     return dict.has('two');
 }
diff --git a/test-packages/cspell-trie-lib/test-cspell-trie-lib/tsconfig.json b/test-packages/cspell-trie-lib/test-cspell-trie-lib/tsconfig.json
index 4fe38af2e39f..478e28cd1ae8 100644
--- a/test-packages/cspell-trie-lib/test-cspell-trie-lib/tsconfig.json
+++ b/test-packages/cspell-trie-lib/test-cspell-trie-lib/tsconfig.json
@@ -3,6 +3,7 @@
     "compilerOptions": {
         "module": "NodeNext",
         "moduleResolution": "NodeNext",
+        "isolatedDeclarations": false,
         "rootDir": "src",
         "outDir": "dist/esm",
         "types": ["node"]
diff --git a/test-packages/cspell/test-cspell-esbuild-cjs/source/src/index.ts b/test-packages/cspell/test-cspell-esbuild-cjs/source/src/index.ts
index a83ca5e87e41..2f25e4dc0c7c 100644
--- a/test-packages/cspell/test-cspell-esbuild-cjs/source/src/index.ts
+++ b/test-packages/cspell/test-cspell-esbuild-cjs/source/src/index.ts
@@ -7,7 +7,7 @@ async function checkFile(fileGlob: string) {
     return result;
 }
 
-export async function run() {
+export async function run(): Promise<void> {
     const fileGlob = process.argv[2];
     assert(fileGlob, 'File to check expected.');
 
diff --git a/test-packages/cspell/test-cspell-esbuild-cjs/source/tsconfig.json b/test-packages/cspell/test-cspell-esbuild-cjs/source/tsconfig.json
index 820a931c2d8e..b2de342a5336 100644
--- a/test-packages/cspell/test-cspell-esbuild-cjs/source/tsconfig.json
+++ b/test-packages/cspell/test-cspell-esbuild-cjs/source/tsconfig.json
@@ -1,8 +1,7 @@
 {
     "extends": "../../../../tsconfig.esm.json",
     "compilerOptions": {
-        "outDir": "dist",
-        "allowJs": true
+        "outDir": "dist"
     },
     "include": ["src"]
 }
diff --git a/test-packages/examples/example-cspell-lib-single-doc/src/index.ts b/test-packages/examples/example-cspell-lib-single-doc/src/index.ts
index 6ec606e5bb43..9d9f5de570bb 100644
--- a/test-packages/examples/example-cspell-lib-single-doc/src/index.ts
+++ b/test-packages/examples/example-cspell-lib-single-doc/src/index.ts
@@ -2,7 +2,7 @@ import * as path from 'node:path';
 
 import { spellCheckDocument } from 'cspell-lib';
 
-export async function run() {
+export async function run(): Promise<void> {
     const app = path.relative(process.cwd(), process.argv[1]);
     const file = process.argv[2];
 
diff --git a/tsconfig.base.json b/tsconfig.base.json
index 9e042a604646..0cb2e7000640 100644
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -1,13 +1,14 @@
 {
     "$schema": "https://json.schemastore.org/tsconfig",
     "display": "CSpell Monorepo Base Config",
-    "extends": "@tsconfig/node18/tsconfig.json",
+    "extends": "@tsconfig/node20/tsconfig.json",
     "compilerOptions": {
         "alwaysStrict": true,
         "declaration": true,
         "declarationMap": true,
         "esModuleInterop": true,
         "exactOptionalPropertyTypes": true,
+        "isolatedDeclarations": true,
         "moduleResolution": "node16",
         "newLine": "LF",
         "noImplicitAny": true,
diff --git a/tsconfig.esm.json b/tsconfig.esm.json
index a4713ead8a12..6430c15403e7 100644
--- a/tsconfig.esm.json
+++ b/tsconfig.esm.json
@@ -3,7 +3,7 @@
     "extends": "./tsconfig.next.json",
     "compilerOptions": {
         "skipLibCheck": true,
-        "module": "Node16",
+        "module": "node18",
         "moduleResolution": "Node16"
     },
     "include": ["src"]