diff --git a/src/BaseAgent.ts b/src/BaseAgent.ts
index 49834eb5..826207e2 100644
--- a/src/BaseAgent.ts
+++ b/src/BaseAgent.ts
@@ -2,7 +2,7 @@ import { AsyncLocalStorage } from 'node:async_hooks';
 
 import { Agent, Dispatcher } from 'undici';
 
-import { FetchOpaque } from './FetchOpaqueInterceptor.js';
+import type { FetchOpaque } from './FetchOpaqueInterceptor.js';
 
 export interface BaseAgentOptions extends Agent.Options {
   opaqueLocalStorage?: AsyncLocalStorage<FetchOpaque>;
diff --git a/src/FormData.ts b/src/FormData.ts
index 3a3cd810..d67d6844 100644
--- a/src/FormData.ts
+++ b/src/FormData.ts
@@ -6,7 +6,7 @@ import _FormData from 'form-data';
 const NON_ASCII_RE = /[^\x00-\x7F]/i;
 
 export class FormData extends _FormData {
-  _getContentDisposition(value: any, options: any) {
+  _getContentDisposition(value: any, options: any): string | undefined {
     // support non-ascii filename
     // https://github.com/form-data/form-data/pull/571
     let filename;
diff --git a/src/HttpAgent.ts b/src/HttpAgent.ts
index cbe6450b..dc04368c 100644
--- a/src/HttpAgent.ts
+++ b/src/HttpAgent.ts
@@ -1,9 +1,11 @@
 import dns from 'node:dns';
-import { LookupFunction, isIP } from 'node:net';
+import { isIP } from 'node:net';
+import type { LookupFunction } from 'node:net';
 
 import { Agent, Dispatcher, buildConnector } from 'undici';
 
-import { BaseAgent, BaseAgentOptions } from './BaseAgent.js';
+import { BaseAgent } from './BaseAgent.js';
+import type { BaseAgentOptions } from './BaseAgent.js';
 
 export type CheckAddressFunction = (ip: string, family: number | string, hostname: string) => boolean;
 
diff --git a/src/HttpClient.ts b/src/HttpClient.ts
index fda8912f..268eb1ee 100644
--- a/src/HttpClient.ts
+++ b/src/HttpClient.ts
@@ -1,8 +1,9 @@
 import diagnosticsChannel from 'node:diagnostics_channel';
+import type { Channel } from 'node:diagnostics_channel';
 import { EventEmitter } from 'node:events';
 import { createReadStream } from 'node:fs';
 import { STATUS_CODES } from 'node:http';
-import { LookupFunction } from 'node:net';
+import type { LookupFunction } from 'node:net';
 import { basename } from 'node:path';
 import { performance } from 'node:perf_hooks';
 import querystring from 'node:querystring';
@@ -23,13 +24,14 @@ import { request as undiciRequest, Dispatcher, Agent, getGlobalDispatcher, Pool
 import undiciSymbols from 'undici/lib/core/symbols.js';
 
 import { initDiagnosticsChannel } from './diagnosticsChannel.js';
-import { FetchOpaque } from './FetchOpaqueInterceptor.js';
+import type { FetchOpaque } from './FetchOpaqueInterceptor.js';
 import { FormData } from './FormData.js';
-import { HttpAgent, CheckAddressFunction } from './HttpAgent.js';
+import { HttpAgent } from './HttpAgent.js';
+import type { CheckAddressFunction } from './HttpAgent.js';
 import { HttpClientConnectTimeoutError, HttpClientRequestTimeoutError } from './HttpClientError.js';
 import type { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
-import { RequestURL, RequestOptions, HttpMethod, RequestMeta } from './Request.js';
-import { RawResponseWithMeta, HttpClientResponse, SocketInfo } from './Response.js';
+import type { RequestURL, RequestOptions, HttpMethod, RequestMeta } from './Request.js';
+import type { RawResponseWithMeta, HttpClientResponse, SocketInfo } from './Response.js';
 import symbols from './symbols.js';
 import { parseJSON, digestAuthHeader, globalId, performanceTime, isReadable, updateSocketInfo } from './utils.js';
 
@@ -38,7 +40,7 @@ type UndiciRequestOption = Exists<Parameters<typeof undiciRequest>[1]>;
 type PropertyShouldBe<T, K extends keyof T, V> = Omit<T, K> & { [P in K]: V };
 type IUndiciRequestOption = PropertyShouldBe<UndiciRequestOption, 'headers', IncomingHttpHeaders>;
 
-export const PROTO_RE = /^https?:\/\//i;
+export const PROTO_RE: RegExp = /^https?:\/\//i;
 
 export interface UndiciTimingInfo {
   startTime: number;
@@ -64,7 +66,7 @@ export interface UndiciTimingInfo {
 // keep typo compatibility
 export interface UnidiciTimingInfo extends UndiciTimingInfo {}
 
-function noop() {
+function noop(): void {
   // noop
 }
 
@@ -108,11 +110,11 @@ export type ClientOptions = {
   };
 };
 
-export const VERSION = 'VERSION';
+export const VERSION: string = 'VERSION';
 // 'node-urllib/4.0.0 Node.js/18.19.0 (darwin; x64)'
-export const HEADER_USER_AGENT = `node-urllib/${VERSION} Node.js/${process.version.substring(1)} (${process.platform}; ${process.arch})`;
+export const HEADER_USER_AGENT: string = `node-urllib/${VERSION} Node.js/${process.version.substring(1)} (${process.platform}; ${process.arch})`;
 
-function getFileName(stream: Readable) {
+function getFileName(stream: Readable): string {
   const filePath: string = (stream as any).path;
   if (filePath) {
     return basename(filePath);
@@ -120,7 +122,7 @@ function getFileName(stream: Readable) {
   return '';
 }
 
-function defaultIsRetry(response: HttpClientResponse) {
+function defaultIsRetry(response: HttpClientResponse): boolean {
   return response.status >= 500;
 }
 
@@ -132,7 +134,12 @@ export type RequestContext = {
   history: string[];
 };
 
-export const channels = {
+export const channels: {
+  request: Channel;
+  response: Channel;
+  fetchRequest: Channel;
+  fetchResponse: Channel;
+} = {
   request: diagnosticsChannel.channel('urllib:request'),
   response: diagnosticsChannel.channel('urllib:response'),
   fetchRequest: diagnosticsChannel.channel('urllib:fetch:request'),
@@ -205,15 +212,15 @@ export class HttpClient extends EventEmitter {
     initDiagnosticsChannel();
   }
 
-  getDispatcher() {
+  getDispatcher(): Dispatcher {
     return this.#dispatcher ?? getGlobalDispatcher();
   }
 
-  setDispatcher(dispatcher: Dispatcher) {
+  setDispatcher(dispatcher: Dispatcher): void {
     this.#dispatcher = dispatcher;
   }
 
-  getDispatcherPoolStats() {
+  getDispatcherPoolStats(): Record<string, PoolStat> {
     const agent = this.getDispatcher();
     // origin => Pool Instance
     const clients: Map<string, WeakRef<Pool>> | undefined = Reflect.get(agent, undiciSymbols.kClients);
@@ -239,12 +246,12 @@ export class HttpClient extends EventEmitter {
     return poolStatsMap;
   }
 
-  async request<T = any>(url: RequestURL, options?: RequestOptions) {
+  async request<T = any>(url: RequestURL, options?: RequestOptions): Promise<HttpClientResponse<T>> {
     return await this.#requestInternal<T>(url, options);
   }
 
   // alias to request, keep compatible with urllib@2 HttpClient.curl
-  async curl<T = any>(url: RequestURL, options?: RequestOptions) {
+  async curl<T = any>(url: RequestURL, options?: RequestOptions): Promise<HttpClientResponse<T>> {
     return await this.request<T>(url, options);
   }
 
diff --git a/src/diagnosticsChannel.ts b/src/diagnosticsChannel.ts
index 507ad7f2..2b07a75e 100644
--- a/src/diagnosticsChannel.ts
+++ b/src/diagnosticsChannel.ts
@@ -3,7 +3,7 @@ import { Socket } from 'node:net';
 import { performance } from 'node:perf_hooks';
 import { debuglog } from 'node:util';
 
-import { DiagnosticsChannel } from 'undici';
+import type { DiagnosticsChannel } from 'undici';
 
 // eslint-disable-next-line @typescript-eslint/ban-ts-comment
 // @ts-ignore
@@ -82,7 +82,7 @@ function getRequestOpaque(request: DiagnosticsChannel.Request, kHandler?: symbol
   return handler?.opts?.opaque ?? handler?.opaque;
 }
 
-export function initDiagnosticsChannel() {
+export function initDiagnosticsChannel(): void {
   // make sure init global DiagnosticsChannel once
   if (initedDiagnosticsChannel) return;
   initedDiagnosticsChannel = true;
diff --git a/src/fetch.ts b/src/fetch.ts
index cb9c042f..e9f0acb5 100644
--- a/src/fetch.ts
+++ b/src/fetch.ts
@@ -1,17 +1,8 @@
 import { AsyncLocalStorage } from 'node:async_hooks';
 import { debuglog } from 'node:util';
 
-import {
-  fetch as UndiciFetch,
-  RequestInfo,
-  RequestInit,
-  Request,
-  Response,
-  Agent,
-  getGlobalDispatcher,
-  Pool,
-  Dispatcher,
-} from 'undici';
+import { fetch as UndiciFetch, Request, Response, Agent, getGlobalDispatcher, Pool, Dispatcher } from 'undici';
+import type { RequestInfo, RequestInit } from 'undici';
 // eslint-disable-next-line @typescript-eslint/ban-ts-comment
 // @ts-ignore
 import undiciSymbols from 'undici/lib/core/symbols.js';
@@ -19,21 +10,23 @@ import undiciSymbols from 'undici/lib/core/symbols.js';
 // @ts-ignore
 import { getResponseState } from 'undici/lib/web/fetch/response.js';
 
-import { BaseAgent, BaseAgentOptions } from './BaseAgent.js';
+import { BaseAgent } from './BaseAgent.js';
+import type { BaseAgentOptions } from './BaseAgent.js';
 import { initDiagnosticsChannel } from './diagnosticsChannel.js';
-import { FetchOpaque } from './FetchOpaqueInterceptor.js';
-import { HttpAgent, HttpAgentOptions } from './HttpAgent.js';
-import {
-  channels,
+import type { FetchOpaque } from './FetchOpaqueInterceptor.js';
+import { HttpAgent } from './HttpAgent.js';
+import type { HttpAgentOptions } from './HttpAgent.js';
+import { channels } from './HttpClient.js';
+import type {
   ClientOptions,
   PoolStat,
   RequestDiagnosticsMessage,
   ResponseDiagnosticsMessage,
   UndiciTimingInfo,
 } from './HttpClient.js';
-import { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
-import { FetchMeta, HttpMethod, RequestMeta } from './Request.js';
-import { RawResponseWithMeta, SocketInfo } from './Response.js';
+import type { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
+import type { FetchMeta, HttpMethod, RequestMeta } from './Request.js';
+import type { RawResponseWithMeta, SocketInfo } from './Response.js';
 import symbols from './symbols.js';
 import { convertHeader, globalId, performanceTime, updateSocketInfo } from './utils.js';
 
@@ -63,7 +56,7 @@ export class FetchFactory {
 
   static #instance = new FetchFactory();
 
-  setClientOptions(clientOptions: ClientOptions) {
+  setClientOptions(clientOptions: ClientOptions): void {
     let dispatcherOption: BaseAgentOptions = {
       opaqueLocalStorage: this.#opaqueLocalStorage,
     };
@@ -96,15 +89,15 @@ export class FetchFactory {
     initDiagnosticsChannel();
   }
 
-  getDispatcher() {
+  getDispatcher(): Dispatcher {
     return this.#dispatcher ?? getGlobalDispatcher();
   }
 
-  setDispatcher(dispatcher: Agent) {
+  setDispatcher(dispatcher: Agent): void {
     this.#dispatcher = dispatcher;
   }
 
-  getDispatcherPoolStats() {
+  getDispatcherPoolStats(): Record<string, PoolStat> {
     const agent = this.getDispatcher();
     // origin => Pool Instance
     const clients: Map<string, WeakRef<Pool>> | undefined = Reflect.get(agent, undiciSymbols.kClients);
@@ -130,11 +123,11 @@ export class FetchFactory {
     return poolStatsMap;
   }
 
-  static setClientOptions(clientOptions: ClientOptions) {
+  static setClientOptions(clientOptions: ClientOptions): void {
     FetchFactory.#instance.setClientOptions(clientOptions);
   }
 
-  static getDispatcherPoolStats() {
+  static getDispatcherPoolStats(): Record<string, PoolStat> {
     return FetchFactory.#instance.getDispatcherPoolStats();
   }
 
@@ -283,11 +276,11 @@ export class FetchFactory {
     return res!;
   }
 
-  static getDispatcher() {
+  static getDispatcher(): Dispatcher {
     return FetchFactory.#instance.getDispatcher();
   }
 
-  static setDispatcher(dispatcher: Agent) {
+  static setDispatcher(dispatcher: Agent): void {
     FetchFactory.#instance.setDispatcher(dispatcher);
   }
 
@@ -296,4 +289,4 @@ export class FetchFactory {
   }
 }
 
-export const fetch = FetchFactory.fetch;
+export const fetch: (input: RequestInfo, init?: UrllibRequestInit) => Promise<Response> = FetchFactory.fetch;
diff --git a/src/index.ts b/src/index.ts
index ad69b5ff..ca917e5f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -5,7 +5,8 @@ import { patchForNode16 } from './utils.js';
 patchForNode16();
 
 import { HttpClient, HEADER_USER_AGENT } from './HttpClient.js';
-import { RequestOptions, RequestURL } from './Request.js';
+import type { RequestOptions, RequestURL } from './Request.js';
+import type { HttpClientResponse } from './Response.js';
 
 let httpClient: HttpClient;
 let allowH2HttpClient: HttpClient;
@@ -63,7 +64,10 @@ interface UrllibRequestOptions extends RequestOptions {
   allowH2?: boolean;
 }
 
-export async function request<T = any>(url: RequestURL, options?: UrllibRequestOptions) {
+export async function request<T = any>(
+  url: RequestURL,
+  options?: UrllibRequestOptions,
+): Promise<HttpClientResponse<T>> {
   if (options?.socketPath) {
     let domainSocketHttpclient = domainSocketHttpClients.get<HttpClient>(options.socketPath);
     if (!domainSocketHttpclient) {
@@ -83,7 +87,7 @@ export async function request<T = any>(url: RequestURL, options?: UrllibRequestO
 // import * as urllib from 'urllib';
 // urllib.curl(url);
 // ```
-export async function curl<T = any>(url: RequestURL, options?: UrllibRequestOptions) {
+export async function curl<T = any>(url: RequestURL, options?: UrllibRequestOptions): Promise<HttpClientResponse<T>> {
   return await request<T>(url, options);
 }
 
@@ -95,25 +99,16 @@ export {
   setGlobalDispatcher,
   getGlobalDispatcher,
   Request,
-  RequestInfo,
-  RequestInit,
   Response,
-  BodyInit,
-  ResponseInit,
   Headers,
   FormData,
 } from 'undici';
+export type { RequestInfo, RequestInit, BodyInit, ResponseInit } from 'undici';
 // HttpClient2 is keep compatible with urllib@2 HttpClient2
-export {
-  HttpClient,
-  HttpClient as HttpClient2,
-  HEADER_USER_AGENT as USER_AGENT,
-  RequestDiagnosticsMessage,
-  ResponseDiagnosticsMessage,
-  ClientOptions,
-} from './HttpClient.js';
+export { HttpClient, HttpClient as HttpClient2, HEADER_USER_AGENT as USER_AGENT } from './HttpClient.js';
+export type { RequestDiagnosticsMessage, ResponseDiagnosticsMessage, ClientOptions } from './HttpClient.js';
 // RequestOptions2 is keep compatible with urllib@2 RequestOptions2
-export {
+export type {
   RequestOptions,
   RequestOptions as RequestOptions2,
   RequestURL,
@@ -122,16 +117,22 @@ export {
   FixJSONCtlChars,
 } from './Request.js';
 
-export { CheckAddressFunction } from './HttpAgent.js';
+export type { CheckAddressFunction } from './HttpAgent.js';
 
-export { SocketInfo, Timing, RawResponseWithMeta, HttpClientResponse } from './Response.js';
-export { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
+export type { SocketInfo, Timing, RawResponseWithMeta, HttpClientResponse } from './Response.js';
+export type { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
 export * from './HttpClientError.js';
 export { FetchFactory, fetch } from './fetch.js';
 export { FormData as WebFormData } from './FormData.js';
 
-export default {
+const urllib: {
+  request: typeof request;
+  curl: typeof curl;
+  USER_AGENT: string;
+} = {
   request,
   curl,
   USER_AGENT: HEADER_USER_AGENT,
 };
+
+export default urllib;
diff --git a/src/symbols.ts b/src/symbols.ts
index 7890ff41..27774b82 100644
--- a/src/symbols.ts
+++ b/src/symbols.ts
@@ -1,4 +1,25 @@
-export default {
+const symbols: {
+  kSocketId: symbol;
+  kSocketStartTime: symbol;
+  kSocketConnectedTime: symbol;
+  kSocketConnectErrorTime: symbol;
+  kSocketRequestEndTime: symbol;
+  kSocketLocalAddress: symbol;
+  kSocketLocalPort: symbol;
+  kSocketConnectHost: symbol;
+  kSocketConnectPort: symbol;
+  kSocketConnectProtocol: symbol;
+  kHandledRequests: symbol;
+  kHandledResponses: symbol;
+  kRequestSocket: symbol;
+  kRequestId: symbol;
+  kRequestStartTime: symbol;
+  kEnableRequestTiming: symbol;
+  kRequestTiming: symbol;
+  kRequestOriginalOpaque: symbol;
+  kRequestInternalOpaque: symbol;
+  kErrorSocket: symbol;
+} = {
   kSocketId: Symbol('socket id'),
   kSocketStartTime: Symbol('socket start time'),
   kSocketConnectedTime: Symbol('socket connected time'),
@@ -20,3 +41,5 @@ export default {
   kRequestInternalOpaque: Symbol('request internal opaque'),
   kErrorSocket: Symbol('socket of error'),
 };
+
+export default symbols;
diff --git a/src/utils.ts b/src/utils.ts
index 603fbe53..56999d07 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -5,9 +5,9 @@ import { Readable } from 'node:stream';
 import { ReadableStream, TransformStream } from 'node:stream/web';
 import { toUSVString } from 'node:util';
 
-import { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
+import type { IncomingHttpHeaders } from './IncomingHttpHeaders.js';
 import type { FixJSONCtlChars } from './Request.js';
-import { SocketInfo } from './Response.js';
+import type { SocketInfo } from './Response.js';
 import symbols from './symbols.js';
 
 const JSONCtlCharsMap: Record<string, string> = {
@@ -30,7 +30,7 @@ function replaceJSONCtlChars(value: string) {
   return value.replace(JSONCtlCharsRE, replaceOneChar);
 }
 
-export function parseJSON(data: string, fixJSONCtlChars?: FixJSONCtlChars) {
+export function parseJSON(data: string, fixJSONCtlChars?: FixJSONCtlChars): unknown {
   if (typeof fixJSONCtlChars === 'function') {
     data = fixJSONCtlChars(data);
   } else if (fixJSONCtlChars) {
@@ -60,7 +60,7 @@ export function parseJSON(data: string, fixJSONCtlChars?: FixJSONCtlChars) {
   return data;
 }
 
-function md5(s: string) {
+function md5(s: string): string {
   const sum = createHash('md5');
   sum.update(s, 'utf8');
   return sum.digest('hex');
@@ -70,7 +70,7 @@ const AUTH_KEY_VALUE_RE = /(\w{1,100})=["']?([^'"]+)["']?/;
 let NC = 0;
 const NC_PAD = '00000000';
 
-export function digestAuthHeader(method: string, uri: string, wwwAuthenticate: string, userpass: string) {
+export function digestAuthHeader(method: string, uri: string, wwwAuthenticate: string, userpass: string): string {
   // WWW-Authenticate: Digest realm="testrealm@host.com",
   //                       qop="auth,auth-int",
   //                       nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
@@ -139,18 +139,18 @@ export function digestAuthHeader(method: string, uri: string, wwwAuthenticate: s
 const MAX_ID_VALUE = Math.pow(2, 31) - 10;
 const globalIds: Record<string, number> = {};
 
-export function globalId(category: string) {
+export function globalId(category: string): number {
   if (!globalIds[category] || globalIds[category] >= MAX_ID_VALUE) {
     globalIds[category] = 0;
   }
   return ++globalIds[category];
 }
 
-export function performanceTime(startTime: number, now?: number) {
+export function performanceTime(startTime: number, now?: number): number {
   return Math.floor(((now ?? performance.now()) - startTime) * 1000) / 1000;
 }
 
-export function isReadable(stream: any) {
+export function isReadable(stream: any): boolean {
   if (typeof Readable.isReadable === 'function') return Readable.isReadable(stream);
   // patch from node
   // https://github.com/nodejs/node/blob/1287530385137dda1d44975063217ccf90759475/lib/internal/streams/utils.js#L119
@@ -165,7 +165,7 @@ export function isReadable(stream: any) {
   );
 }
 
-export function updateSocketInfo(socketInfo: SocketInfo, internalOpaque: any, err?: any) {
+export function updateSocketInfo(socketInfo: SocketInfo, internalOpaque: any, err?: any): void {
   const socket = internalOpaque[symbols.kRequestSocket] ?? err?.[symbols.kErrorSocket];
 
   if (socket) {
@@ -218,7 +218,7 @@ export function convertHeader(headers: Headers): IncomingHttpHeaders {
 }
 
 // support require from Node.js 16
-export function patchForNode16() {
+export function patchForNode16(): void {
   if (typeof global.ReadableStream === 'undefined') {
     // eslint-disable-next-line @typescript-eslint/ban-ts-comment
     // @ts-ignore
diff --git a/test/HttpClient.test.ts b/test/HttpClient.test.ts
index 72911b75..4b6e4c91 100644
--- a/test/HttpClient.test.ts
+++ b/test/HttpClient.test.ts
@@ -2,14 +2,15 @@ import { strict as assert } from 'node:assert';
 import dns from 'node:dns';
 import { once } from 'node:events';
 import { sensitiveHeaders, createSecureServer } from 'node:http2';
-import { AddressInfo } from 'node:net';
+import type { AddressInfo } from 'node:net';
 import { PerformanceObserver } from 'node:perf_hooks';
 import { setTimeout as sleep } from 'node:timers/promises';
 
 import selfsigned from 'selfsigned';
 import { describe, it, beforeAll, afterAll } from 'vitest';
 
-import { HttpClient, RawResponseWithMeta, getGlobalDispatcher } from '../src/index.js';
+import { HttpClient, getGlobalDispatcher } from '../src/index.js';
+import type { RawResponseWithMeta } from '../src/index.js';
 import { startServer } from './fixtures/server.js';
 import { nodeMajorVersion } from './utils.js';
 
diff --git a/test/diagnostics_channel.test.ts b/test/diagnostics_channel.test.ts
index 8d1377dc..7dd0fa6a 100644
--- a/test/diagnostics_channel.test.ts
+++ b/test/diagnostics_channel.test.ts
@@ -2,7 +2,7 @@ import { strict as assert } from 'node:assert';
 import diagnosticsChannel from 'node:diagnostics_channel';
 import { once } from 'node:events';
 import { createSecureServer } from 'node:http2';
-import { AddressInfo } from 'node:net';
+import type { AddressInfo } from 'node:net';
 import { setTimeout as sleep } from 'node:timers/promises';
 
 import selfsigned from 'selfsigned';
diff --git a/test/fetch.test.ts b/test/fetch.test.ts
index b1d53d96..411f789a 100644
--- a/test/fetch.test.ts
+++ b/test/fetch.test.ts
@@ -5,8 +5,9 @@ import { setTimeout as sleep } from 'node:timers/promises';
 import { Request } from 'undici';
 import { describe, it, beforeAll, afterAll } from 'vitest';
 
-import { fetch, FetchDiagnosticsMessage, FetchFactory, FetchResponseDiagnosticsMessage } from '../src/fetch.js';
-import { RequestDiagnosticsMessage, ResponseDiagnosticsMessage } from '../src/HttpClient.js';
+import { fetch, FetchFactory } from '../src/fetch.js';
+import type { FetchDiagnosticsMessage, FetchResponseDiagnosticsMessage } from '../src/fetch.js';
+import type { RequestDiagnosticsMessage, ResponseDiagnosticsMessage } from '../src/HttpClient.js';
 import { startServer } from './fixtures/server.js';
 
 describe('fetch.test.ts', () => {
diff --git a/test/fixtures/BufferStream.ts b/test/fixtures/BufferStream.ts
index df60f8b3..78242f37 100644
--- a/test/fixtures/BufferStream.ts
+++ b/test/fixtures/BufferStream.ts
@@ -11,12 +11,12 @@ export class BufferStream extends Transform {
     this.realloc();
   }
 
-  realloc() {
+  realloc(): void {
     this.buf = Buffer.alloc(BUF_SIZE);
     this.offset = 0;
   }
 
-  _transform(chunk: Buffer, _: any, callback: any) {
+  _transform(chunk: Buffer, _: any, callback: any): void {
     const currentLength = this.offset;
     const chunkSize = chunk.length;
     const newSize = currentLength + chunkSize;
@@ -60,7 +60,7 @@ export class BufferStream extends Transform {
     return callback(null);
   }
 
-  _flush(callback: any) {
+  _flush(callback: any): void {
     if (this.offset) {
       const chunk = Buffer.alloc(this.offset);
       this.buf.copy(chunk);
diff --git a/test/options.timeout.test.ts b/test/options.timeout.test.ts
index 48848200..551f82ba 100644
--- a/test/options.timeout.test.ts
+++ b/test/options.timeout.test.ts
@@ -1,7 +1,7 @@
 import { strict as assert } from 'node:assert';
 import { once } from 'node:events';
 import { createSecureServer } from 'node:http2';
-import { AddressInfo } from 'node:net';
+import type { AddressInfo } from 'node:net';
 
 import selfsigned from 'selfsigned';
 import { describe, it, beforeAll, afterAll } from 'vitest';
diff --git a/test/options.timing.test.ts b/test/options.timing.test.ts
index 40811f89..9a92399d 100644
--- a/test/options.timing.test.ts
+++ b/test/options.timing.test.ts
@@ -3,7 +3,8 @@ import { setTimeout as sleep } from 'node:timers/promises';
 
 import { describe, it, beforeAll, afterAll } from 'vitest';
 
-import urllib, { RawResponseWithMeta } from '../src/index.js';
+import urllib from '../src/index.js';
+import type { RawResponseWithMeta } from '../src/index.js';
 import { startServer } from './fixtures/server.js';
 
 describe('options.timing.test.ts', () => {
diff --git a/test/urllib.options.rejectUnauthorized-false.test.ts b/test/urllib.options.rejectUnauthorized-false.test.ts
index c64ede7c..62cb2547 100644
--- a/test/urllib.options.rejectUnauthorized-false.test.ts
+++ b/test/urllib.options.rejectUnauthorized-false.test.ts
@@ -1,7 +1,7 @@
 import { strict as assert } from 'node:assert';
 import { once } from 'node:events';
 import { createSecureServer } from 'node:http2';
-import { AddressInfo } from 'node:net';
+import type { AddressInfo } from 'node:net';
 
 import selfsigned from 'selfsigned';
 import { describe, it, beforeAll, afterAll } from 'vitest';
diff --git a/test/utils.ts b/test/utils.ts
index 589a71f7..bf1d621c 100644
--- a/test/utils.ts
+++ b/test/utils.ts
@@ -5,7 +5,7 @@ import { join } from 'node:path';
 import { Readable } from 'node:stream';
 import { ReadableStream } from 'node:stream/web';
 
-export async function readableToBytes(stream: Readable | ReadableStream) {
+export async function readableToBytes(stream: Readable | ReadableStream): Promise<Buffer> {
   const chunks: Buffer[] = [];
   let chunk: Buffer;
   for await (chunk of stream) {
@@ -14,12 +14,14 @@ export async function readableToBytes(stream: Readable | ReadableStream) {
   return Buffer.concat(chunks);
 }
 
-export async function readableToString(stream: Readable | ReadableStream) {
+export async function readableToString(stream: Readable | ReadableStream): Promise<string> {
   const bytes = await readableToBytes(stream);
   return bytes.toString();
 }
 
-export async function createTempfile(content?: Buffer | string) {
+export async function createTempfile(
+  content?: Buffer | string,
+): Promise<{ tmpfile: string; cleanup: () => Promise<void> }> {
   const tmpfile = join(tmpdir(), randomUUID());
   if (typeof content === 'string') {
     content = Buffer.from(content);
@@ -35,10 +37,10 @@ export async function createTempfile(content?: Buffer | string) {
   };
 }
 
-export function nodeMajorVersion() {
+export function nodeMajorVersion(): number {
   return parseInt(process.versions.node.split('.')[0]);
 }
 
-export function isWindows() {
+export function isWindows(): boolean {
   return platform() === 'win32';
 }
diff --git a/tsconfig.json b/tsconfig.json
index a83410c1..148d5b50 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,6 +6,10 @@
     "target": "ES2022",
     "module": "NodeNext",
     "moduleResolution": "NodeNext",
+    // https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-5.html#isolated-declarations
+    "isolatedDeclarations": true,
+    // verbatimModuleSyntax is incompatible with multi-dialect builds
+    // "verbatimModuleSyntax": true
   },
   "exclude": ["test/fixtures/ts*"],
 }
diff --git a/vite.config.ts b/vite.config.ts
index 5dcbc4a8..b4ef3725 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -1,6 +1,7 @@
 import { defineConfig } from 'vitest/config';
+import type { UserConfig } from 'vitest/config';
 
-export default defineConfig({
+const config: UserConfig = defineConfig({
   test: {
     include: ['test/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}'],
     testTimeout: 60000,
@@ -11,3 +12,5 @@ export default defineConfig({
     setupFiles: ['test/setup.ts'],
   },
 });
+
+export default config;