fix: add explicit .js extensions to relative imports so declarations resolve under NodeNext/Node16

The emitted .d.ts copy relative specifiers verbatim, and the extensionless form
(e.g. `from "./events"`) doesn't resolve under Node16/NodeNext, hiding inherited
members such as `addEventListener` (TS2339). Appending `.js` matches the existing
SDK-import style and works with both bundler-mode type-checking and Bun.build.
Types-only; runtime is unaffected.

Fixes #704

src/app-bridge.test.ts

@@ -3,8 +3,8 @@ import { Client, InMemoryTransport } from "@modelcontextprotocol/client";
 import { Server, type ServerCapabilities } from "@modelcontextprotocol/server";
 import { z } from "zod/v4";
 
-import { App } from "./app";
-import { LATEST_PROTOCOL_VERSION } from "./types";
+import { App } from "./app.js";
+import { LATEST_PROTOCOL_VERSION } from "./types.js";
 import {
   AppBridge,
   buildAllowAttribute,
@@ -13,7 +13,7 @@ import {
   isToolVisibilityAppOnly,
   McpUiOpenLinkResultSchema,
   type McpUiHostCapabilities,
-} from "./app-bridge";
+} from "./app-bridge.js";
 
 /** Wait for pending microtasks to complete */
 const flush = () => new Promise((resolve) => setTimeout(resolve, 0));

src/app-bridge.ts

@@ -34,7 +34,7 @@ import {
   EmptyResultSchema,
   LoggingMessageNotificationSchema,
 } from "@modelcontextprotocol/core";
-import { EventDispatcher } from "./events";
+import { EventDispatcher } from "./events.js";
 import type { ZodLiteral, ZodObject, ZodType } from "zod/v4";
 
 type MethodSchema = ZodObject<{
@@ -87,11 +87,11 @@ import {
   McpUiRequestDisplayModeResultSchema,
   McpUiResourcePermissions,
   McpUiToolMeta,
-} from "./types";
-export * from "./types";
-export { RESOURCE_URI_META_KEY, RESOURCE_MIME_TYPE } from "./constants";
-import { RESOURCE_URI_META_KEY } from "./constants";
-export { PostMessageTransport } from "./message-transport";
+} from "./types.js";
+export * from "./types.js";
+export { RESOURCE_URI_META_KEY, RESOURCE_MIME_TYPE } from "./constants.js";
+import { RESOURCE_URI_META_KEY } from "./constants.js";
+export { PostMessageTransport } from "./message-transport.js";
 
 /**
  * Extract UI resource URI from tool metadata.

src/app.ts

@@ -22,10 +22,10 @@ import {
   type Transport,
 } from "@modelcontextprotocol/client";
 import { EmptyResultSchema } from "@modelcontextprotocol/core";
-export { RESOURCE_MIME_TYPE, RESOURCE_URI_META_KEY } from "./constants";
-import { EventDispatcher } from "./events";
-export { EventDispatcher } from "./events";
-import { PostMessageTransport } from "./message-transport";
+export { RESOURCE_MIME_TYPE, RESOURCE_URI_META_KEY } from "./constants.js";
+import { EventDispatcher } from "./events.js";
+export { EventDispatcher } from "./events.js";
+import { PostMessageTransport } from "./message-transport.js";
 import {
   LATEST_PROTOCOL_VERSION,
   McpUiAppCapabilities,
@@ -59,12 +59,12 @@ import {
   McpUiToolResultNotificationSchema,
   McpUiRequestDisplayModeRequest,
   McpUiRequestDisplayModeResultSchema,
-} from "./types";
+} from "./types.js";
 import {
   StandardSchemaV1,
   standardSchemaToJsonSchema,
   validateStandardSchema,
-} from "./standard-schema";
+} from "./standard-schema.js";
 import { z, type ZodLiteral, type ZodObject, type ZodType } from "zod/v4";
 
 type MethodSchema = ZodObject<{
@@ -92,16 +92,16 @@ function mergeAppCapabilities(
 export type {
   StandardSchemaV1,
   StandardSchemaWithJSON,
-} from "./standard-schema";
+} from "./standard-schema.js";
 
-export { PostMessageTransport } from "./message-transport";
-export * from "./types";
+export { PostMessageTransport } from "./message-transport.js";
+export * from "./types.js";
 export {
   applyHostStyleVariables,
   applyHostFonts,
   getDocumentTheme,
   applyDocumentTheme,
-} from "./styles";
+} from "./styles.js";
 
 /**
  * Metadata key for associating a UI resource URI with a tool.

src/message-transport.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect, beforeEach, afterEach, mock } from "bun:test";
 import type { JSONRPCMessage } from "@modelcontextprotocol/client";
 
-import { PostMessageTransport } from "./message-transport";
+import { PostMessageTransport } from "./message-transport.js";
 
 /**
  * Minimal `window` stub for bun's DOM-less test environment.

src/message-transport.ts

@@ -5,7 +5,7 @@ import type {
   Transport,
   TransportSendOptions,
 } from "@modelcontextprotocol/client";
-import { TOOL_INPUT_PARTIAL_METHOD } from "./spec.types";
+import { TOOL_INPUT_PARTIAL_METHOD } from "./spec.types.js";
 
 /**
  * JSON-RPC transport using `window.postMessage` for iframe↔parent communication.

src/react/index.tsx

@@ -31,7 +31,7 @@
  * }
  * ```
  */
-export * from "./useApp";
-export * from "./useAutoResize";
-export * from "./useDocumentTheme";
-export * from "./useHostStyles";
+export * from "./useApp.js";
+export * from "./useAutoResize.js";
+export * from "./useDocumentTheme.js";
+export * from "./useHostStyles.js";

src/react/useApp.tsx

@@ -5,8 +5,8 @@ import {
   AppOptions,
   McpUiAppCapabilities,
   PostMessageTransport,
-} from "../app";
-export * from "../app";
+} from "../app.js";
+export * from "../app.js";
 
 /**
  * Options for configuring the {@link useApp `useApp`} hook.

src/react/useAutoResize.ts

@@ -1,5 +1,5 @@
 import { useEffect, RefObject } from "react";
-import { App } from "../app";
+import { App } from "../app.js";
 
 /**
  * React hook that automatically reports UI size changes to the host.

src/react/useDocumentTheme.ts

@@ -1,6 +1,6 @@
 import { useEffect, useState } from "react";
-import { getDocumentTheme } from "../styles";
-import { McpUiTheme } from "../types";
+import { getDocumentTheme } from "../styles.js";
+import { McpUiTheme } from "../types.js";
 
 /**
  * React hook that provides the current document theme reactively.

src/react/useHostStyles.ts

@@ -1,11 +1,11 @@
 import { useEffect, useRef } from "react";
-import { App } from "../app";
+import { App } from "../app.js";
 import {
   applyDocumentTheme,
   applyHostFonts,
   applyHostStyleVariables,
-} from "../styles";
-import { McpUiHostContext } from "../types";
+} from "../styles.js";
+import { McpUiHostContext } from "../types.js";
 
 /**
  * React hook that applies host style variables and theme as CSS custom properties.

src/server/index.test.ts

@@ -6,7 +6,7 @@ import {
   RESOURCE_MIME_TYPE,
   getUiCapability,
   EXTENSION_ID,
-} from "./index";
+} from "./index.js";
 import type { McpServer } from "@modelcontextprotocol/server";
 
 describe("registerAppTool", () => {

src/styles.ts

@@ -1,4 +1,4 @@
-import { McpUiStyles, McpUiTheme } from "./types";
+import { McpUiStyles, McpUiTheme } from "./types.js";
 
 /**
  * Get the current document theme from the root HTML element.