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fix(client,server): browser-conditional export for stdio transports
Adds browser/workerd conditions to the package root export that resolve to a build whose StdioClientTransport / StdioServerTransport are throwing stubs, so browser and Cloudflare Workers bundlers no longer pull node:child_process, node:stream, or cross-spawn. Node.js, Bun, and Deno continue to resolve the default condition and get the real implementation. Adds SdkErrorCode.TransportNotSupported for the stub error.
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‎.changeset/browser-stdio-conditional-export.md‎

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---
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'@modelcontextprotocol/client': patch
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'@modelcontextprotocol/server': patch
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---
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Add `browser`/`workerd` export conditions for the package root that swap the stdio transport for a throwing stub, so browser and Cloudflare Workers bundlers no longer pull `node:child_process`, `node:stream`, or `cross-spawn`. Node.js, Bun, and Deno continue to resolve the real `StdioClientTransport`/`StdioServerTransport`.

‎packages/client/package.json‎

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],
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"exports": {
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".": {
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"workerd": {
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"types": "./dist/index.browser.d.mts",
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"import": "./dist/index.browser.mjs"
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},
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"browser": {
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"types": "./dist/index.browser.d.mts",
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"import": "./dist/index.browser.mjs"
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},
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"types": "./dist/index.d.mts",
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"import": "./dist/index.mjs"
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},

‎packages/client/src/index.browser.ts‎

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// Public API for @modelcontextprotocol/client (browser / Cloudflare Workers entry).
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//
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// Mirrors ./index.ts exactly, except stdio symbols come from ./stdioStub.js so
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// the bundle does not pull `node:child_process`, `node:stream`, or `cross-spawn`.
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// Selected via the `browser` and `workerd` export conditions.
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export type {
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AddClientAuthentication,
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AuthProvider,
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AuthResult,
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ClientAuthMethod,
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OAuthClientProvider,
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OAuthDiscoveryState,
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OAuthServerInfo
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} from './client/auth.js';
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export {
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auth,
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buildDiscoveryUrls,
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discoverAuthorizationServerMetadata,
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discoverOAuthMetadata,
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discoverOAuthProtectedResourceMetadata,
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discoverOAuthServerInfo,
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exchangeAuthorization,
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extractResourceMetadataUrl,
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extractWWWAuthenticateParams,
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fetchToken,
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isHttpsUrl,
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parseErrorResponse,
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prepareAuthorizationCodeRequest,
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refreshAuthorization,
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registerClient,
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selectClientAuthMethod,
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selectResourceURL,
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startAuthorization,
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UnauthorizedError
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} from './client/auth.js';
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export type {
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AssertionCallback,
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ClientCredentialsProviderOptions,
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CrossAppAccessContext,
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CrossAppAccessProviderOptions,
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PrivateKeyJwtProviderOptions,
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StaticPrivateKeyJwtProviderOptions
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} from './client/authExtensions.js';
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export {
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ClientCredentialsProvider,
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createPrivateKeyJwtAuth,
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CrossAppAccessProvider,
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PrivateKeyJwtProvider,
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StaticPrivateKeyJwtProvider
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} from './client/authExtensions.js';
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export type { ClientOptions } from './client/client.js';
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export { Client } from './client/client.js';
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export { getSupportedElicitationModes } from './client/client.js';
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export type { DiscoverAndRequestJwtAuthGrantOptions, JwtAuthGrantResult, RequestJwtAuthGrantOptions } from './client/crossAppAccess.js';
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export { discoverAndRequestJwtAuthGrant, exchangeJwtAuthGrant, requestJwtAuthorizationGrant } from './client/crossAppAccess.js';
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export type { LoggingOptions, Middleware, RequestLogger } from './client/middleware.js';
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export { applyMiddlewares, createMiddleware, withLogging, withOAuth } from './client/middleware.js';
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export type { SSEClientTransportOptions } from './client/sse.js';
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export { SSEClientTransport, SseError } from './client/sse.js';
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export type {
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ReconnectionScheduler,
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StartSSEOptions,
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StreamableHTTPClientTransportOptions,
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StreamableHTTPReconnectionOptions
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} from './client/streamableHttp.js';
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export { StreamableHTTPClientTransport } from './client/streamableHttp.js';
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// stdio: stubbed for browser / Cloudflare Workers — throws on use
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export type { StdioServerParameters } from './stdioStub.js';
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export { DEFAULT_INHERITED_ENV_VARS, getDefaultEnvironment, StdioClientTransport } from './stdioStub.js';
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// experimental exports
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export { ExperimentalClientTasks } from './experimental/tasks/client.js';
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// runtime-aware wrapper (shadows core/public's fromJsonSchema with optional validator)
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export { fromJsonSchema } from './fromJsonSchema.js';
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// re-export curated public API from core
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export * from '@modelcontextprotocol/core/public';

‎packages/client/src/stdioStub.ts‎

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/**
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* Browser/Workers stub for {@linkcode StdioClientTransport}.
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*
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* Selected via the `browser` / `workerd` export conditions on the package root.
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* The real implementation in `./client/stdio.ts` imports `node:child_process`,
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* `node:stream`, and `cross-spawn`, which are unavailable in browsers and
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* Cloudflare Workers. Node.js, Bun, and Deno resolve the default condition and
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* get the real implementation.
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*/
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import type { JSONRPCMessage, Transport } from '@modelcontextprotocol/core';
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import { SdkError, SdkErrorCode } from '@modelcontextprotocol/core';
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/**
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* Parameters for launching an MCP server over stdio.
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*
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* Browser stub: `stderr` is loosely typed to avoid pulling Node-only types.
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*/
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export type StdioServerParameters = {
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command: string;
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args?: string[];
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env?: Record<string, string>;
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stderr?: unknown;
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cwd?: string;
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};
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const UNSUPPORTED_MESSAGE =
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'StdioClientTransport requires a process-spawning runtime (Node.js, Bun, or Deno) and is not available in browsers or Cloudflare Workers. Use StreamableHTTPClientTransport instead.';
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function unsupported(): never {
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throw new SdkError(SdkErrorCode.TransportNotSupported, UNSUPPORTED_MESSAGE);
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}
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/** Browser stub: empty in environments without a process-level env. */
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export const DEFAULT_INHERITED_ENV_VARS: string[] = [];
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/** Browser stub: throws — there is no process environment to inherit. */
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export function getDefaultEnvironment(): Record<string, string> {
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return unsupported();
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}
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/**
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* Browser stub for `StdioClientTransport`. Throws on construction with a message
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* directing the caller to `StreamableHTTPClientTransport`.
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*/
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export class StdioClientTransport implements Transport {
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onclose?: () => void;
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onerror?: (error: Error) => void;
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onmessage?: (message: JSONRPCMessage) => void;
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constructor(_server: StdioServerParameters) {
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unsupported();
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}
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start(): Promise<void> {
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return unsupported();
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}
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send(_message: JSONRPCMessage): Promise<void> {
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return unsupported();
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}
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close(): Promise<void> {
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return Promise.resolve();
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}
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}

‎packages/client/src/validators/cfWorker.ts‎

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* import { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/client/validators/cf-worker';
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* ```
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*/
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export { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/core';
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export type { CfWorkerSchemaDraft } from '@modelcontextprotocol/core';
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export { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/core';

‎packages/client/test/client/browserBundle.test.ts‎

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import { existsSync, readFileSync } from 'node:fs';
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import { dirname, join } from 'node:path';
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import { fileURLToPath } from 'node:url';
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import { describe, expect, test } from 'vitest';
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const distDir = join(dirname(fileURLToPath(import.meta.url)), '../../dist');
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const browserBundle = join(distDir, 'index.browser.mjs');
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const nodeBundle = join(distDir, 'index.mjs');
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const built = existsSync(browserBundle) && existsSync(nodeBundle);
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describe.skipIf(!built)('browser-conditional bundle (requires `pnpm build`)', () => {
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test('index.browser.mjs has no Node-only stdio dependencies', () => {
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const src = readFileSync(browserBundle, 'utf8');
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expect(src).not.toMatch(/\bchild_process\b/);
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expect(src).not.toMatch(/\bcross-spawn\b/);
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expect(src).not.toMatch(/\bnode:stream\b/);
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});
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test('index.browser.mjs still exports StdioClientTransport (stubbed)', () => {
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const src = readFileSync(browserBundle, 'utf8');
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expect(src).toContain('StdioClientTransport');
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});
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test('default index.mjs still includes the real stdio transport', () => {
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const src = readFileSync(nodeBundle, 'utf8');
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expect(src).toContain('cross-spawn');
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});
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});

‎packages/client/tsdown.config.ts‎

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failOnWarn: 'ci-only',
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// 1. Entry Points
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// Directly matches package.json include/exclude globs
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entry: ['src/index.ts', 'src/shimsNode.ts', 'src/shimsWorkerd.ts', 'src/shimsBrowser.ts', 'src/validators/cfWorker.ts'],
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entry: [
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'src/index.ts',
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'src/index.browser.ts',
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'src/shimsNode.ts',
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'src/shimsWorkerd.ts',
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'src/shimsBrowser.ts',
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'src/validators/cfWorker.ts'
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],
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// 2. Output Configuration
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format: ['esm'],

‎packages/core/src/errors/sdkErrors.ts‎

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CapabilityNotSupported = 'CAPABILITY_NOT_SUPPORTED',
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// Transport errors
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/** Transport is not available in the current runtime (e.g. stdio in a browser) */
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TransportNotSupported = 'TRANSPORT_NOT_SUPPORTED',
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/** Request timed out waiting for response */
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RequestTimeout = 'REQUEST_TIMEOUT',
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/** Connection was closed */

‎packages/server/package.json‎

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],
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"exports": {
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".": {
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"workerd": {
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"types": "./dist/index.browser.d.mts",
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"import": "./dist/index.browser.mjs"
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},
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"browser": {
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"types": "./dist/index.browser.d.mts",
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"import": "./dist/index.browser.mjs"
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},
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"types": "./dist/index.d.mts",
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"import": "./dist/index.mjs"
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},

‎packages/server/src/index.browser.ts‎

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// Public API for @modelcontextprotocol/server (browser / Cloudflare Workers entry).
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//
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// Mirrors ./index.ts exactly, except stdio symbols come from ./stdioStub.js so
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// the bundle does not pull `node:stream`.
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// Selected via the `browser` and `workerd` export conditions.
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export type { CompletableSchema, CompleteCallback } from './server/completable.js';
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export { completable, isCompletable } from './server/completable.js';
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export type {
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AnyToolHandler,
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BaseToolCallback,
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CompleteResourceTemplateCallback,
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ListResourcesCallback,
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PromptCallback,
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ReadResourceCallback,
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ReadResourceTemplateCallback,
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RegisteredPrompt,
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RegisteredResource,
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RegisteredResourceTemplate,
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RegisteredTool,
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ResourceMetadata,
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ToolCallback
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} from './server/mcp.js';
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export { McpServer, ResourceTemplate } from './server/mcp.js';
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export type { HostHeaderValidationResult } from './server/middleware/hostHeaderValidation.js';
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export { hostHeaderValidationResponse, localhostAllowedHostnames, validateHostHeader } from './server/middleware/hostHeaderValidation.js';
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export type { ServerOptions } from './server/server.js';
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export { Server } from './server/server.js';
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export type {
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EventId,
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EventStore,
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HandleRequestOptions,
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StreamId,
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WebStandardStreamableHTTPServerTransportOptions
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} from './server/streamableHttp.js';
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export { WebStandardStreamableHTTPServerTransport } from './server/streamableHttp.js';
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// stdio: stubbed for browser / Cloudflare Workers — throws on use
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export { StdioServerTransport } from './stdioStub.js';
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// experimental exports
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export type { CreateTaskRequestHandler, TaskRequestHandler, ToolTaskHandler } from './experimental/tasks/interfaces.js';
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export { ExperimentalMcpServerTasks } from './experimental/tasks/mcpServer.js';
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export { ExperimentalServerTasks } from './experimental/tasks/server.js';
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// runtime-aware wrapper (shadows core/public's fromJsonSchema with optional validator)
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export { fromJsonSchema } from './fromJsonSchema.js';
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// re-export curated public API from core
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export * from '@modelcontextprotocol/core/public';

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