simplify: explicit kind brand on transports replaces duck-typing discriminator

ChannelTransport gets `readonly kind?: 'channel'` (optional, back-compat).
RequestTransport / ClientTransport get `readonly kind: 'request'` (required).
isRequestTransport / isChannelTransport check the brand instead of probing
for `onrequest` / `fetch` / `start` / `send`.

The SHTTP transport classes now `implements RequestTransport` only (not also
ChannelTransport — the brands are mutually exclusive). They keep the
start/send/onmessage methods for back-compat with v1 callers, but
McpServer/Client.connect() routes them via the request-shaped path.

Author: felixweinberger

packages/client/src/client/clientTransport.ts

@@ -65,6 +65,9 @@ export type ClientFetchOptions = {
  * interface is adapted via {@linkcode channelAsClientTransport}.
  */
 export interface ClientTransport {
+    /** Explicit shape brand. Required so {@linkcode isChannelTransport} can discriminate without duck-typing. */
+    readonly kind: 'request';
+
     /**
      * Send one JSON-RPC request and resolve with the terminal response.
      * Any progress/notifications received before the response are surfaced
@@ -104,8 +107,7 @@ export interface ClientTransport {
  * request-shaped path.
  */
 export function isChannelTransport(t: Transport | ClientTransport): t is Transport {
-    if (typeof (t as ClientTransport).fetch === 'function') return false;
-    return typeof (t as Transport).start === 'function' && typeof (t as Transport).send === 'function';
+    return (t as ClientTransport).kind !== 'request';
 }
 
 /**
@@ -134,6 +136,7 @@ export function channelAsClientTransport(pipe: Transport, dispatcher: Dispatcher
         }
     };
     return {
+        kind: 'request',
         driver,
         async fetch(request, opts) {
             await ensureStarted();

packages/client/src/client/streamableHttp.ts

@@ -7,8 +7,7 @@ import type {
     JSONRPCNotification,
     JSONRPCRequest,
     JSONRPCResultResponse,
-    Notification,
-    Transport
+    Notification
 } from '@modelcontextprotocol/core';
 import {
     createFetchWithInit,
@@ -194,7 +193,9 @@ export type StreamableHTTPClientTransportOptions = {
  * {@linkcode Client.connect}) and the legacy pipe-shaped {@linkcode Transport} (deprecated; kept for
  * direct callers and v1 compat).
  */
-export class StreamableHTTPClientTransport implements ClientTransport, Transport {
+export class StreamableHTTPClientTransport implements ClientTransport {
+    readonly kind = 'request' as const;
+
     private _abortController?: AbortController;
     private _url: URL;
     private _resourceMetadataUrl?: URL;

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

@@ -24,6 +24,7 @@ function mockTransport(handler: (req: JSONRPCRequest, opts?: ClientFetchOptions)
     const sent: JSONRPCRequest[] = [];
     const notified: Notification[] = [];
     const ct: ClientTransport = {
+        kind: 'request',
         async fetch(req, opts) {
             sent.push(req);
             return handler(req, opts);

packages/core/src/shared/transport.ts

@@ -86,6 +86,12 @@ export type TransportSendOptions = {
  * For request/response-shaped transports (Streamable HTTP), see {@linkcode RequestTransport}.
  */
 export interface ChannelTransport {
+    /**
+     * Explicit shape brand. Optional (defaults to `'channel'`) so existing
+     * `Transport` implementations don't need to declare it.
+     */
+    readonly kind?: 'channel';
+
     /**
      * Starts processing messages on the transport, including any connection steps that might need to be taken.
      *
@@ -173,13 +179,13 @@ export type AttachOptions = {
  * per inbound message. The transport itself never imports or references a `Dispatcher`.
  */
 export interface RequestTransport {
+    /** Explicit shape brand. Required so {@linkcode isRequestTransport} can discriminate without duck-typing. */
+    readonly kind: 'request';
+
     /**
      * Callback slot for inbound JSON-RPC requests. Set by `McpServer.connect()`.
      * The transport calls this per request and writes the yielded messages
      * (notifications + one terminal response) to the HTTP response stream.
-     *
-     * Transports MUST declare this property (initialised to `undefined`) so
-     * {@linkcode isRequestTransport} can discriminate before `connect()` runs.
      */
     onrequest?: ((req: JSONRPCRequest, env?: RequestEnv) => AsyncIterable<JSONRPCMessage>) | undefined;
 
@@ -218,5 +224,5 @@ export interface RequestTransport {
 
 /** Type guard distinguishing {@linkcode RequestTransport} from {@linkcode ChannelTransport}. */
 export function isRequestTransport(t: ChannelTransport | RequestTransport): t is RequestTransport {
-    return 'onrequest' in t;
+    return (t as RequestTransport).kind === 'request';
 }

packages/middleware/node/src/streamableHttp.ts

@@ -12,7 +12,6 @@ import type { IncomingMessage, ServerResponse } from 'node:http';
 import { getRequestListener } from '@hono/node-server';
 import type {
     AuthInfo,
-    ChannelTransport,
     JSONRPCErrorResponse,
     JSONRPCMessage,
     JSONRPCNotification,
@@ -96,7 +95,9 @@ export function toNodeHttpHandler(
  * });
  * ```
  */
-export class NodeStreamableHTTPServerTransport implements ChannelTransport, RequestTransport {
+export class NodeStreamableHTTPServerTransport implements RequestTransport {
+    readonly kind = 'request' as const;
+
     private _webStandardTransport: WebStandardStreamableHTTPServerTransport;
     private _requestListener: ReturnType<typeof getRequestListener>;
     // Store auth and parsedBody per request for passing through to handleRequest

packages/server/src/server/streamableHttp.ts

@@ -14,7 +14,6 @@
 
 import type {
     AuthInfo,
-    ChannelTransport,
     JSONRPCErrorResponse,
     JSONRPCMessage,
     JSONRPCNotification,
@@ -157,7 +156,9 @@ export interface HandleRequestOptions {
  * {@linkcode Transport} interface methods route outbound messages through the
  * per-session {@linkcode Backchannel2511}.
  */
-export class WebStandardStreamableHTTPServerTransport implements ChannelTransport, RequestTransport {
+export class WebStandardStreamableHTTPServerTransport implements RequestTransport {
+    readonly kind = 'request' as const;
+
     private _options: WebStandardStreamableHTTPServerTransportOptions;
     private _session?: SessionCompat;
     private _backchannel = new Backchannel2511();
@@ -171,7 +172,7 @@ export class WebStandardStreamableHTTPServerTransport implements ChannelTranspor
     onerror?: (error: Error) => void;
     onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;
 
-    /** {@linkcode RequestTransport.onrequest} — set by `McpServer.connect()`. Declared so {@linkcode isRequestTransport} matches. */
+    /** {@linkcode RequestTransport.onrequest} — set by `McpServer.connect()`. */
     onrequest: ((req: JSONRPCRequest, env?: RequestEnv) => AsyncIterable<JSONRPCMessage>) | undefined = undefined;
     /** {@linkcode RequestTransport.onnotification} — set by `McpServer.connect()`. */
     onnotification?: (n: JSONRPCNotification) => void | Promise<void>;