fix: support exactOptionalPropertyTypes in Transport interface

Adds explicit `| undefined` to all optional callback properties on the
Transport interface and all implementing classes. This allows consumers
with `exactOptionalPropertyTypes: true` in their tsconfig.json to use
the SDK without type errors.

Without this fix, class properties initialized as `undefined` are
incompatible with the interface under exactOptionalPropertyTypes, since
`T | undefined` is not assignable to `T` when the property is present.

Fixes #1314

Author: naman10parikh

packages/client/src/client/sse.ts

@@ -76,9 +76,9 @@ export class SSEClientTransport implements Transport {
     private _fetchWithInit: FetchLike;
     private _protocolVersion?: string;
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage) => void) | undefined;
 
     constructor(url: URL, opts?: SSEClientTransportOptions) {
         this._url = url;

packages/client/src/client/stdio.ts

@@ -96,9 +96,9 @@ export class StdioClientTransport implements Transport {
     private _serverParams: StdioServerParameters;
     private _stderrStream: PassThrough | null = null;
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage) => void) | undefined;
 
     constructor(server: StdioServerParameters) {
         this._serverParams = server;

packages/client/src/client/streamableHttp.ts

@@ -149,9 +149,9 @@ export class StreamableHTTPClientTransport implements Transport {
     private _serverRetryMs?: number; // Server-provided retry delay from SSE retry field
     private _reconnectionTimeout?: ReturnType<typeof setTimeout>;
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage) => void) | undefined;
 
     constructor(url: URL, opts?: StreamableHTTPClientTransportOptions) {
         this._url = url;

packages/client/src/client/websocket.ts

@@ -10,9 +10,9 @@ export class WebSocketClientTransport implements Transport {
     private _socket?: WebSocket;
     private _url: URL;
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage) => void) | undefined;
 
     constructor(url: URL) {
         this._url = url;

packages/core/src/shared/transport.ts

@@ -98,14 +98,14 @@ export interface Transport {
      *
      * This should be invoked when {@linkcode Transport.close | close()} is called as well.
      */
-    onclose?: () => void;
+    onclose?: (() => void) | undefined;
 
     /**
      * Callback for when an error occurs.
      *
      * Note that errors are not necessarily fatal; they are used for reporting any kind of exceptional condition out of band.
      */
-    onerror?: (error: Error) => void;
+    onerror?: ((error: Error) => void) | undefined;
 
     /**
      * Callback for when a message (request or response) is received over the connection.
@@ -114,21 +114,21 @@ export interface Transport {
      *
      * The {@linkcode MessageExtraInfo.requestInfo | requestInfo} can be used to get the original request information (headers, etc.)
      */
-    onmessage?: <T extends JSONRPCMessage>(message: T, extra?: MessageExtraInfo) => void;
+    onmessage?: (<T extends JSONRPCMessage>(message: T, extra?: MessageExtraInfo) => void) | undefined;
 
     /**
      * The session ID generated for this connection.
      */
-    sessionId?: string;
+    sessionId?: string | undefined;
 
     /**
      * Sets the protocol version used for the connection (called when the initialize response is received).
      */
-    setProtocolVersion?: (version: string) => void;
+    setProtocolVersion?: ((version: string) => void) | undefined;
 
     /**
      * Sets the supported protocol versions for header validation (called during connect).
      * This allows the server to pass its supported versions to the transport.
      */
-    setSupportedProtocolVersions?: (versions: string[]) => void;
+    setSupportedProtocolVersions?: ((versions: string[]) => void) | undefined;
 }

packages/core/src/util/inMemory.ts

@@ -14,10 +14,10 @@ export class InMemoryTransport implements Transport {
     private _otherTransport?: InMemoryTransport;
     private _messageQueue: QueuedMessage[] = [];
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void;
-    sessionId?: string;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage, extra?: { authInfo?: AuthInfo }) => void) | undefined;
+    sessionId?: string | undefined;
 
     /**
      * Creates a pair of linked in-memory transports that can communicate with each other. One should be passed to a {@linkcode @modelcontextprotocol/client!client/client.Client | Client} and one to a {@linkcode @modelcontextprotocol/server!server/server.Server | Server}.

packages/core/test/shared/protocol.test.ts

@@ -54,9 +54,9 @@ interface TestProtocol {
 
 // Mock Transport class
 class MockTransport implements Transport {
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: unknown) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: unknown) => void) | undefined;
 
     async start(): Promise<void> {}
     async close(): Promise<void> {

packages/core/test/shared/protocolTransportHandling.test.ts

@@ -8,9 +8,9 @@ import type { EmptyResult, JSONRPCMessage, Notification, Request, Result } from
 // Mock Transport class
 class MockTransport implements Transport {
     id: string;
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: unknown) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: unknown) => void) | undefined;
     sentMessages: JSONRPCMessage[] = [];
 
     constructor(id: string) {

packages/server/src/server/stdio.ts

@@ -25,9 +25,9 @@ export class StdioServerTransport implements Transport {
         private _stdout: Writable = process.stdout
     ) {}
 
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage) => void;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage) => void) | undefined;
 
     // Arrow functions to bind `this` properly, while maintaining function identity.
     _ondata = (chunk: Buffer) => {

packages/server/src/server/streamableHttp.ts

@@ -240,10 +240,10 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _retryInterval?: number;
     private _supportedProtocolVersions: string[];
 
-    sessionId?: string;
-    onclose?: () => void;
-    onerror?: (error: Error) => void;
-    onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;
+    sessionId?: string | undefined;
+    onclose?: (() => void) | undefined;
+    onerror?: ((error: Error) => void) | undefined;
+    onmessage?: ((message: JSONRPCMessage, extra?: MessageExtraInfo) => void) | undefined;
 
     constructor(options: WebStandardStreamableHTTPServerTransportOptions = {}) {
         this.sessionIdGenerator = options.sessionIdGenerator;