fix: add | undefined to transport option callbacks for exactOptionalPropertyTypes (#1855)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Christian-Sidak

.changeset/gentle-planets-rest.md

@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Add `| undefined` to optional callback and function properties on `WebStandardStreamableHTTPServerTransportOptions` (`sessionIdGenerator`, `onsessioninitialized`, `onsessionclosed`) and corresponding private fields.
+
+This fixes TS2430 errors for consumers using `exactOptionalPropertyTypes: true` without `skipLibCheck`, where optional properties with function types need explicit `| undefined` to match their emitted declarations.

packages/middleware/node/test/streamableHttp.test.ts

@@ -42,8 +42,8 @@ interface TestServerConfig {
     enableJsonResponse?: boolean;
     customRequestHandler?: (req: IncomingMessage, res: ServerResponse, parsedBody?: unknown) => Promise<void>;
     eventStore?: EventStore;
-    onsessioninitialized?: (sessionId: string) => void | Promise<void>;
-    onsessionclosed?: (sessionId: string) => void | Promise<void>;
+    onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined;
+    onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined;
     retryInterval?: number;
 }
 

packages/server/src/server/streamableHttp.ts

@@ -77,7 +77,7 @@ export interface WebStandardStreamableHTTPServerTransportOptions {
      *
      * If not provided, session management is disabled (stateless mode).
      */
-    sessionIdGenerator?: () => string;
+    sessionIdGenerator?: (() => string) | undefined;
 
     /**
      * A callback for session initialization events
@@ -86,7 +86,7 @@ export interface WebStandardStreamableHTTPServerTransportOptions {
      * and need to keep track of them.
      * @param sessionId The generated session ID
      */
-    onsessioninitialized?: (sessionId: string) => void | Promise<void>;
+    onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined;
 
     /**
      * A callback for session close events
@@ -98,7 +98,7 @@ export interface WebStandardStreamableHTTPServerTransportOptions {
      * session open/running.
      * @param sessionId The session ID that was closed
      */
-    onsessionclosed?: (sessionId: string) => void | Promise<void>;
+    onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined;
 
     /**
      * If `true`, the server will return JSON responses instead of starting an SSE stream.
@@ -233,8 +233,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _enableJsonResponse: boolean = false;
     private _standaloneSseStreamId: string = '_GET_stream';
     private _eventStore?: EventStore;
-    private _onsessioninitialized?: (sessionId: string) => void | Promise<void>;
-    private _onsessionclosed?: (sessionId: string) => void | Promise<void>;
+    private _onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined;
+    private _onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined;
     private _allowedHosts?: string[];
     private _allowedOrigins?: string[];
     private _enableDnsRebindingProtection: boolean;