fix: support exactOptionalPropertyTypes for optional callbacks

Add `| undefined` to optional callback properties to support TypeScript
projects using `exactOptionalPropertyTypes: true`.

Changes:
- Transport interface: onclose, onerror, onmessage
- WebStandardStreamableHTTPServerTransportOptions: sessionIdGenerator,
  onsessioninitialized, onsessionclosed

This is the official TypeScript pattern for exactOptionalPropertyTypes
compatibility, used by Radix UI, Material UI, and other major libraries.

Fixes #1397

Author: codewithkenzo

.changeset/exact-optional-property-types.md

@@ -0,0 +1,15 @@
+---
+"@modelcontextprotocol/core": patch
+"@modelcontextprotocol/server": patch
+---
+
+fix: support `exactOptionalPropertyTypes` TypeScript config
+
+Adds `| undefined` to optional callback properties in the `Transport` interface
+and `WebStandardStreamableHTTPServerTransportOptions` to support projects using
+`exactOptionalPropertyTypes: true` in their tsconfig.
+
+This allows explicitly passing `undefined` for optional callbacks like
+`sessionIdGenerator`, `onclose`, `onerror`, and `onmessage`.
+
+Fixes #1397

packages/core/src/shared/transport.ts

@@ -98,14 +98,14 @@ export interface Transport {
      *
      * This should be invoked when 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.
@@ -114,7 +114,7 @@ export interface Transport {
      *
      * The 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.

packages/server/src/server/streamableHttp.ts

@@ -79,7 +79,7 @@ export interface WebStandardStreamableHTTPServerTransportOptions {
      *
      * If not provided, session management is disabled (stateless mode).
      */
-    sessionIdGenerator?: () => string;
+    sessionIdGenerator?: (() => string) | undefined;
 
     /**
      * A callback for session initialization events
@@ -88,7 +88,7 @@ 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
@@ -100,7 +100,7 @@ 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.