fix: support exactOptionalPropertyTypes in Transport interface

[corvid-agent]

Summary

• Add explicit | undefined to all optional properties in the Transport interface, TransportSendOptions type, and WebStandardStreamableHTTPServerTransportOptions interface
• This ensures compatibility with TypeScript's exactOptionalPropertyTypes compiler option, which otherwise causes TS2420: Class incorrectly implements interface errors when implementing Transport

Details

When exactOptionalPropertyTypes: true is enabled in tsconfig.json, TypeScript distinguishes between "property may be missing" (prop?: T) and "property may be undefined" (prop?: T | undefined). Without the | undefined union, classes that declare optional properties (e.g., onclose?: () => void) cannot be assigned undefined at runtime, triggering type errors.

Affected types:

• Transport.onclose, Transport.onerror, Transport.onmessage, Transport.sessionId, Transport.setProtocolVersion, Transport.setSupportedProtocolVersions
• TransportSendOptions.onresumptiontoken
• WebStandardStreamableHTTPServerTransportOptions.sessionIdGenerator, .onsessioninitialized, .onsessionclosed

Fixes #1314

Test plan

• tsc --noEmit passes for all packages (core, server, client, middleware/node, middleware/express, middleware/hono)
• No functional changes -- only type-level additions of | undefined to optional property types
• Existing tests remain unaffected (this is a purely additive type change)

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: c725192

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/node	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/hono	Patch

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[pkg-pr-new]

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@modelcontextprotocol/client

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1507

@modelcontextprotocol/server

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1507

@modelcontextprotocol/express

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1507

@modelcontextprotocol/hono

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1507

@modelcontextprotocol/node

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1507

commit: c725192

[NatoBoram]

In case anyone wants to fix this locally in advance, here's a pnpm patch:

patches/@modelcontextprotocol__sdk.patch:

diff --git a/dist/esm/shared/transport.d.ts b/dist/esm/shared/transport.d.ts
index 1bd95efb41fcfd1ae09712ca968c57d828e0acaf..62ec4c109392f04141074d444b386882dfd2b052 100644
--- a/dist/esm/shared/transport.d.ts
+++ b/dist/esm/shared/transport.d.ts
@@ -62,13 +62,13 @@ 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.
      *
@@ -76,11 +76,11 @@ 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.
      */
-    sessionId?: string;
+    sessionId?: string | undefined;
     /**
      * Sets the protocol version used for the connection (called when the initialize response is received).
      */

[felixweinberger]

Thanks for the contribution! Closing in favor of #1766.