fix: support exactOptionalPropertyTypes for optional callbacks

[codewithkenzo]

Summary

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

Fixes #1397

Changes

Transport interface (packages/core/src/shared/transport.ts):

• onclose?: (() => void) | undefined
• onerror?: ((error: Error) => void) | undefined
• onmessage?: (<T extends JSONRPCMessage>(...) => void) | undefined

WebStandardStreamableHTTPServerTransportOptions (packages/server/src/server/streamableHttp.ts):

• sessionIdGenerator?: (() => string) | undefined
• onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined
• onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined

Why This Pattern

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

Without this fix, users with strict TypeScript configs get errors like:

error TS2379: Argument of type '{ sessionIdGenerator: undefined; ... }'
is not assignable to parameter of type 'WebStandardStreamableHTTPServerTransportOptions' with
'exactOptionalPropertyTypes: true'.
  Types of property 'sessionIdGenerator' are incompatible.
    Type 'undefined' is not assignable to type '() => string'.

Testing

• ✅ pnpm typecheck:all passes
• ✅ No new test failures (1 pre-existing failure in authExtensions.test.ts unrelated to this change)

[changeset-bot]

🦋 Changeset detected

Latest commit: e943b7a

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1439

@modelcontextprotocol/server

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1439

@modelcontextprotocol/express

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1439

@modelcontextprotocol/hono

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1439

@modelcontextprotocol/node

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1439

commit: e943b7a

[codewithkenzo]

Note: This PR is independent from #1433 (onerror callback fix) - they touch different parts of the code:

• fix(server): call onerror callback for all transport errors #1433 adds runtime code (this.onerror?.() calls in method bodies)
• fix: support exactOptionalPropertyTypes for optional callbacks #1439 fixes type definitions (adding | undefined to interface properties)

No conflicts - both can be merged independently.