fix: add explicit | undefined to Transport optional properties (v1.x backport of #1766)

[kishormorol]

Backport of #1766 to the v1.x line. Fixes #2083 for v1.

Problem

Under exactOptionalPropertyTypes: true, onclose?: () => void means the property may be absent, but never explicitly undefined. The concrete transports declare these members as accessors typed (() => void) | undefined, so the SDK's own transports are not assignable to the SDK's own Transport interface:

TS2379: Argument of type 'StreamableHTTPServerTransport' is not assignable to parameter
of type 'Transport' with 'exactOptionalPropertyTypes: true'.
  Types of property 'onclose' are incompatible.
    Type '(() => void) | undefined' is not assignable to type '() => void'.

Reproduce with strict + exactOptionalPropertyTypes: true, then server.connect(new StreamableHTTPServerTransport({ sessionIdGenerator: () => randomUUID() })).

Consumers currently have to cast at the call site or disable the flag for their whole project.

Change

Four optional members of Transport gain an explicit | undefined:

onclose?: (() => void) | undefined;
onerror?: ((error: Error) => void) | undefined;
onmessage?: (<T extends JSONRPCMessage>(message: T, extra?: MessageExtraInfo) => void) | undefined;
sessionId?: string | undefined;

Source-compatible for every existing consumer: it only permits an explicit undefined that the interface previously rejected. This is the same change already accepted on main in #1766, which has not reached v1.x — so 1.30.0 still ships the issue.

Verification

• npm run typecheck clean
• npm test — 1647 tests across 52 files, all passing
• Built this branch, installed the tarball into a real MCP server project that had been forced to set exactOptionalPropertyTypes: false because of this, turned the flag back on, and typechecked clean
• Changeset added as a patch, matching fix(core): add explicit | undefined to Transport interface optional properties #1766

Happy to adjust or close if a v1 patch release isn't planned — filing it because #2083 notes a backport is what's needed.

[changeset-bot]

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