bug: invalid types for sessionIdGenerator in 1.25.2

[domdomegg]

SDK 1.25.2 introduced types that break projects using sessionIdGenerator: undefined with strict TypeScript configs.

Errors

error TS2379: Argument of type '{ sessionIdGenerator: undefined; enableJsonResponse: true; }'
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'.

error 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'.
      Type 'undefined' is not assignable to type '() => void'.

Cause

Optional properties like sessionIdGenerator?: () => string need to be typed as sessionIdGenerator?: (() => string) | undefined to allow explicitly passing undefined.

Similarly, the Transport interface's onclose property needs the same treatment.

I beleive this was recently changed in #1326

[aldipower]

Confirm with 1.25.3 !

[stomde]

Supported versions

…

On Mon, Jan 26, 2026, 11:09 AM Felix Gertz ***@***.***> wrote: *aldipower* left a comment (modelcontextprotocol/typescript-sdk#1397) <#1397 (comment)> Confirm with 1.25.3 ! — Reply to this email directly, view it on GitHub <#1397 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ATZ5V7735JGS3MXIZFE3KXT4IXDSVAVCNFSM6AAAAACR6YD27CVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTOOJYGMZTENBXG4> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.*** com>

[codewithkenzo]

Hey, I can take this on.

The fix is adding | undefined to optional callback properties - this is the official TypeScript pattern for exactOptionalPropertyTypes compatibility:

sessionIdGenerator?: (() => string) | undefined

I'll audit the transport interfaces for other optional callbacks that need the same treatment.

Is this available?

[mozmo15]

This also happens with me when sessionIdGenerator is defined, example:

 const transport = new StreamableHTTPServerTransport({
                sessionIdGenerator: () => randomUUID(),
                onsessioninitialized: (sessionId) => {
                    transportsBySessionId.set(sessionId, transport);
                },
                onsessionclosed: (sessionId) => {
                    transportsBySessionId.delete(sessionId);
                },
 });
const mcpServer = getMcpServer();
await mcp.connect(transport);

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

Turns out, this only occurs when exactOptionalPropertyTypes is set to true in your tsconfig, which is why not everyone will run across this (exactOptionalPropertyTypes: Differentiate between undefined and not present when type checking)

Edit: nvm I am a bit late to state the obvious, @codewithkenzo changes should fix it

[mattzcarey]

I'm a bit worried we wont be able to maintain this compatibility. Let me know what you think @felixweinberger, otherwise #1439 looks ready to merge.