handle async onmessage hook

[fabio-sassi-spotview]

Motivation and Context

Using the example code from README:

// Legacy message endpoint for older clients
app.post('/messages', async (req, res) => {
  const sessionId = req.query.sessionId as string;
  const transport = transports.sse[sessionId];
  if (transport) {
    await transport.handlePostMessage(req, res, req.body);
  } else {
    res.status(400).send('No transport found for sessionId');
  }
});

I notice that "await" is not respected

    await transport.handlePostMessage(req, res, req.body);

The reason is a missing await in handlePostMessage 

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• [ x] I have read the MCP Documentation
• [ x] My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[felixweinberger]

typescript-sdk/src/server/sse.ts

Line 22 in 606c278

onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;

onmessage is not an Awaitable, so adding await does nothing here.

[fabio-sassi-spotview]

typescript-sdk/src/server/sse.ts

Line 22 in 606c278

onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;

onmessage is not an Awaitable, so adding await does nothing here.

Hello @felixweinberger,

Based on my understanding of the code, it seems to me that (async) handlePostMessage calls (async) handleMessage, which then calls without await onmessage.

So if I use await handlePostMessage in the code, the hook is not actually called with await.

[felixweinberger]

typescript-sdk/src/server/sse.ts

Line 22 in 606c278

onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;

onmessage is not an Awaitable, so adding await does nothing here.

Hello @felixweinberger,

Based on my understanding of the code, it seems to me that (async) handlePostMessage calls (async) handleMessage, which then calls without await onmessage.

So if I use await handlePostMessage in the code, the hook is not actually called with await.

Are you running into an actual issue with the example code? Adding await here should do nothing, given this.onmessage does not return a promise.