[IMPROVEMENT]: Add a promise to the intializeServer

[aeciolevy]

The problem is that if you have any resource or tool that needs to run an async operation through a callback or a promise, you can run into an issue like this:

unhandledRejection: Error: Cannot register capabilities after connecting to transport

One way to handle this is bypassing the server with the necessary capabilities upfront.
Alternatively, we could pass a callback or a promise handler that can manage this situation gracefully to invoke the connection with transport after the async operation is out of the stack.

[quuu]

hey @aeciolevy can you share a code snippet of this?

[aeciolevy]

hey @aeciolevy can you share a code snippet of this?

Yes.

I was running this to add the resources to the database, and every time I used it, I got the error.

  const client = await pool.connect();
  let tables: string[] = [];
  try {
    const res = await client.query(
      "SELECT table_name FROM information_schema.tables WHERE table_schema = 'public'",
    );
    tables = res.rows.map((r) => r.table_name);
  } finally {
    client.release();
  }

  for (const tableName of tables) {
    const uri = `${resourceBaseUrl.href}${tableName}/${SCHEMA_PATH}`;
    server.resource(tableName, uri, async () => {
      const client = await pool.connect();
      try {
        const res = await client.query(
          "SELECT column_name, data_type FROM information_schema.columns WHERE table_name = $1",
          [tableName],
        );
        return {
          contents: [
            {
              uri,
              mimeType: "application/json",
              text: JSON.stringify(res.rows, null, 2),
            },
          ],
        };
      } finally {
        client.release();
      }
    });
 

From the source code, it looks like it calls initializeServer in a sync first, and then connect. Probably, if there’s any async operation inside, connect ends up being called before it has been completed.

initializeServer(statelessServer);
yield statelessServer.connect(statelessTransport);

[aomi]

Also running into this issue. Is it as simple as adding an await to initializeServer in the mcpApiHandler?
It seems to work under chose changes locally.

[quuu]

hey @aomi @aeciolevy thanks for the bump!

Just implemented and tested the change, works fine!

please try upgrading to version 0.6.1 and reporting back if you still can't perform your async calls within the handler:

#22

[aomi]

Thanks @quuu
I've gone ahead and tested it with the HTTP streamable transport and works but it's still broken for SSE.
There's another initializeServer in the SSE path which needs to be fixed as well I believe.

[quuu]

Thanks @quuu I've gone ahead and tested it with the HTTP streamable transport and works but it's still broken for SSE. There's another initializeServer in the SSE path which needs to be fixed as well I believe.

good catch, hoping SSE goes away soon :) fixed SSE in 0.6.2

let me know!

[aomi]

Changes look good and I've tested SSE and seems to work for me.
Ditto on SSE. We've been having issues with clients of MCP not being entirely clear with their compatibility so unfortunately need SSE just to be safe :(

[quuu]

@aomi the best workaround to get clients to support StreamableHTTP is by using mcp-remote!

eg: a user of the mcp-adapter https://mcp.solana.com/ has instructions that uses StreamableHTTP (not SSE) on the backend, but allows existing clients like Claude/Cursor/Windsurf to consume the server using Stdio :)

[aeciolevy]

Thank a lot @quuu 🙏

[quuu]

let me know how else I can help! would love to see what you build with the adapter :D