bun run express SSEServerTransport BUG

[phpmac]

What version of Bun is running?

1.2.5

What platform is your computer?

Darwin M3Pro.local 24.4.0 Darwin Kernel Version 24.4.0: Wed Mar 19 21:17:32 PDT 2025; root:xnu-11417.101.15~1/RELEASE_ARM64_T6030 arm64

What steps can reproduce the bug?

import express from "express";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import { z } from "zod";

const app = express();

const server = new McpServer(
    {
        name: "example-server",
        version: "1.0.0",
    },
    {
        capabilities: {},
    }
);

// Add an addition tool
server.tool("add", { a: z.number(), b: z.number() }, async ({ a, b }) => ({
    content: [{ type: "text", text: String(a + b) }],
}));

let transport: SSEServerTransport | null = null;

app.get("/sse", (req, res) => {
    transport = new SSEServerTransport("/messages", res);
    server.connect(transport);
});

app.post("/messages", (req, res) => {
    if (transport) {
        transport.handlePostMessage(req, res);
    }
});

app.listen(3000);

What is the expected behavior?

No response
bun run main.ts will not be able to access /sse
npx tsx main.ts is normal

What do you see instead?

No response

Additional information

No response

[cirospaciari]

This was fixed by #18804 and is present in bun v1.2.9

[phpmac]

This was fixed by #18804 and is present in bun v1.2.9

Unfortunately, I found a bug that will cause some programs to fail to connect and not respond.