MCP server fails after first request: stateless transport reuse bug

[seth-with-zest]

Bug Description

Happy's MCP server (which provides the change_title tool) returns 500 errors on every request after startup, causing the mcp__happy__change_title tool to never register in Claude Code.

Root Cause

In startHappyServer() (dist/index-B3gQr6vs.mjs:4779), the StreamableHTTPServerTransport is created with sessionIdGenerator: void 0 (stateless mode):

const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: void 0
});
await mcp.connect(transport);
const server = createServer(async (req, res) => {
    try {
        await transport.handleRequest(req, res);
    } catch (error) {
        logger.debug("Error handling request:", error);
        if (!res.headersSent) {
            res.writeHead(500).end();
        }
    }
});

In @modelcontextprotocol/sdk v1.26.0 (the version Happy ships), stateless mode makes the transport single-use. After handling one request, subsequent calls to handleRequest() throw:

"Stateless transport cannot be reused across requests. Create a new transport per request."

This is from webStandardStreamableHttp.js:139-140:

if (!this.sessionIdGenerator && this._hasHandledRequest) {
    throw new Error('Stateless transport cannot be reused across requests. Create a new transport per request.');
}

The thrown error is caught by Happy's server handler, which returns an empty 500 response. Claude Code then sees the MCP server as broken, the change_title tool never registers, and ToolSearch finds nothing.

Expected Behavior

The MCP server should handle multiple requests throughout the session lifetime, allowing change_title to be discovered and called.

Suggested Fixes

Either:

1. Create a new transport per request (what stateless mode requires):

const server = createServer(async (req, res) => {
    const transport = new StreamableHTTPServerTransport({
        sessionIdGenerator: void 0
    });
    await mcp.connect(transport);
    try {
        await transport.handleRequest(req, res);
    } catch (error) { ... }
});

2. Use stateful mode by providing a sessionIdGenerator:

const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: () => randomUUID()
});

Environment

• happy-coder: 0.13.0
• @modelcontextprotocol/sdk: 1.26.0
• Claude Code: 2.1.39
• macOS (darwin arm64)
• Node: v25.2.1

Reproduction

Every session started via Happy exhibits this behavior. The symptom from the user's perspective is:

ToolSearch query: "select:mcp__happy__change_title"
→ "No matching deferred tools found"

The daemon log shows the MCP server starting successfully, but any curl to the server returns 500:

curl -v -X POST http://127.0.0.1:<port>/ \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","method":"initialize","params":{...},"id":1}'
# → HTTP/1.1 500 Internal Server Error (empty body)

[VrianCao]

#584

[LightYear512]

I solved it by downgrade @modelcontextprotocol/sdk to 2.4.0 temporarily.
#585

[darkmirage]

+1 — experiencing the same issue on happy-coder 0.13.0 / macOS. The mcp__happy__change_title tool never registers because the stateless transport returns 500 after the first request.

[andreadellacorte]

#619 #620

[EdamAme-x]

same problem at latest version