Stateless HTTP mode reuses one MCP server across requests -> "Already connected to a transport" (breaks multi-request clients)

[Defilan]

Summary

In HTTP mode (src/http.ts / dist/http.js), createHttpApp creates a single McpServer at construction time and calls mcpServer.connect(transport) on every /mcp request:

const mcpServer = createPerplexityServer();          // one shared server
app.all("/mcp", async (req, res) => {
  const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: undefined, ... });
  await mcpServer.connect(transport);                // 2nd concurrent connect throws
  ...
});

Because the streamable-HTTP transport is stateless (sessionIdGenerator: undefined), there is no session to reuse. Once one request's transport is connected, any request that arrives before it closes calls connect() on the already-connected server and throws:

Error: Already connected to a transport. Call close() before connecting to a new transport, or use a separate Protocol instance per connection.

and the handler returns HTTP 500.

Impact

This breaks real MCP clients over HTTP. A standards-compliant client sends initialize, then tools/list (and typically holds a stream open), so its second request 500s and no tools ever register. A strictly-sequential caller happens to dodge it, because res.on("close") closes the transport and frees the shared server between requests — which is why a single one-shot curl "works" while an actual client does not.

Reproduce

Run the HTTP server and point any real MCP client (or two overlapping requests) at it:

PERPLEXITY_API_KEY=... npm run start:http

initialize succeeds; the follow-up request returns 500 and the server logs:

ERROR: Error handling MCP request {"error":"Error: Already connected to a transport. ..."}

Fix

Create the McpServer (and transport) per request — the SDK's documented stateless pattern — and close it on response close. PR attached.

Environment

• @perplexity-ai/mcp-server 0.9.0, streamable-HTTP transport (start:http), stateless mode.