Debug HTTP Connections and Sessions

When an HTTP client cannot connect, debug the transport before you debug your tools.

Turn on transport logs

app.listen({
  port: Number(process.env.PORT ?? 3000),
  hostname: "0.0.0.0",
  cors: true,
  debug: true,
});

That prints request, session, initialize, and response logs from the HTTP transport.

Check the endpoint first

The MCP URL is:

http://localhost:3000/mcp

Do not point clients at /mcp/health or /mcp/schema. The current transport does not mount those routes by default. If you enabled health, use that custom path for platform checks only, not as the MCP client endpoint.

What the current transport serves

POST /mcp for JSON-RPC (and subscriptions/listen SSE on MCP 2026-07-28)
GET /mcp for the legacy sessioned event stream
DELETE /mcp for legacy session termination
an optional health route when you enable health

Practical compatibility notes

the transport negotiates MCP protocol versions with the client (2026-07-28 through 2024-11-05)
MCP 2026-07-28 is stateless: no Mcp-Session-Id, but Mcp-Method / Mcp-Name are required
older clients still use sessions and Inspector-style GET SSE flows
it accepts client-provided session IDs for better tooling interoperability on legacy versions

Common checks

make sure the client uses the /mcp path
keep one simple tool like ping until the handshake succeeds
if you are using stdio, send logs to stderr, not stdout

​Turn on transport logs

​Check the endpoint first

​What the current transport serves

​Practical compatibility notes

​Common checks

Turn on transport logs

Check the endpoint first

What the current transport serves

Practical compatibility notes

Common checks