Old MCP vs new MCP, on a real (local) serverless platform

A runnable proof of what the 2026-07-28 MCP revision means by "stateless", built on the official mcp Python SDK (2.0.0). A tiny but honest serverless platform cold-starts real server processes on demand, scales out under load, and kills anything idle for 4 seconds — then a narrated client runs the old protocol and the new protocol against that same platform. The old one dies (Session not found); the new one never notices, including a mid-call user question whose asking instance is dead before the answer arrives.

git clone https://github.com/jobvancreij/demo-mcp-stateless
cd demo-mcp-stateless
./run_demo.sh        # needs uv + Python >= 3.11; creates .venv on first run

Binds 127.0.0.1:9000 (the gateway) and 127.0.0.1:9102+ (instances); the script refuses to start if a previous gateway still holds :9000. The whole run takes about 40 seconds — most of it deliberate idling, waiting for the reaper.

What's in the box

The five acts

1. Old pattern — mode="legacy": handshake, session, tool call works; 4s of idleness later the platform reaps the instance and the next call is MCPError: Session not found. That error is the whole reason remote MCP needed sticky, always-on servers.
2. New pattern — mode="2026-07-28": first packet is a real request; three concurrent calls scale the fleet to 3; idleness scales it to 0; the same client object keeps working via fresh cold starts.
3. MRTR across instance death — book_flight pauses with input_required + AES-GCM-sealed requestState; the user "thinks" for 6s; the asking instance is reaped; the retry cold-starts a new process that unseals the state and completes the booking. Works only because all instances share the RequestStateSecurity key.
4. Durable state — the process-memory job handle dies with its instance; the SQLite-backed handle survives (started on 'i7', fetched from 'i8').
5. MCP Apps — boarding_pass is bound to ui://flight-desk/boarding-pass (text/html;profile=mcp-app) via the SDK's Apps extension; the demo writes boarding-pass.html with the tool's structured result injected the way a host's postMessage bridge would.

Fidelity notes

Server and client are the unmodified official SDK, so the wire format is the SDK's real 2026-07-28 implementation (and its real legacy implementation in Act 1). The platform is the toy part: a ~180-line process manager standing in for Lambda/Cloud Run/Workers. Out of scope: OAuth, Origin validation hardening, SSE streaming responses, subscriptions/listen.

Two consequences of json_response=True (every reply a plain JSON object, which keeps the gateway trivially bufferable) are worth naming. book_flight is 2026-07-28-only: a legacy session would need the SSE back-channel to deliver elicitation/create, and answers no back-channel for server-initiated requests without it — so Act 1 uses whoami. And because there is no authenticated transport here, the sealed requestState is bound to its originating request, expiry and audience, but not to a principal; add OAuth and the SDK binds that too.