The 2026-07-28 protocol did not work over stdio: subscriptions/listen
was hard-refused, a legacy initialize arriving during an in-flight
modern request was accepted and re-locked the connection, and a peer
cancel produced a trailing "Request cancelled" frame. All three share
one root cause: the connection's era was derived from which requests
had completed instead of being decided once, in wire order, from how
the client opened the connection.
Replace serve_dual_era_loop and serve_loop with a single serve_stream
driver that decides the era synchronously in the dispatcher's read
loop, before the request body is spawned. initialize (or any
envelope-less request) opens the legacy era, an enveloped request opens
the modern era, server/discover is answered without pinning, and a
stray leading notification opens nothing. A conflicting era claim on a
committed connection is refused (-32022 or -32600) rather than
silently switching.
Cancel silence is structural: each request answers through a one-shot
channel whose write target becomes a powerless void on a peer cancel,
so there is no cancelled-check at any write site. The generic JSON-RPC
dispatcher loses the code-0 cancel frame, the code-0 str(exc)
catch-all, and the inline_methods knob, and documents that handlers
are invoked synchronously in receive order with the returned awaitable
as the body.
Add a Posture enum (DUAL default, LEGACY_ONLY, MODERN_ONLY) on the
Server and MCPServer constructors, honoured by the stream driver and
the streamable-HTTP manager alike. Server.run(read, write) now stands
alone, Server.lifespan() is a bound context manager, and
serve_listener / newline_json_transport / close_subscriptions() give a
straightforward path for custom transports. See docs/migration.md for
the full list of observable changes.
Max Isbey