Serve the 2026-07-28 era over stdio and other stream-pair transports

src/mcp/server/connection.py

@@ -100,6 +100,30 @@ async def notify(self, method: str, params: Mapping[str, Any] | None, opts: Call
 _NO_CHANNEL = _NoChannelOutbound()
 
 
+class NotifyOnlyOutbound:
+    """Connection-scoped `Outbound` that forwards notifications and refuses requests.
+
+    Installed by `serve_dual_era_loop` for modern (2026-07-28+) connections
+    over duplex stream transports: the pipe is real, so server notifications
+    ride it, but the modern protocol forbids server-initiated JSON-RPC
+    requests, so `send_raw_request` refuses by construction.
+    """
+
+    def __init__(self, outbound: Outbound) -> None:
+        self._outbound = outbound
+
+    async def send_raw_request(
+        self,
+        method: str,
+        params: Mapping[str, Any] | None,
+        opts: CallOptions | None = None,
+    ) -> dict[str, Any]:
+        raise NoBackChannelError(method)
+
+    async def notify(self, method: str, params: Mapping[str, Any] | None, opts: CallOptions | None = None) -> None:
+        await self._outbound.notify(method, params, opts)
+
+
 class Connection:
     """Per-client connection state and standalone-stream `Outbound`.
 
@@ -167,7 +191,8 @@ def from_envelope(
         both supplied) are recorded as `client_params` so capability checks
         work. `outbound` defaults to the no-channel sentinel for the
         single-exchange HTTP path; duplex modern transports (e.g. stdio) pass
-        the dispatcher so server-initiated messages have a back-channel.
+        a notify-only wrapper around the dispatcher so server notifications
+        ride the pipe while server-initiated requests stay refused.
         """
         client_params = None
         if client_info is not None and client_capabilities is not None:

src/mcp/server/lowlevel/server.py

@@ -62,7 +62,7 @@ async def main():
 from mcp.server.caching import CacheableMethod, CacheHint, validate_cache_hints
 from mcp.server.context import HandlerResult, ServerMiddleware, ServerRequestContext
 from mcp.server.models import InitializationOptions
-from mcp.server.runner import serve_loop
+from mcp.server.runner import serve_dual_era_loop
 from mcp.server.streamable_http import EventStore
 from mcp.server.streamable_http_manager import StreamableHTTPASGIApp, StreamableHTTPSessionManager
 from mcp.server.transport_security import TransportSecuritySettings
@@ -689,12 +689,14 @@ async def run(
     ) -> None:
         """Serve a single connection over the given streams until the read side closes.
 
-        Thin wrapper over `serve_loop`: enters the server lifespan,
-        then drives the loop. Transports with their own lifespan owner
-        (the streamable-HTTP manager) call `serve_loop` directly instead.
+        Thin wrapper over `serve_dual_era_loop`: enters the server lifespan,
+        then drives the loop, serving the legacy handshake era and the modern
+        per-request-envelope era (the first era-distinctive message locks the
+        connection). Transports with their own lifespan owner (the
+        streamable-HTTP manager) call `serve_loop` directly instead.
         """
         async with self.lifespan(self) as lifespan_context:
-            await serve_loop(
+            await serve_dual_era_loop(
                 self,
                 read_stream,
                 write_stream,

src/mcp/server/runner.py

@@ -5,8 +5,8 @@
 pure kernel: it holds a pre-populated `Connection` and reads
 `connection.protocol_version` / `connection.outbound` as facts. Driving a
 dispatcher loop and tearing down the connection live in the free-function
-drivers (`serve_connection`, `serve_loop`, `serve_one`); the entry constructs
-the `Connection`, the driver tears it down.
+drivers (`serve_connection`, `serve_loop`, `serve_dual_era_loop`, `serve_one`);
+the entry constructs the `Connection`, the driver tears it down.
 
 `ServerRunner` holds a `Server` directly - `Server` is the registry.
 """
@@ -17,7 +17,7 @@
 from collections.abc import Awaitable, Mapping
 from dataclasses import KW_ONLY, dataclass
 from functools import cached_property, partial
-from typing import TYPE_CHECKING, Any, Generic, cast
+from typing import TYPE_CHECKING, Any, Generic, Literal, cast
 
 import anyio
 import anyio.abc
@@ -26,31 +26,41 @@
     CLIENT_INFO_META_KEY,
     INTERNAL_ERROR,
     INVALID_PARAMS,
+    INVALID_REQUEST,
     METHOD_NOT_FOUND,
     PROTOCOL_VERSION_META_KEY,
+    UNSUPPORTED_PROTOCOL_VERSION,
     CacheableResult,
     ErrorData,
     Implementation,
     InitializeRequestParams,
     InitializeResult,
+    RequestId,
     RequestParams,
     RequestParamsMeta,
+    UnsupportedProtocolVersionErrorData,
 )
 from mcp_types import methods as _methods
-from mcp_types.version import HANDSHAKE_PROTOCOL_VERSIONS, LATEST_HANDSHAKE_VERSION, LATEST_MODERN_VERSION
+from mcp_types.version import (
+    HANDSHAKE_PROTOCOL_VERSIONS,
+    LATEST_HANDSHAKE_VERSION,
+    LATEST_MODERN_VERSION,
+    MODERN_PROTOCOL_VERSIONS,
+)
 from pydantic import BaseModel, ValidationError
 from typing_extensions import TypeVar
 
 from mcp.server.caching import apply_cache_hint
-from mcp.server.connection import Connection
+from mcp.server.connection import Connection, NotifyOnlyOutbound
 from mcp.server.context import CallNext, HandlerResult, ServerMiddleware, ServerRequestContext
 from mcp.server.models import InitializationOptions
 from mcp.server.session import ServerSession
 from mcp.shared._stream_protocols import ReadStream, WriteStream
-from mcp.shared.dispatcher import DispatchContext, Dispatcher, OnNotify, OnRequest
-from mcp.shared.exceptions import MCPError
+from mcp.shared.dispatcher import CallOptions, DispatchContext, Dispatcher, OnNotify, OnRequest
+from mcp.shared.exceptions import MCPError, NoBackChannelError
+from mcp.shared.inbound import InboundLadderRejection, classify_inbound_request
 from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
-from mcp.shared.message import ServerMessageMetadata, SessionMessage
+from mcp.shared.message import MessageMetadata, ServerMessageMetadata, SessionMessage
 from mcp.shared.transport_context import TransportContext
 
 if TYPE_CHECKING:
@@ -63,6 +73,7 @@
     "aclose_shielded",
     "modern_on_request",
     "serve_connection",
+    "serve_dual_era_loop",
     "serve_loop",
     "serve_one",
 ]
@@ -427,6 +438,206 @@ async def serve_loop(
     )
 
 
+_MODERN_ENVELOPE_KEYS = (PROTOCOL_VERSION_META_KEY, CLIENT_INFO_META_KEY, CLIENT_CAPABILITIES_META_KEY)
+
+
+def _has_modern_envelope(params: Mapping[str, Any] | None) -> bool:
+    """Whether `params._meta` carries every reserved modern-envelope key.
+
+    Era evidence is the FULL key triple - bare `_meta` is not (legacy traffic
+    carries `progressToken` there).
+    """
+    if not params:
+        return False
+    meta = params.get("_meta")
+    return isinstance(meta, Mapping) and all(key in meta for key in _MODERN_ENVELOPE_KEYS)
+
+
+def _initialize_after_modern_data(params: Mapping[str, Any] | None) -> dict[str, Any]:
+    """Error data for an `initialize` arriving on a modern-locked connection.
+
+    The typed -32022 payload when the client's proposed version is parseable;
+    otherwise just the supported list (the point is naming what we serve).
+    """
+    requested = (params or {}).get("protocolVersion")
+    if isinstance(requested, str):
+        return UnsupportedProtocolVersionErrorData(
+            supported=list(MODERN_PROTOCOL_VERSIONS), requested=requested
+        ).model_dump(mode="json")
+    return {"supported": list(MODERN_PROTOCOL_VERSIONS)}
+
+
+@dataclass
+class _NoServerRequestsDispatchContext:
+    """Delegating `DispatchContext` that refuses server-initiated requests.
+
+    Wraps the loop dispatcher's per-message context for modern-era dispatch:
+    the modern protocol forbids server-initiated JSON-RPC requests, so
+    `send_raw_request` refuses while notifications and progress still ride
+    the duplex pipe.
+    """
+
+    _inner: DispatchContext[TransportContext]
+
+    @property
+    def transport(self) -> TransportContext:
+        return self._inner.transport
+
+    @property
+    def can_send_request(self) -> bool:
+        return False
+
+    @property
+    def request_id(self) -> RequestId | None:
+        return self._inner.request_id
+
+    @property
+    def message_metadata(self) -> MessageMetadata:
+        return self._inner.message_metadata
+
+    @property
+    def cancel_requested(self) -> anyio.Event:
+        return self._inner.cancel_requested
+
+    async def send_raw_request(
+        self,
+        method: str,
+        params: Mapping[str, Any] | None,
+        opts: CallOptions | None = None,
+    ) -> dict[str, Any]:
+        raise NoBackChannelError(method)
+
+    async def notify(self, method: str, params: Mapping[str, Any] | None, opts: CallOptions | None = None) -> None:
+        await self._inner.notify(method, params, opts)
+
+    async def progress(self, progress: float, total: float | None = None, message: str | None = None) -> None:
+        await self._inner.progress(progress, total, message)
+
+
+async def serve_dual_era_loop(
+    server: Server[LifespanT],
+    read_stream: ReadStream[SessionMessage | Exception],
+    write_stream: WriteStream[SessionMessage],
+    *,
+    lifespan_state: LifespanT,
+    session_id: str | None = None,
+    init_options: InitializationOptions | None = None,
+    raise_exceptions: bool = False,
+) -> None:
+    """Drive `server` over a duplex stream pair, serving both protocol eras.
+
+    The stream-pair counterpart of the modern HTTP entry's era router. Era is
+    a property of the connection, decided by how the client opens it, and
+    mid-stream switching is undefined - so the first era-distinctive message
+    locks the connection (matching the typescript-sdk):
+
+    - `initialize` locks legacy: the connection behaves exactly like
+      `serve_loop` for its lifetime, and modern envelope traffic is rejected
+      with INVALID_REQUEST.
+    - A request carrying the modern `_meta` envelope triple - or
+      `server/discover`, a modern-only method - locks modern: every request is
+      classified (`classify_inbound_request`) and served single-exchange via
+      `serve_one` with a born-ready per-request `Connection`, the same
+      dispatch model as the modern HTTP entry. A later `initialize` is
+      rejected with UNSUPPORTED_PROTOCOL_VERSION naming the modern versions.
+
+    Modern connections push notifications over the duplex pipe but refuse
+    server-initiated requests on both channels (the modern protocol forbids
+    them). A rejected classification (malformed envelope, unsupported version)
+    never locks the era, so a failed probe leaves the legacy handshake
+    available - released auto-negotiating clients fall back on any error code
+    except -32022.
+    """
+    dispatcher: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(
+        read_stream,
+        write_stream,
+        raise_handler_exceptions=raise_exceptions,
+        # `initialize` inline for the same pipelining reason as `serve_loop`;
+        # `server/discover` inline so the modern era lock commits before the
+        # next pipelined message is read.
+        inline_methods=frozenset({"initialize", "server/discover"}),
+    )
+    loop_connection = Connection.for_loop(dispatcher, session_id=session_id)
+    loop_runner = ServerRunner(server, loop_connection, lifespan_state, init_options=init_options)
+    standalone_outbound = NotifyOnlyOutbound(dispatcher)
+    era: Literal["unlocked", "legacy", "modern"] = "unlocked"
+    modern_version = LATEST_MODERN_VERSION
+
+    async def serve_modern(
+        dctx: DispatchContext[TransportContext], method: str, params: Mapping[str, Any] | None
+    ) -> dict[str, Any]:
+        nonlocal era, modern_version
+        route = classify_inbound_request({"method": method, "params": params})
+        if isinstance(route, InboundLadderRejection):
+            raise MCPError(code=route.code, message=route.message, data=route.data)
+        if era != "modern":
+            era, modern_version = "modern", route.protocol_version
+        if method == "subscriptions/listen":
+            # The registered listen handler assumes the HTTP entry's stream
+            # semantics; served over a stream pair it would wedge. Reject until
+            # this transport grows its own listen design.
+            raise MCPError(
+                code=METHOD_NOT_FOUND, message="subscriptions/listen is not served over this transport", data=method
+            )
+        connection = Connection.from_envelope(
+            route.protocol_version,
+            route.client_info,
+            route.client_capabilities,
+            outbound=standalone_outbound,
+        )
+        return await serve_one(
+            server,
+            _NoServerRequestsDispatchContext(dctx),
+            method,
+            params,
+            connection=connection,
+            lifespan_state=lifespan_state,
+        )
+
+    async def on_request(
+        dctx: DispatchContext[TransportContext], method: str, params: Mapping[str, Any] | None
+    ) -> dict[str, Any]:
+        nonlocal era
+        if era == "legacy":
+            if method == "server/discover" or _has_modern_envelope(params):
+                raise MCPError(
+                    code=INVALID_REQUEST,
+                    message="connection is locked to the legacy handshake era; "
+                    "modern envelope requests are not accepted",
+                )
+            return await loop_runner.on_request(dctx, method, params)
+        if era == "modern" and method == "initialize":
+            raise MCPError(
+                code=UNSUPPORTED_PROTOCOL_VERSION,
+                message="connection already negotiated a modern protocol version",
+                data=_initialize_after_modern_data(params),
+            )
+        if era == "modern" or method == "server/discover" or _has_modern_envelope(params):
+            return await serve_modern(dctx, method, params)
+        result = await loop_runner.on_request(dctx, method, params)
+        if method == "initialize":
+            # Lock only on success: a failed handshake leaves both eras open.
+            era = "legacy"
+        return result
+
+    async def on_notify(dctx: DispatchContext[TransportContext], method: str, params: Mapping[str, Any] | None) -> None:
+        if era != "modern":
+            return await loop_runner.on_notify(dctx, method, params)
+        # The envelope is request-only, so notifications inherit the
+        # connection's locked version.
+        connection = Connection.from_envelope(modern_version, None, None, outbound=standalone_outbound)
+        notify_runner = ServerRunner(server, connection, lifespan_state)
+        try:
+            await notify_runner.on_notify(_NoServerRequestsDispatchContext(dctx), method, params)
+        finally:
+            await aclose_shielded(connection)
+
+    try:
+        await dispatcher.run(on_request, on_notify)
+    finally:
+        await aclose_shielded(loop_connection)
+
+
 async def serve_one(
     server: Server[LifespanT],
     dctx: DispatchContext[TransportContext],