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Propagate contextvars.Context through anyio streams without modifying SessionMessage (#2298)

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Marcelo Trylesinski 2026-03-31 12:49:38 -04:00 • committed by GitHub
commit e6235d1667
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1 changed files with 310 additions and 94 deletions
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23
src/mcp/client/streamable_http.py
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@ -11,11 +11,11 @@ from dataclasses import dataclass
import anyio
import httpx
from anyio.abc import TaskGroup
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from httpx_sse import EventSource, ServerSentEvent, aconnect_sse
from pydantic import ValidationError
from mcp.client._transport import TransportStreams
from mcp.shared._context_streams import ContextReceiveStream, ContextSendStream, create_context_streams
from mcp.shared._httpx_utils import create_mcp_http_client
from mcp.shared.message import ClientMessageMetadata, SessionMessage
from mcp.types import (
@ -38,8 +38,8 @@ logger = logging.getLogger(__name__)
# TODO(Marcelo): Put the TransportStreams in a module under shared, so we can import here.
SessionMessageOrError = SessionMessage | Exception
StreamWriter = MemoryObjectSendStream[SessionMessageOrError]
StreamReader = MemoryObjectReceiveStream[SessionMessage]
StreamWriter = ContextSendStream[SessionMessageOrError]
StreamReader = ContextReceiveStream[SessionMessage]
MCP_SESSION_ID = "mcp-session-id"
MCP_PROTOCOL_VERSION = "mcp-protocol-version"
@ -434,14 +434,15 @@ class StreamableHTTPTransport:
client: httpx.AsyncClient,
write_stream_reader: StreamReader,
read_stream_writer: StreamWriter,
write_stream: MemoryObjectSendStream[SessionMessage],
write_stream: ContextSendStream[SessionMessage],
start_get_stream: Callable[[], None],
tg: TaskGroup,
) -> None:
"""Handle writing requests to the server."""
try:
async with write_stream_reader, read_stream_writer, write_stream:
async for session_message in write_stream_reader:
async def _handle_message(session_message: SessionMessage) -> None:
message = session_message.message
metadata = (
session_message.metadata
@ -478,6 +479,14 @@ class StreamableHTTPTransport:
else:
await handle_request_async()
async for session_message in write_stream_reader:
sender_ctx = write_stream_reader.last_context
if sender_ctx is not None:
async with anyio.create_task_group() as tg_local:
sender_ctx.run(tg_local.start_soon, _handle_message, session_message)
else:
await _handle_message(session_message) # pragma: no cover
except Exception: # pragma: lax no cover
logger.exception("Error in post_writer")
@ -547,8 +556,8 @@ async def streamable_http_client(
if not client_provided:
await stack.enter_async_context(client)
read_stream_writer, read_stream = anyio.create_memory_object_stream[SessionMessage | Exception](0)
write_stream, write_stream_reader = anyio.create_memory_object_stream[SessionMessage](0)
read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
write_stream, write_stream_reader = create_context_streams[SessionMessage](0)
async with (
read_stream_writer,