python-sdk/src/mcp/client/streamable_http.py

"""
StreamableHTTP Client Transport Module

This module implements the StreamableHTTP transport for MCP clients,
providing support for HTTP POST requests with optional SSE streaming responses
and session management.
"""

import logging
from collections.abc import AsyncGenerator, Awaitable, Callable
from contextlib import asynccontextmanager
from dataclasses import dataclass
from datetime import timedelta
from typing import Any

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 mcp.shared._httpx_utils import create_mcp_http_client
from mcp.shared.message import ClientMessageMetadata, SessionMessage
from mcp.types import (
    ErrorData,
    JSONRPCError,
    JSONRPCMessage,
    JSONRPCNotification,
    JSONRPCRequest,
    JSONRPCResponse,
    RequestId,
)

logger = logging.getLogger(__name__)


SessionMessageOrError = SessionMessage | Exception
StreamWriter = MemoryObjectSendStream[SessionMessageOrError]
StreamReader = MemoryObjectReceiveStream[SessionMessage]
GetSessionIdCallback = Callable[[], str | None]

MCP_SESSION_ID = "mcp-session-id"
LAST_EVENT_ID = "last-event-id"
CONTENT_TYPE = "content-type"
ACCEPT = "Accept"


JSON = "application/json"
SSE = "text/event-stream"


class StreamableHTTPError(Exception):
    """Base exception for StreamableHTTP transport errors."""

    pass


class ResumptionError(StreamableHTTPError):
    """Raised when resumption request is invalid."""

    pass


@dataclass
class RequestContext:
    """Context for a request operation."""

    client: httpx.AsyncClient
    headers: dict[str, str]
    session_id: str | None
    session_message: SessionMessage
    metadata: ClientMessageMetadata | None
    read_stream_writer: StreamWriter
    sse_read_timeout: timedelta


class StreamableHTTPTransport:
    """StreamableHTTP client transport implementation."""

    def __init__(
        self,
        url: str,
        headers: dict[str, Any] | None = None,
        timeout: timedelta = timedelta(seconds=30),
        sse_read_timeout: timedelta = timedelta(seconds=60 * 5),
    ) -> None:
        """Initialize the StreamableHTTP transport.

        Args:
            url: The endpoint URL.
            headers: Optional headers to include in requests.
            timeout: HTTP timeout for regular operations.
            sse_read_timeout: Timeout for SSE read operations.
        """
        self.url = url
        self.headers = headers or {}
        self.timeout = timeout
        self.sse_read_timeout = sse_read_timeout
        self.session_id: str | None = None
        self.request_headers = {
            ACCEPT: f"{JSON}, {SSE}",
            CONTENT_TYPE: JSON,
            **self.headers,
        }

    def _update_headers_with_session(
        self, base_headers: dict[str, str]
    ) -> dict[str, str]:
        """Update headers with session ID if available."""
        headers = base_headers.copy()
        if self.session_id:
            headers[MCP_SESSION_ID] = self.session_id
        return headers

    def _is_initialization_request(self, message: JSONRPCMessage) -> bool:
        """Check if the message is an initialization request."""
        return (
            isinstance(message.root, JSONRPCRequest)
            and message.root.method == "initialize"
        )

    def _is_initialized_notification(self, message: JSONRPCMessage) -> bool:
        """Check if the message is an initialized notification."""
        return (
            isinstance(message.root, JSONRPCNotification)
            and message.root.method == "notifications/initialized"
        )

    def _maybe_extract_session_id_from_response(
        self,
        response: httpx.Response,
    ) -> None:
        """Extract and store session ID from response headers."""
        new_session_id = response.headers.get(MCP_SESSION_ID)
        if new_session_id:
            self.session_id = new_session_id
            logger.info(f"Received session ID: {self.session_id}")

    async def _handle_sse_event(
        self,
        sse: ServerSentEvent,
        read_stream_writer: StreamWriter,
        original_request_id: RequestId | None = None,
        resumption_callback: Callable[[str], Awaitable[None]] | None = None,
    ) -> bool:
        """Handle an SSE event, returning True if the response is complete."""
        if sse.event == "message":
            try:
                message = JSONRPCMessage.model_validate_json(sse.data)
                logger.debug(f"SSE message: {message}")

                # If this is a response and we have original_request_id, replace it
                if original_request_id is not None and isinstance(
                    message.root, JSONRPCResponse | JSONRPCError
                ):
                    message.root.id = original_request_id

                session_message = SessionMessage(message)
                await read_stream_writer.send(session_message)

                # Call resumption token callback if we have an ID
                if sse.id and resumption_callback:
                    await resumption_callback(sse.id)

                # If this is a response or error return True indicating completion
                # Otherwise, return False to continue listening
                return isinstance(message.root, JSONRPCResponse | JSONRPCError)

            except Exception as exc:
                logger.error(f"Error parsing SSE message: {exc}")
                await read_stream_writer.send(exc)
                return False
        else:
            logger.warning(f"Unknown SSE event: {sse.event}")
            return False

    async def handle_get_stream(
        self,
        client: httpx.AsyncClient,
        read_stream_writer: StreamWriter,
    ) -> None:
        """Handle GET stream for server-initiated messages."""
        try:
            if not self.session_id:
                return

            headers = self._update_headers_with_session(self.request_headers)

            async with aconnect_sse(
                client,
                "GET",
                self.url,
                headers=headers,
                timeout=httpx.Timeout(
                    self.timeout.seconds, read=self.sse_read_timeout.seconds
                ),
            ) as event_source:
                event_source.response.raise_for_status()
                logger.debug("GET SSE connection established")

                async for sse in event_source.aiter_sse():
                    await self._handle_sse_event(sse, read_stream_writer)

        except Exception as exc:
            logger.debug(f"GET stream error (non-fatal): {exc}")

    async def _handle_resumption_request(self, ctx: RequestContext) -> None:
        """Handle a resumption request using GET with SSE."""
        headers = self._update_headers_with_session(ctx.headers)
        if ctx.metadata and ctx.metadata.resumption_token:
            headers[LAST_EVENT_ID] = ctx.metadata.resumption_token
        else:
            raise ResumptionError("Resumption request requires a resumption token")

        # Extract original request ID to map responses
        original_request_id = None
        if isinstance(ctx.session_message.message.root, JSONRPCRequest):
            original_request_id = ctx.session_message.message.root.id

        async with aconnect_sse(
            ctx.client,
            "GET",
            self.url,
            headers=headers,
            timeout=httpx.Timeout(
                self.timeout.seconds, read=ctx.sse_read_timeout.seconds
            ),
        ) as event_source:
            event_source.response.raise_for_status()
            logger.debug("Resumption GET SSE connection established")

            async for sse in event_source.aiter_sse():
                is_complete = await self._handle_sse_event(
                    sse,
                    ctx.read_stream_writer,
                    original_request_id,
                    ctx.metadata.on_resumption_token_update if ctx.metadata else None,
                )
                if is_complete:
                    break

    async def _handle_post_request(self, ctx: RequestContext) -> None:
        """Handle a POST request with response processing."""
        headers = self._update_headers_with_session(ctx.headers)
        message = ctx.session_message.message
        is_initialization = self._is_initialization_request(message)

        async with ctx.client.stream(
            "POST",
            self.url,
            json=message.model_dump(by_alias=True, mode="json", exclude_none=True),
            headers=headers,
        ) as response:
            if response.status_code == 202:
                logger.debug("Received 202 Accepted")
                return

            if response.status_code == 404:
                if isinstance(message.root, JSONRPCRequest):
                    await self._send_session_terminated_error(
                        ctx.read_stream_writer,
                        message.root.id,
                    )
                return

            response.raise_for_status()
            if is_initialization:
                self._maybe_extract_session_id_from_response(response)

            content_type = response.headers.get(CONTENT_TYPE, "").lower()

            if content_type.startswith(JSON):
                await self._handle_json_response(response, ctx.read_stream_writer)
            elif content_type.startswith(SSE):
                await self._handle_sse_response(response, ctx)
            else:
                await self._handle_unexpected_content_type(
                    content_type,
                    ctx.read_stream_writer,
                )

    async def _handle_json_response(
        self,
        response: httpx.Response,
        read_stream_writer: StreamWriter,
    ) -> None:
        """Handle JSON response from the server."""
        try:
            content = await response.aread()
            message = JSONRPCMessage.model_validate_json(content)
            session_message = SessionMessage(message)
            await read_stream_writer.send(session_message)
        except Exception as exc:
            logger.error(f"Error parsing JSON response: {exc}")
            await read_stream_writer.send(exc)

    async def _handle_sse_response(
        self, response: httpx.Response, ctx: RequestContext
    ) -> None:
        """Handle SSE response from the server."""
        try:
            event_source = EventSource(response)
            async for sse in event_source.aiter_sse():
                is_complete = await self._handle_sse_event(
                    sse,
                    ctx.read_stream_writer,
                    resumption_callback=(
                        ctx.metadata.on_resumption_token_update
                        if ctx.metadata
                        else None
                    ),
                )
                # If the SSE event indicates completion, like returning respose/error
                # break the loop
                if is_complete:
                    break
        except Exception as e:
            logger.exception("Error reading SSE stream:")
            await ctx.read_stream_writer.send(e)

    async def _handle_unexpected_content_type(
        self,
        content_type: str,
        read_stream_writer: StreamWriter,
    ) -> None:
        """Handle unexpected content type in response."""
        error_msg = f"Unexpected content type: {content_type}"
        logger.error(error_msg)
        await read_stream_writer.send(ValueError(error_msg))

    async def _send_session_terminated_error(
        self,
        read_stream_writer: StreamWriter,
        request_id: RequestId,
    ) -> None:
        """Send a session terminated error response."""
        jsonrpc_error = JSONRPCError(
            jsonrpc="2.0",
            id=request_id,
            error=ErrorData(code=32600, message="Session terminated"),
        )
        session_message = SessionMessage(JSONRPCMessage(jsonrpc_error))
        await read_stream_writer.send(session_message)

    async def post_writer(
        self,
        client: httpx.AsyncClient,
        write_stream_reader: StreamReader,
        read_stream_writer: StreamWriter,
        write_stream: MemoryObjectSendStream[SessionMessage],
        start_get_stream: Callable[[], None],
        tg: TaskGroup,
    ) -> None:
        """Handle writing requests to the server."""
        try:
            async with write_stream_reader:
                async for session_message in write_stream_reader:
                    message = session_message.message
                    metadata = (
                        session_message.metadata
                        if isinstance(session_message.metadata, ClientMessageMetadata)
                        else None
                    )

                    # Check if this is a resumption request
                    is_resumption = bool(metadata and metadata.resumption_token)

                    logger.debug(f"Sending client message: {message}")

                    # Handle initialized notification
                    if self._is_initialized_notification(message):
                        start_get_stream()

                    ctx = RequestContext(
                        client=client,
                        headers=self.request_headers,
                        session_id=self.session_id,
                        session_message=session_message,
                        metadata=metadata,
                        read_stream_writer=read_stream_writer,
                        sse_read_timeout=self.sse_read_timeout,
                    )

                    async def handle_request_async():
                        if is_resumption:
                            await self._handle_resumption_request(ctx)
                        else:
                            await self._handle_post_request(ctx)

                    # If this is a request, start a new task to handle it
                    if isinstance(message.root, JSONRPCRequest):
                        tg.start_soon(handle_request_async)
                    else:
                        await handle_request_async()

        except Exception as exc:
            logger.error(f"Error in post_writer: {exc}")
        finally:
            await read_stream_writer.aclose()
            await write_stream.aclose()

    async def terminate_session(self, client: httpx.AsyncClient) -> None:
        """Terminate the session by sending a DELETE request."""
        if not self.session_id:
            return

        try:
            headers = self._update_headers_with_session(self.request_headers)
            response = await client.delete(self.url, headers=headers)

            if response.status_code == 405:
                logger.debug("Server does not allow session termination")
            elif response.status_code != 200:
                logger.warning(f"Session termination failed: {response.status_code}")
        except Exception as exc:
            logger.warning(f"Session termination failed: {exc}")

    def get_session_id(self) -> str | None:
        """Get the current session ID."""
        return self.session_id


@asynccontextmanager
async def streamablehttp_client(
    url: str,
    headers: dict[str, Any] | None = None,
    timeout: timedelta = timedelta(seconds=30),
    sse_read_timeout: timedelta = timedelta(seconds=60 * 5),
    terminate_on_close: bool = True,
) -> AsyncGenerator[
    tuple[
        MemoryObjectReceiveStream[SessionMessage | Exception],
        MemoryObjectSendStream[SessionMessage],
        GetSessionIdCallback,
    ],
    None,
]:
    """
    Client transport for StreamableHTTP.

    `sse_read_timeout` determines how long (in seconds) the client will wait for a new
    event before disconnecting. All other HTTP operations are controlled by `timeout`.

    Yields:
        Tuple containing:
            - read_stream: Stream for reading messages from the server
            - write_stream: Stream for sending messages to the server
            - get_session_id_callback: Function to retrieve the current session ID
    """
    transport = StreamableHTTPTransport(url, headers, timeout, sse_read_timeout)

    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)

    async with anyio.create_task_group() as tg:
        try:
            logger.info(f"Connecting to StreamableHTTP endpoint: {url}")

            async with create_mcp_http_client(
                headers=transport.request_headers,
                timeout=httpx.Timeout(
                    transport.timeout.seconds, read=transport.sse_read_timeout.seconds
                ),
            ) as client:
                # Define callbacks that need access to tg
                def start_get_stream() -> None:
                    tg.start_soon(
                        transport.handle_get_stream, client, read_stream_writer
                    )

                tg.start_soon(
                    transport.post_writer,
                    client,
                    write_stream_reader,
                    read_stream_writer,
                    write_stream,
                    start_get_stream,
                    tg,
                )

                try:
                    yield (
                        read_stream,
                        write_stream,
                        transport.get_session_id,
                    )
                finally:
                    if transport.session_id and terminate_on_close:
                        await transport.terminate_session(client)
                    tg.cancel_scope.cancel()
        finally:
            await read_stream_writer.aclose()
            await write_stream.aclose()