StreamableHttp - stateless server support

examples/servers/simple-streamablehttp-stateless/README.md

@@ -0,0 +1,41 @@
+# MCP Simple StreamableHttp Stateless Server Example
+
+A stateless MCP server example demonstrating the StreamableHttp transport without maintaining session state. This example is ideal for understanding how to deploy MCP servers in multi-node environments where requests can be routed to any instance.
+
+## Features
+
+- Uses the StreamableHTTP transport in stateless mode (mcp_session_id=None)
+- Each request creates a new ephemeral connection
+- No session state maintained between requests
+- Task lifecycle scoped to individual requests
+- Suitable for deployment in multi-node environments
+
+
+## Usage
+
+Start the server:
+
+```bash
+# Using default port 3000
+uv run mcp-simple-streamablehttp-stateless
+
+# Using custom port
+uv run mcp-simple-streamablehttp-stateless --port 3000
+
+# Custom logging level
+uv run mcp-simple-streamablehttp-stateless --log-level DEBUG
+
+# Enable JSON responses instead of SSE streams
+uv run mcp-simple-streamablehttp-stateless --json-response
+```
+
+The server exposes a tool named "start-notification-stream" that accepts three arguments:
+
+- `interval`: Time between notifications in seconds (e.g., 1.0)
+- `count`: Number of notifications to send (e.g., 5)
+- `caller`: Identifier string for the caller
+
+
+## Client
+
+You can connect to this server using an HTTP client. For now, only the TypeScript SDK has streamable HTTP client examples, or you can use [Inspector](https://github.com/modelcontextprotocol/inspector) for testing.

examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/__main__.py

@@ -0,0 +1,4 @@
+from .server import main
+
+if __name__ == "__main__":
+    main()

examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py

@@ -0,0 +1,171 @@
+import contextlib
+import logging
+
+import anyio
+import click
+import mcp.types as types
+from mcp.server.lowlevel import Server
+from mcp.server.streamableHttp import (
+    StreamableHTTPServerTransport,
+)
+from starlette.applications import Starlette
+from starlette.routing import Mount
+
+logger = logging.getLogger(__name__)
+# Global task group that will be initialized in the lifespan
+task_group = None
+
+
+@contextlib.asynccontextmanager
+async def lifespan(app):
+    """Application lifespan context manager for managing task group."""
+    global task_group
+
+    async with anyio.create_task_group() as tg:
+        task_group = tg
+        logger.info("Application started, task group initialized!")
+        try:
+            yield
+        finally:
+            logger.info("Application shutting down, cleaning up resources...")
+            if task_group:
+                tg.cancel_scope.cancel()
+                task_group = None
+            logger.info("Resources cleaned up successfully.")
+
+
+@click.command()
+@click.option("--port", default=3000, help="Port to listen on for HTTP")
+@click.option(
+    "--log-level",
+    default="INFO",
+    help="Logging level (DEBUG, INFO, WARNING, ERROR, CRITICAL)",
+)
+@click.option(
+    "--json-response",
+    is_flag=True,
+    default=False,
+    help="Enable JSON responses instead of SSE streams",
+)
+def main(
+    port: int,
+    log_level: str,
+    json_response: bool,
+) -> int:
+    # Configure logging
+    logging.basicConfig(
+        level=getattr(logging, log_level.upper()),
+        format="%(asctime)s - %(name)s - %(levelname)s - %(message)s",
+    )
+
+    app = Server("mcp-streamable-http-stateless-demo")
+
+    @app.call_tool()
+    async def call_tool(
+        name: str, arguments: dict
+    ) -> list[types.TextContent | types.ImageContent | types.EmbeddedResource]:
+        ctx = app.request_context
+        interval = arguments.get("interval", 1.0)
+        count = arguments.get("count", 5)
+        caller = arguments.get("caller", "unknown")
+
+        # Send the specified number of notifications with the given interval
+        for i in range(count):
+            await ctx.session.send_log_message(
+                level="info",
+                data=f"Notification {i+1}/{count} from caller: {caller}",
+                logger="notification_stream",
+                related_request_id=ctx.request_id,
+            )
+            if i < count - 1:  # Don't wait after the last notification
+                await anyio.sleep(interval)
+
+        return [
+            types.TextContent(
+                type="text",
+                text=(
+                    f"Sent {count} notifications with {interval}s interval"
+                    f" for caller: {caller}"
+                ),
+            )
+        ]
+
+    @app.list_tools()
+    async def list_tools() -> list[types.Tool]:
+        return [
+            types.Tool(
+                name="start-notification-stream",
+                description=(
+                    "Sends a stream of notifications with configurable count"
+                    " and interval"
+                ),
+                inputSchema={
+                    "type": "object",
+                    "required": ["interval", "count", "caller"],
+                    "properties": {
+                        "interval": {
+                            "type": "number",
+                            "description": "Interval between notifications in seconds",
+                        },
+                        "count": {
+                            "type": "number",
+                            "description": "Number of notifications to send",
+                        },
+                        "caller": {
+                            "type": "string",
+                            "description": (
+                                "Identifier of the caller to include in notifications"
+                            ),
+                        },
+                    },
+                },
+            )
+        ]
+
+    # ASGI handler for stateless HTTP connections
+    async def handle_streamable_http(scope, receive, send):
+        logger.debug("Creating new transport")
+        # Use lock to prevent race conditions when creating new sessions
+        http_transport = StreamableHTTPServerTransport(
+            mcp_session_id=None,
+            is_json_response_enabled=json_response,
+        )
+        async with http_transport.connect() as streams:
+            read_stream, write_stream = streams
+
+            if not task_group:
+                raise RuntimeError("Task group is not initialized")
+
+            async def run_server():
+                await app.run(
+                    read_stream,
+                    write_stream,
+                    app.create_initialization_options(),
+                    # This allows the server to run without waiting for initialization
+                    require_initialization=False,
+                )
+
+            # Start server task
+            task_group.start_soon(run_server)
+
+            # Small delay to allow the server task to start
+            # This helps prevent race conditions in stateless mode
+            await anyio.sleep(0.001)
+
+            # Handle the HTTP request and return the response
+            await http_transport.handle_request(scope, receive, send)
+
+    # Create an ASGI application using the transport
+    starlette_app = Starlette(
+        debug=True,
+        routes=[
+            Mount("/mcp", app=handle_streamable_http),
+        ],
+        lifespan=lifespan,
+    )
+
+    import uvicorn
+
+    uvicorn.run(starlette_app, host="0.0.0.0", port=port)
+
+    return 0

examples/servers/simple-streamablehttp-stateless/pyproject.toml

@@ -0,0 +1,36 @@
+[project]
+name = "mcp-simple-streamablehttp-stateless"
+version = "0.1.0"
+description = "A simple MCP server exposing a StreamableHttp transport in stateless mode"
+readme = "README.md"
+requires-python = ">=3.10"
+authors = [{ name = "Anthropic, PBC." }]
+keywords = ["mcp", "llm", "automation", "web", "fetch", "http", "streamable", "stateless"]
+license = { text = "MIT" }
+dependencies = ["anyio>=4.5", "click>=8.1.0", "httpx>=0.27", "mcp", "starlette", "uvicorn"]
+
+[project.scripts]
+mcp-simple-streamablehttp-stateless = "mcp_simple_streamablehttp_stateless.server:main"
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["mcp_simple_streamablehttp_stateless"]
+
+[tool.pyright]
+include = ["mcp_simple_streamablehttp_stateless"]
+venvPath = "."
+venv = ".venv"
+
+[tool.ruff.lint]
+select = ["E", "F", "I"]
+ignore = []
+
+[tool.ruff]
+line-length = 88
+target-version = "py310"
+
+[tool.uv]
+dev-dependencies = ["pyright>=1.1.378", "pytest>=8.3.3", "ruff>=0.6.9"]

src/mcp/server/lowlevel/server.py

@@ -479,11 +479,20 @@ async def run(
         # but also make tracing exceptions much easier during testing and when using
         # in-process servers.
         raise_exceptions: bool = False,
+        # When True, the server will wait for the client to send an initialization
+        # message before processing any other messages.
+        # False should be used for stateless servers.
+        require_initialization: bool = True,
     ):
         async with AsyncExitStack() as stack:
             lifespan_context = await stack.enter_async_context(self.lifespan(self))
             session = await stack.enter_async_context(
-                ServerSession(read_stream, write_stream, initialization_options)
+                ServerSession(
+                    read_stream,
+                    write_stream,
+                    initialization_options,
+                    require_initialization,
+                )
             )
 
             async with anyio.create_task_group() as tg:

src/mcp/server/session.py

@@ -85,11 +85,15 @@ def __init__(
         read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception],
         write_stream: MemoryObjectSendStream[types.JSONRPCMessage],
         init_options: InitializationOptions,
+        require_initialization: bool = True,
     ) -> None:
         super().__init__(
             read_stream, write_stream, types.ClientRequest, types.ClientNotification
         )
-        self._initialization_state = InitializationState.NotInitialized
+        if require_initialization:
+            self._initialization_state = InitializationState.NotInitialized
+        else:
+            self._initialization_state = InitializationState.Initialized
         self._init_options = init_options
         self._incoming_message_stream_writer, self._incoming_message_stream_reader = (
             anyio.create_memory_object_stream[ServerRequestResponder](0)