StreamableHttp - GET request standalone SSE (#561)

examples/servers/simple-streamablehttp/mcp_simple_streamablehttp/server.py

@@ -11,6 +11,7 @@ from mcp.server.streamableHttp import (
     MCP_SESSION_ID_HEADER,
     StreamableHTTPServerTransport,
 )
+from pydantic import AnyUrl
 from starlette.applications import Starlette
 from starlette.requests import Request
 from starlette.responses import Response
@@ -92,6 +93,9 @@ def main(
             if i < count - 1:  # Don't wait after the last notification
                 await anyio.sleep(interval)
 
+        # This will send a resource notificaiton though standalone SSE
+        # established by GET request
+        await ctx.session.send_resource_updated(uri=AnyUrl("http:///test_resource"))
         return [
             types.TextContent(
                 type="text",

src/mcp/server/streamableHttp.py

@@ -50,6 +50,9 @@ LAST_EVENT_ID_HEADER = "last-event-id"
 CONTENT_TYPE_JSON = "application/json"
 CONTENT_TYPE_SSE = "text/event-stream"
 
+# Special key for the standalone GET stream
+GET_STREAM_KEY = "_GET_stream"
+
 # Session ID validation pattern (visible ASCII characters ranging from 0x21 to 0x7E)
 # Pattern ensures entire string contains only valid characters by using ^ and $ anchors
 SESSION_ID_PATTERN = re.compile(r"^[\x21-\x7E]+$")
@@ -443,10 +446,19 @@ class StreamableHTTPServerTransport:
             return
 
     async def _handle_get_request(self, request: Request, send: Send) -> None:
-        """Handle GET requests for SSE stream establishment."""
-        # Validate session ID if server has one
-        if not await self._validate_session(request, send):
-            return
+        """
+        Handle GET request to establish SSE.
+
+        This allows the server to communicate to the client without the client
+        first sending data via HTTP POST. The server can send JSON-RPC requests
+        and notifications on this stream.
+        """
+        writer = self._read_stream_writer
+        if writer is None:
+            raise ValueError(
+                "No read stream writer available. Ensure connect() is called first."
+            )
+
         # Validate Accept header - must include text/event-stream
         _, has_sse = self._check_accept_headers(request)
 
@@ -458,13 +470,80 @@ class StreamableHTTPServerTransport:
             await response(request.scope, request.receive, send)
             return
 
-        # TODO: Implement SSE stream for GET requests
-        # For now, return 405 Method Not Allowed
-        response = self._create_error_response(
-            "SSE stream from GET request not implemented yet",
-            HTTPStatus.METHOD_NOT_ALLOWED,
+        if not await self._validate_session(request, send):
+            return
+
+        headers = {
+            "Cache-Control": "no-cache, no-transform",
+            "Connection": "keep-alive",
+            "Content-Type": CONTENT_TYPE_SSE,
+        }
+
+        if self.mcp_session_id:
+            headers[MCP_SESSION_ID_HEADER] = self.mcp_session_id
+
+        # Check if we already have an active GET stream
+        if GET_STREAM_KEY in self._request_streams:
+            response = self._create_error_response(
+                "Conflict: Only one SSE stream is allowed per session",
+                HTTPStatus.CONFLICT,
+            )
+            await response(request.scope, request.receive, send)
+            return
+
+        # Create SSE stream
+        sse_stream_writer, sse_stream_reader = anyio.create_memory_object_stream[
+            dict[str, Any]
+        ](0)
+
+        async def standalone_sse_writer():
+            try:
+                # Create a standalone message stream for server-initiated messages
+                standalone_stream_writer, standalone_stream_reader = (
+                    anyio.create_memory_object_stream[JSONRPCMessage](0)
+                )
+
+                # Register this stream using the special key
+                self._request_streams[GET_STREAM_KEY] = standalone_stream_writer
+
+                async with sse_stream_writer, standalone_stream_reader:
+                    # Process messages from the standalone stream
+                    async for received_message in standalone_stream_reader:
+                        # For the standalone stream, we handle:
+                        # - JSONRPCNotification (server sends notifications to client)
+                        # - JSONRPCRequest (server sends requests to client)
+                        # We should NOT receive JSONRPCResponse
+
+                        # Send the message via SSE
+                        event_data = {
+                            "event": "message",
+                            "data": received_message.model_dump_json(
+                                by_alias=True, exclude_none=True
+                            ),
+                        }
+
+                        await sse_stream_writer.send(event_data)
+            except Exception as e:
+                logger.exception(f"Error in standalone SSE writer: {e}")
+            finally:
+                logger.debug("Closing standalone SSE writer")
+                # Remove the stream from request_streams
+                self._request_streams.pop(GET_STREAM_KEY, None)
+
+        # Create and start EventSourceResponse
+        response = EventSourceResponse(
+            content=sse_stream_reader,
+            data_sender_callable=standalone_sse_writer,
+            headers=headers,
         )
-        await response(request.scope, request.receive, send)
+
+        try:
+            # This will send headers immediately and establish the SSE connection
+            await response(request.scope, request.receive, send)
+        except Exception as e:
+            logger.exception(f"Error in standalone SSE response: {e}")
+            # Clean up the request stream
+            self._request_streams.pop(GET_STREAM_KEY, None)
 
     async def _handle_delete_request(self, request: Request, send: Send) -> None:
         """Handle DELETE requests for explicit session termination."""
@@ -611,13 +690,10 @@ class StreamableHTTPServerTransport:
                         else:
                             target_request_id = str(message.root.id)
 
-                        # Send to the specific request stream if available
-                        if (
-                            target_request_id
-                            and target_request_id in self._request_streams
-                        ):
+                        request_stream_id = target_request_id or GET_STREAM_KEY
+                        if request_stream_id in self._request_streams:
                             try:
-                                await self._request_streams[target_request_id].send(
+                                await self._request_streams[request_stream_id].send(
                                     message
                                 )
                             except (
@@ -625,7 +701,7 @@ class StreamableHTTPServerTransport:
                                 anyio.ClosedResourceError,
                             ):
                                 # Stream might be closed, remove from registry
-                                self._request_streams.pop(target_request_id, None)
+                                self._request_streams.pop(request_stream_id, None)
                 except Exception as e:
                     logger.exception(f"Error in message router: {e}")
 

tests/server/test_streamableHttp.py

@@ -541,3 +541,92 @@ def test_json_response(json_response_server, json_server_url):
     )
     assert response.status_code == 200
     assert response.headers.get("Content-Type") == "application/json"
+
+
+def test_get_sse_stream(basic_server, basic_server_url):
+    """Test establishing an SSE stream via GET request."""
+    # First, we need to initialize a session
+    mcp_url = f"{basic_server_url}/mcp"
+    init_response = requests.post(
+        mcp_url,
+        headers={
+            "Accept": "application/json, text/event-stream",
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert init_response.status_code == 200
+
+    # Get the session ID
+    session_id = init_response.headers.get(MCP_SESSION_ID_HEADER)
+    assert session_id is not None
+
+    # Now attempt to establish an SSE stream via GET
+    get_response = requests.get(
+        mcp_url,
+        headers={
+            "Accept": "text/event-stream",
+            MCP_SESSION_ID_HEADER: session_id,
+        },
+        stream=True,
+    )
+
+    # Verify we got a successful response with the right content type
+    assert get_response.status_code == 200
+    assert get_response.headers.get("Content-Type") == "text/event-stream"
+
+    # Test that a second GET request gets rejected (only one stream allowed)
+    second_get = requests.get(
+        mcp_url,
+        headers={
+            "Accept": "text/event-stream",
+            MCP_SESSION_ID_HEADER: session_id,
+        },
+        stream=True,
+    )
+
+    # Should get CONFLICT (409) since there's already a stream
+    # Note: This might fail if the first stream fully closed before this runs,
+    # but generally it should work in the test environment where it runs quickly
+    assert second_get.status_code == 409
+
+
+def test_get_validation(basic_server, basic_server_url):
+    """Test validation for GET requests."""
+    # First, we need to initialize a session
+    mcp_url = f"{basic_server_url}/mcp"
+    init_response = requests.post(
+        mcp_url,
+        headers={
+            "Accept": "application/json, text/event-stream",
+            "Content-Type": "application/json",
+        },
+        json=INIT_REQUEST,
+    )
+    assert init_response.status_code == 200
+
+    # Get the session ID
+    session_id = init_response.headers.get(MCP_SESSION_ID_HEADER)
+    assert session_id is not None
+
+    # Test without Accept header
+    response = requests.get(
+        mcp_url,
+        headers={
+            MCP_SESSION_ID_HEADER: session_id,
+        },
+        stream=True,
+    )
+    assert response.status_code == 406
+    assert "Not Acceptable" in response.text
+
+    # Test with wrong Accept header
+    response = requests.get(
+        mcp_url,
+        headers={
+            "Accept": "application/json",
+            MCP_SESSION_ID_HEADER: session_id,
+        },
+    )
+    assert response.status_code == 406
+    assert "Not Acceptable" in response.text