feat: Make maximum message size configurable

Author: pantanurag555

src/mcp/server/fastmcp/server.py

@@ -96,6 +96,7 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     # StreamableHTTP settings
     json_response: bool = False
     stateless_http: bool = False  # If True, uses true stateless mode (new transport per request)
+    maximum_message_size: int | None = None  # Specified in bytes
 
     # resource settings
     warn_on_duplicate_resources: bool = True
@@ -815,6 +816,7 @@ def streamable_http_app(self) -> Starlette:
                 json_response=self.settings.json_response,
                 stateless=self.settings.stateless_http,  # Use the stateless setting
                 security_settings=self.settings.transport_security,
+                maximum_message_size=self.settings.maximum_message_size,
             )
 
         # Create the ASGI handler

src/mcp/server/streamable_http.py

@@ -135,13 +135,15 @@ class StreamableHTTPServerTransport:
     _write_stream: MemoryObjectSendStream[SessionMessage] | None = None
     _write_stream_reader: MemoryObjectReceiveStream[SessionMessage] | None = None
     _security: TransportSecurityMiddleware
+    _maximum_message_size: int = MAXIMUM_MESSAGE_SIZE
 
     def __init__(
         self,
         mcp_session_id: str | None,
         is_json_response_enabled: bool = False,
         event_store: EventStore | None = None,
         security_settings: TransportSecuritySettings | None = None,
+        maximum_message_size: int | None = None,
     ) -> None:
         """
         Initialize a new StreamableHTTP server transport.
@@ -155,6 +157,8 @@ def __init__(
                         resumability will be enabled, allowing clients to
                         reconnect and resume messages.
             security_settings: Optional security settings for DNS rebinding protection.
+            maximum_message_size: Optional configurable maximum message size specified
+                                in bytes
 
         Raises:
             ValueError: If the session ID contains invalid characters.
@@ -166,6 +170,7 @@ def __init__(
         self.is_json_response_enabled = is_json_response_enabled
         self._event_store = event_store
         self._security = TransportSecurityMiddleware(security_settings)
+        self._maximum_message_size = maximum_message_size if maximum_message_size else MAXIMUM_MESSAGE_SIZE
         self._request_streams: dict[
             RequestId,
             tuple[
@@ -329,7 +334,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
 
             # Parse the body - only read it once
             body = await request.body()
-            if len(body) > MAXIMUM_MESSAGE_SIZE:
+            if len(body) > self._maximum_message_size:
                 response = self._create_error_response(
                     "Payload Too Large: Message exceeds maximum size",
                     HTTPStatus.REQUEST_ENTITY_TOO_LARGE,

src/mcp/server/streamable_http_manager.py

@@ -52,7 +52,8 @@ class StreamableHTTPSessionManager:
         json_response: Whether to use JSON responses instead of SSE streams
         stateless: If True, creates a completely fresh transport for each request
                    with no session tracking or state persistence between requests.
-
+        maximum_message_size: Optional configurable maximum message size specified
+                              in bytes
     """
 
     def __init__(
@@ -62,12 +63,14 @@ def __init__(
         json_response: bool = False,
         stateless: bool = False,
         security_settings: TransportSecuritySettings | None = None,
+        maximum_message_size: int | None = None,
     ):
         self.app = app
         self.event_store = event_store
         self.json_response = json_response
         self.stateless = stateless
         self.security_settings = security_settings
+        self.maximum_message_size = maximum_message_size
 
         # Session tracking (only used if not stateless)
         self._session_creation_lock = anyio.Lock()
@@ -166,6 +169,7 @@ async def _handle_stateless_request(
             is_json_response_enabled=self.json_response,
             event_store=None,  # No event store in stateless mode
             security_settings=self.security_settings,
+            maximum_message_size=self.maximum_message_size,
         )
 
         # Start server in a new task
@@ -222,6 +226,7 @@ async def _handle_stateful_request(
                     is_json_response_enabled=self.json_response,
                     event_store=self.event_store,  # May be None (no resumability)
                     security_settings=self.security_settings,
+                    maximum_message_size=self.maximum_message_size,
                 )
 
                 assert http_transport.mcp_session_id is not None

tests/server/fastmcp/test_configure_input_size.py

@@ -0,0 +1,23 @@
+"""Test that the maximum http input size is configurable via FastMCP settings"""
+
+import pytest
+
+from mcp.server.fastmcp import FastMCP
+
+
+@pytest.mark.anyio
+async def test_configure_input_size():
+    """Create a FastMCP server with StreamableHTTP transport."""
+    configured_input_size = 1024
+    mcp = FastMCP("Test Server", maximum_message_size=configured_input_size)
+
+    # Add a simple tool
+    @mcp.tool(description="A simple echo tool")
+    def echo(message: str) -> str:
+        return f"Echo: {message}"
+
+    # Create the StreamableHTTP app
+    _ = mcp.streamable_http_app()
+
+    # Check that the maximum input size is set correctly
+    assert mcp.session_manager.maximum_message_size == configured_input_size

tests/shared/test_streamable_http.py

@@ -236,6 +236,7 @@ def create_app(is_json_response_enabled=False, event_store: EventStore | None =
         event_store=event_store,
         json_response=is_json_response_enabled,
         security_settings=security_settings,
+        maximum_message_size=1024,
     )
 
     # Create an ASGI application that uses the session manager
@@ -491,6 +492,27 @@ def test_method_not_allowed(basic_server, basic_server_url):
     assert "Method Not Allowed" in response.text
 
 
+def test_maximum_message_size_validation(basic_server, basic_server_url):
+    """Test maximum allowed input size validation."""
+
+    # Test with input greater than 1024 bytes
+    large_input = "A" * 1300  # string of size 1300 bytes
+    response = requests.post(
+        f"{basic_server_url}/mcp",
+        headers={
+            "Accept": "application/json, text/event-stream",
+            "Content-Type": "application/json",
+        },
+        json={
+            "jsonrpc": "2.0",
+            "method": "call_tool",
+            "body": {"name": "test_tool_with_standalone_notification", "args": {"text": large_input}},
+        },
+    )
+    assert response.status_code == 413
+    assert "Payload Too Large" in response.text
+
+
 def test_session_validation(basic_server, basic_server_url):
     """Test session ID validation."""
     # session_id not used directly in this test