types: Setting default value for jsonrpc

src/mcp/client/streamable_http.py

@@ -356,7 +356,6 @@ async def _send_session_terminated_error(
     ) -> None:
         """Send a session terminated error response."""
         jsonrpc_error = JSONRPCError(
-            jsonrpc="2.0",
             id=request_id,
             error=ErrorData(code=32600, message="Session terminated"),
         )

src/mcp/server/streamable_http.py

@@ -195,7 +195,6 @@ def _create_error_response(
 
         # Return a properly formatted JSON error response
         error_response = JSONRPCError(
-            jsonrpc="2.0",
             id="server-error",  # We don't have a request ID for general errors
             error=ErrorData(
                 code=error_code,
@@ -872,7 +871,7 @@ async def message_router():
                                 self._request_streams.pop(request_stream_id, None)
                         else:
                             logging.debug(
-                                f"""Request stream {request_stream_id} not found 
+                                f"""Request stream {request_stream_id} not found
                                 for message. Still processing message as the client
                                 might reconnect and replay."""
                             )

src/mcp/shared/session.py

@@ -253,7 +253,6 @@ async def send_request(
 
         try:
             jsonrpc_request = JSONRPCRequest(
-                jsonrpc="2.0",
                 id=request_id,
                 **request_data,
             )
@@ -305,7 +304,6 @@ async def send_notification(
         # Some transport implementations may need to set the related_request_id
         # to attribute to the notifications to the request that triggered them.
         jsonrpc_notification = JSONRPCNotification(
-            jsonrpc="2.0",
             **notification.model_dump(by_alias=True, mode="json", exclude_none=True),
         )
         session_message = SessionMessage(
@@ -316,12 +314,11 @@ async def send_notification(
 
     async def _send_response(self, request_id: RequestId, response: SendResultT | ErrorData) -> None:
         if isinstance(response, ErrorData):
-            jsonrpc_error = JSONRPCError(jsonrpc="2.0", id=request_id, error=response)
+            jsonrpc_error = JSONRPCError(id=request_id, error=response)
             session_message = SessionMessage(message=JSONRPCMessage(jsonrpc_error))
             await self._write_stream.send(session_message)
         else:
             jsonrpc_response = JSONRPCResponse(
-                jsonrpc="2.0",
                 id=request_id,
                 result=response.model_dump(by_alias=True, mode="json", exclude_none=True),
             )
@@ -363,7 +360,6 @@ async def _receive_loop(self) -> None:
                             logging.warning(f"Failed to validate request: {e}")
                             logging.debug(f"Message that failed validation: {message.message.root}")
                             error_response = JSONRPCError(
-                                jsonrpc="2.0",
                                 id=message.message.root.id,
                                 error=ErrorData(
                                     code=INVALID_PARAMS,
@@ -428,7 +424,7 @@ async def _receive_loop(self) -> None:
                 for id, stream in self._response_streams.items():
                     error = ErrorData(code=CONNECTION_CLOSED, message="Connection closed")
                     try:
-                        await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))
+                        await stream.send(JSONRPCError(id=id, error=error))
                         await stream.aclose()
                     except Exception:
                         # Stream might already be closed

src/mcp/types.py

@@ -33,6 +33,12 @@
 """
 DEFAULT_NEGOTIATED_VERSION = "2025-03-26"
 
+"""
+The JSON-RPC version (fixed at "2.0") to use for MCP messages.
+"""
+JSONRPCVersionType: TypeAlias = Literal["2.0"]
+JSON_RPC_VERSION: JSONRPCVersionType = "2.0"
+
 ProgressToken = str | int
 Cursor = str
 Role = Literal["user", "assistant"]
@@ -124,7 +130,7 @@ class PaginatedResult(Result):
 class JSONRPCRequest(Request[dict[str, Any] | None, str]):
     """A request that expects a response."""
 
-    jsonrpc: Literal["2.0"]
+    jsonrpc: JSONRPCVersionType = JSON_RPC_VERSION
     id: RequestId
     method: str
     params: dict[str, Any] | None = None
@@ -133,14 +139,14 @@ class JSONRPCRequest(Request[dict[str, Any] | None, str]):
 class JSONRPCNotification(Notification[dict[str, Any] | None, str]):
     """A notification which does not expect a response."""
 
-    jsonrpc: Literal["2.0"]
+    jsonrpc: JSONRPCVersionType = JSON_RPC_VERSION
     params: dict[str, Any] | None = None
 
 
 class JSONRPCResponse(BaseModel):
     """A successful (non-error) response to a request."""
 
-    jsonrpc: Literal["2.0"]
+    jsonrpc: JSONRPCVersionType = JSON_RPC_VERSION
     id: RequestId
     result: dict[str, Any]
     model_config = ConfigDict(extra="allow")
@@ -182,7 +188,7 @@ class ErrorData(BaseModel):
 class JSONRPCError(BaseModel):
     """A response to a request that indicates an error occurred."""
 
-    jsonrpc: Literal["2.0"]
+    jsonrpc: JSONRPCVersionType = JSON_RPC_VERSION
     id: str | int
     error: ErrorData
     model_config = ConfigDict(extra="allow")

tests/client/test_session.py

@@ -65,7 +65,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -146,7 +145,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -207,7 +205,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -268,7 +265,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -327,7 +323,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -386,7 +381,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )
@@ -464,7 +458,6 @@ async def mock_server():
                 SessionMessage(
                     JSONRPCMessage(
                         JSONRPCResponse(
-                            jsonrpc="2.0",
                             id=jsonrpc_request.root.id,
                             result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
                         )

tests/client/test_stdio.py

@@ -41,8 +41,8 @@ async def test_stdio_client():
     async with stdio_client(server_parameters) as (read_stream, write_stream):
         # Test sending and receiving messages
         messages = [
-            JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")),
-            JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=2, result={})),
+            JSONRPCMessage(root=JSONRPCRequest(id=1, method="ping")),
+            JSONRPCMessage(root=JSONRPCResponse(id=2, result={})),
         ]
 
         async with write_stream:
@@ -61,8 +61,8 @@ async def test_stdio_client():
                     break
 
         assert len(read_messages) == 2
-        assert read_messages[0] == JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=1, method="ping"))
-        assert read_messages[1] == JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=2, result={}))
+        assert read_messages[0] == JSONRPCMessage(root=JSONRPCRequest(id=1, method="ping"))
+        assert read_messages[1] == JSONRPCMessage(root=JSONRPCResponse(id=2, result={}))
 
 
 @pytest.mark.anyio

tests/issues/test_192_request_id.py

@@ -63,7 +63,6 @@ async def run_server():
                 capabilities=ClientCapabilities(),
                 clientInfo=Implementation(name="test-client", version="1.0.0"),
             ).model_dump(by_alias=True, exclude_none=True),
-            jsonrpc="2.0",
         )
 
         await client_writer.send(SessionMessage(JSONRPCMessage(root=init_req)))
@@ -73,12 +72,11 @@ async def run_server():
         initialized_notification = JSONRPCNotification(
             method="notifications/initialized",
             params=NotificationParams().model_dump(by_alias=True, exclude_none=True),
-            jsonrpc="2.0",
         )
         await client_writer.send(SessionMessage(JSONRPCMessage(root=initialized_notification)))
 
         # Send ping request with custom ID
-        ping_request = JSONRPCRequest(id=custom_request_id, method="ping", params={}, jsonrpc="2.0")
+        ping_request = JSONRPCRequest(id=custom_request_id, method="ping", params={})
 
         await client_writer.send(SessionMessage(JSONRPCMessage(root=ping_request)))
 

tests/issues/test_malformed_input.py

@@ -11,6 +11,7 @@
 from mcp.shared.message import SessionMessage
 from mcp.types import (
     INVALID_PARAMS,
+    JSON_RPC_VERSION,
     JSONRPCError,
     JSONRPCMessage,
     JSONRPCRequest,
@@ -31,7 +32,6 @@ async def test_malformed_initialize_request_does_not_crash_server():
     try:
         # Create a malformed initialize request (missing required params field)
         malformed_request = JSONRPCRequest(
-            jsonrpc="2.0",
             id="f20fe86132ed4cd197f89a7134de5685",
             method="initialize",
             # params=None  # Missing required params field
@@ -63,15 +63,14 @@ async def test_malformed_initialize_request_does_not_crash_server():
 
                 # Verify it's a proper JSON-RPC error response
                 assert isinstance(response, JSONRPCError)
-                assert response.jsonrpc == "2.0"
+                assert response.jsonrpc == JSON_RPC_VERSION, f"jsonrpc should be set to '{JSON_RPC_VERSION}'"
                 assert response.id == "f20fe86132ed4cd197f89a7134de5685"
                 assert response.error.code == INVALID_PARAMS
                 assert "Invalid request parameters" in response.error.message
 
                 # Verify the session is still alive and can handle more requests
                 # Send another malformed request to confirm server stability
                 another_malformed_request = JSONRPCRequest(
-                    jsonrpc="2.0",
                     id="test_id_2",
                     method="tools/call",
                     # params=None  # Missing required params
@@ -123,7 +122,6 @@ async def test_multiple_concurrent_malformed_requests():
             malformed_requests: list[SessionMessage] = []
             for i in range(10):
                 malformed_request = JSONRPCRequest(
-                    jsonrpc="2.0",
                     id=f"malformed_{i}",
                     method="initialize",
                     # params=None  # Missing required params

tests/server/test_lifespan.py

@@ -84,7 +84,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCRequest(
-                        jsonrpc="2.0",
                         id=1,
                         method="initialize",
                         params=TypeAdapter(InitializeRequestParams).dump_python(params),
@@ -100,7 +99,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCNotification(
-                        jsonrpc="2.0",
                         method="notifications/initialized",
                     )
                 )
@@ -112,7 +110,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCRequest(
-                        jsonrpc="2.0",
                         id=2,
                         method="tools/call",
                         params={"name": "check_lifespan", "arguments": {}},
@@ -190,7 +187,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCRequest(
-                        jsonrpc="2.0",
                         id=1,
                         method="initialize",
                         params=TypeAdapter(InitializeRequestParams).dump_python(params),
@@ -206,7 +202,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCNotification(
-                        jsonrpc="2.0",
                         method="notifications/initialized",
                     )
                 )
@@ -218,7 +213,6 @@ async def run_server():
             SessionMessage(
                 JSONRPCMessage(
                     root=JSONRPCRequest(
-                        jsonrpc="2.0",
                         id=2,
                         method="tools/call",
                         params={"name": "check_lifespan", "arguments": {}},

tests/server/test_session.py

@@ -166,7 +166,6 @@ async def mock_client():
             SessionMessage(
                 types.JSONRPCMessage(
                     types.JSONRPCRequest(
-                        jsonrpc="2.0",
                         id=1,
                         method="initialize",
                         params=types.InitializeRequestParams(
@@ -194,7 +193,6 @@ async def mock_client():
             SessionMessage(
                 types.JSONRPCMessage(
                     types.JSONRPCNotification(
-                        jsonrpc="2.0",
                         method="notifications/initialized",
                     )
                 )

tests/server/test_stdio.py

@@ -14,8 +14,8 @@ async def test_stdio_server():
     stdout = io.StringIO()
 
     messages = [
-        JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")),
-        JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=2, result={})),
+        JSONRPCMessage(root=JSONRPCRequest(id=1, method="ping")),
+        JSONRPCMessage(root=JSONRPCResponse(id=2, result={})),
     ]
 
     for message in messages:
@@ -37,13 +37,13 @@ async def test_stdio_server():
 
         # Verify received messages
         assert len(received_messages) == 2
-        assert received_messages[0] == JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=1, method="ping"))
-        assert received_messages[1] == JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=2, result={}))
+        assert received_messages[0] == JSONRPCMessage(root=JSONRPCRequest(id=1, method="ping"))
+        assert received_messages[1] == JSONRPCMessage(root=JSONRPCResponse(id=2, result={}))
 
         # Test sending responses from the server
         responses = [
-            JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=3, method="ping")),
-            JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=4, result={})),
+            JSONRPCMessage(root=JSONRPCRequest(id=3, method="ping")),
+            JSONRPCMessage(root=JSONRPCResponse(id=4, result={})),
         ]
 
         async with write_stream:
@@ -57,5 +57,5 @@ async def test_stdio_server():
 
     received_responses = [JSONRPCMessage.model_validate_json(line.strip()) for line in output_lines]
     assert len(received_responses) == 2
-    assert received_responses[0] == JSONRPCMessage(root=JSONRPCRequest(jsonrpc="2.0", id=3, method="ping"))
-    assert received_responses[1] == JSONRPCMessage(root=JSONRPCResponse(jsonrpc="2.0", id=4, result={}))
+    assert received_responses[0] == JSONRPCMessage(root=JSONRPCRequest(id=3, method="ping"))
+    assert received_responses[1] == JSONRPCMessage(root=JSONRPCResponse(id=4, result={}))

tests/shared/test_sse.py

@@ -478,11 +478,11 @@ def test_sse_message_id_coercion():
     """
     json_message = '{"jsonrpc": "2.0", "id": "123", "method": "ping", "params": null}'
     msg = types.JSONRPCMessage.model_validate_json(json_message)
-    assert msg == snapshot(types.JSONRPCMessage(root=types.JSONRPCRequest(method="ping", jsonrpc="2.0", id="123")))
+    assert msg == snapshot(types.JSONRPCMessage(root=types.JSONRPCRequest(method="ping", id="123")))
 
     json_message = '{"jsonrpc": "2.0", "id": 123, "method": "ping", "params": null}'
     msg = types.JSONRPCMessage.model_validate_json(json_message)
-    assert msg == snapshot(types.JSONRPCMessage(root=types.JSONRPCRequest(method="ping", jsonrpc="2.0", id=123)))
+    assert msg == snapshot(types.JSONRPCMessage(root=types.JSONRPCRequest(method="ping", id=123)))
 
 
 @pytest.mark.parametrize(