Make sure RequestId is not coerced as int

src/mcp/types.py

@@ -36,7 +36,7 @@
 ProgressToken = str | int
 Cursor = str
 Role = Literal["user", "assistant"]
-RequestId = Annotated[int | str, Field(union_mode="left_to_right")]
+RequestId = Annotated[int, Field(strict=True)] | str
 AnyFunction: TypeAlias = Callable[..., Any]
 
 
@@ -849,7 +849,7 @@ class Tool(BaseMetadata):
     """A JSON Schema object defining the expected parameters for the tool."""
     outputSchema: dict[str, Any] | None = None
     """
-    An optional JSON Schema object defining the structure of the tool's output 
+    An optional JSON Schema object defining the structure of the tool's output
     returned in the structuredContent field of a CallToolResult.
     """
     annotations: ToolAnnotations | None = None

tests/shared/test_sse.py

@@ -466,12 +466,21 @@ async def test_request_context_isolation(context_server: None, server_url: str)
 
 
 def test_sse_message_id_coercion():
-    """Test that string message IDs that look like integers are parsed as integers.
+    """Previously, the `RequestId` would coerce a string that looked like an integer into an integer.
 
     See <https://github.com/modelcontextprotocol/python-sdk/pull/851> for more details.
+
+    As per the JSON-RPC 2.0 specification, the id in the response object needs to be the same type as the id in the
+    request object. In other words, we can't perform the coercion.
+
+    See <https://www.jsonrpc.org/specification#response_object> for more details.
     """
     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")))
+
+    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)))