Completely drop RootModel from types module (#1910)

Author: Kludex

docs/migration.md

@@ -179,6 +179,60 @@ app = Starlette(routes=[Mount("/", app=mcp.streamable_http_app(json_response=Tru
 
 **Note:** DNS rebinding protection is automatically enabled when `host` is `127.0.0.1`, `localhost`, or `::1`. This now happens in `sse_app()` and `streamable_http_app()` instead of the constructor.
 
+### Replace `RootModel` by union types with `TypeAdapter` validation
+
+The following union types are no longer `RootModel` subclasses:
+
+- `ClientRequest`
+- `ServerRequest`
+- `ClientNotification`
+- `ServerNotification`
+- `ClientResult`
+- `ServerResult`
+- `JSONRPCMessage`
+
+This means you can no longer access `.root` on these types or use `model_validate()` directly on them. Instead, use the provided `TypeAdapter` instances for validation.
+
+**Before (v1):**
+
+```python
+from mcp.types import ClientRequest, ServerNotification
+
+# Using RootModel.model_validate()
+request = ClientRequest.model_validate(data)
+actual_request = request.root  # Accessing the wrapped value
+
+notification = ServerNotification.model_validate(data)
+actual_notification = notification.root
+```
+
+**After (v2):**
+
+```python
+from mcp.types import client_request_adapter, server_notification_adapter
+
+# Using TypeAdapter.validate_python()
+request = client_request_adapter.validate_python(data)
+# No .root access needed - request is the actual type
+
+notification = server_notification_adapter.validate_python(data)
+# No .root access needed - notification is the actual type
+```
+
+**Available adapters:**
+
+| Union Type | Adapter |
+|------------|---------|
+| `ClientRequest` | `client_request_adapter` |
+| `ServerRequest` | `server_request_adapter` |
+| `ClientNotification` | `client_notification_adapter` |
+| `ServerNotification` | `server_notification_adapter` |
+| `ClientResult` | `client_result_adapter` |
+| `ServerResult` | `server_result_adapter` |
+| `JSONRPCMessage` | `jsonrpc_message_adapter` |
+
+All adapters are exported from `mcp.types`.
+
 ### Resource URI type changed from `AnyUrl` to `str`
 
 The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the [MCP specification schema](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/draft/schema.ts) which defines URIs as plain strings (`uri: string`) without strict URL validation. This change allows relative paths like `users/me` that were previously rejected.

pyproject.toml

@@ -125,18 +125,23 @@ extend-exclude = ["README.md"]
 
 [tool.ruff.lint]
 select = [
-    "C4",   # flake8-comprehensions
-    "C90",  # mccabe
-    "D212", # pydocstyle: multi-line docstring summary should start at the first line
-    "E",    # pycodestyle
-    "F",    # pyflakes
-    "I",    # isort
-    "PERF", # Perflint
-    "PL",   # Pylint
-    "UP",   # pyupgrade
+    "C4",     # flake8-comprehensions
+    "C90",    # mccabe
+    "D212",   # pydocstyle: multi-line docstring summary should start at the first line
+    "E",      # pycodestyle
+    "F",      # pyflakes
+    "I",      # isort
+    "PERF",   # Perflint
+    "PL",     # Pylint
+    "UP",     # pyupgrade
+    "TID251", # https://docs.astral.sh/ruff/rules/banned-api/
 ]
 ignore = ["PERF203", "PLC0415", "PLR0402"]
 
+[tool.ruff.lint.flake8-tidy-imports.banned-api]
+"pydantic.RootModel".msg = "Use `pydantic.TypeAdapter` instead."
+
+
 [tool.ruff.lint.mccabe]
 max-complexity = 24 # Default is 10
 

src/mcp/client/experimental/task_handlers.py

@@ -242,7 +242,7 @@ def build_capability(self) -> types.ClientTasksCapability | None:
     def handles_request(request: types.ServerRequest) -> bool:
         """Check if this handler handles the given request type."""
         return isinstance(
-            request.root,
+            request,
             types.GetTaskRequest | types.GetTaskPayloadRequest | types.ListTasksRequest | types.CancelTaskRequest,
         )
 
@@ -259,7 +259,7 @@ async def handle_request(
             types.ClientResult | types.ErrorData
         )
 
-        match responder.request.root:
+        match responder.request:
             case types.GetTaskRequest(params=params):
                 response = await self.get_task(ctx, params)
                 client_response = client_response_type.validate_python(response)
@@ -281,7 +281,7 @@ async def handle_request(
                 await responder.respond(client_response)
 
             case _:  # pragma: no cover
-                raise ValueError(f"Unhandled request type: {type(responder.request.root)}")
+                raise ValueError(f"Unhandled request type: {type(responder.request)}")
 
 
 # Backwards compatibility aliases

src/mcp/client/experimental/tasks.py

@@ -92,15 +92,13 @@ async def call_tool_as_task(
             _meta = types.RequestParams.Meta(**meta)
 
         return await self._session.send_request(
-            types.ClientRequest(
-                types.CallToolRequest(
-                    params=types.CallToolRequestParams(
-                        name=name,
-                        arguments=arguments,
-                        task=types.TaskMetadata(ttl=ttl),
-                        _meta=_meta,
-                    ),
-                )
+            types.CallToolRequest(
+                params=types.CallToolRequestParams(
+                    name=name,
+                    arguments=arguments,
+                    task=types.TaskMetadata(ttl=ttl),
+                    _meta=_meta,
+                ),
             ),
             types.CreateTaskResult,
         )
@@ -115,10 +113,8 @@ async def get_task(self, task_id: str) -> types.GetTaskResult:
             GetTaskResult containing the task status and metadata
         """
         return await self._session.send_request(
-            types.ClientRequest(
-                types.GetTaskRequest(
-                    params=types.GetTaskRequestParams(task_id=task_id),
-                )
+            types.GetTaskRequest(
+                params=types.GetTaskRequestParams(task_id=task_id),
             ),
             types.GetTaskResult,
         )
@@ -142,10 +138,8 @@ async def get_task_result(
             The task result, validated against result_type
         """
         return await self._session.send_request(
-            types.ClientRequest(
-                types.GetTaskPayloadRequest(
-                    params=types.GetTaskPayloadRequestParams(task_id=task_id),
-                )
+            types.GetTaskPayloadRequest(
+                params=types.GetTaskPayloadRequestParams(task_id=task_id),
             ),
             result_type,
         )
@@ -164,9 +158,7 @@ async def list_tasks(
         """
         params = types.PaginatedRequestParams(cursor=cursor) if cursor else None
         return await self._session.send_request(
-            types.ClientRequest(
-                types.ListTasksRequest(params=params),
-            ),
+            types.ListTasksRequest(params=params),
             types.ListTasksResult,
         )
 
@@ -180,10 +172,8 @@ async def cancel_task(self, task_id: str) -> types.CancelTaskResult:
             CancelTaskResult with the updated task state
         """
         return await self._session.send_request(
-            types.ClientRequest(
-                types.CancelTaskRequest(
-                    params=types.CancelTaskRequestParams(task_id=task_id),
-                )
+            types.CancelTaskRequest(
+                params=types.CancelTaskRequestParams(task_id=task_id),
             ),
             types.CancelTaskResult,
         )