chore: add D212 lint rule to enforce Google-style docstrings (#1892)

src/mcp/client/_memory.py

@@ -16,8 +16,7 @@ from mcp.shared.message import SessionMessage
 
 
 class InMemoryTransport:
-    """
-    In-memory transport for testing MCP servers without network overhead.
+    """In-memory transport for testing MCP servers without network overhead.
 
     This transport starts the server in a background task and provides
     streams for client-side communication. The server is automatically
@@ -43,8 +42,7 @@ class InMemoryTransport:
         *,
         raise_exceptions: bool = False,
     ) -> None:
-        """
-        Initialize the in-memory transport.
+        """Initialize the in-memory transport.
 
         Args:
             server: The MCP server to connect to (Server or FastMCP instance)
@@ -63,8 +61,7 @@ class InMemoryTransport:
         ],
         None,
     ]:
-        """
-        Connect to the server and return streams for communication.
+        """Connect to the server and return streams for communication.
 
         Yields:
             A tuple of (read_stream, write_stream) for bidirectional communication

src/mcp/client/auth/__init__.py

@@ -1,5 +1,4 @@
-"""
-OAuth2 Authentication implementation for HTTPX.
+"""OAuth2 Authentication implementation for HTTPX.
 
 Implements authorization code flow with PKCE and automatic token refresh.
 """

src/mcp/client/auth/extensions/client_credentials.py

@@ -1,5 +1,4 @@
-"""
-OAuth client credential extensions for MCP.
+"""OAuth client credential extensions for MCP.
 
 Provides OAuth providers for machine-to-machine authentication flows:
 - ClientCredentialsOAuthProvider: For client_credentials with client_id + client_secret

src/mcp/client/auth/oauth2.py

@@ -1,5 +1,4 @@
-"""
-OAuth2 Authentication implementation for HTTPX.
+"""OAuth2 Authentication implementation for HTTPX.
 
 Implements authorization code flow with PKCE and automatic token refresh.
 """
@@ -215,8 +214,7 @@ class OAuthContext:
 
 
 class OAuthClientProvider(httpx.Auth):
-    """
-    OAuth2 authentication for httpx.
+    """OAuth2 authentication for httpx.
     Handles OAuth flow with automatic client registration and token storage.
     """
 
@@ -268,8 +266,7 @@ class OAuthClientProvider(httpx.Auth):
         self._initialized = False
 
     async def _handle_protected_resource_response(self, response: httpx.Response) -> bool:
-        """
-        Handle protected resource metadata discovery response.
+        """Handle protected resource metadata discovery response.
 
         Per SEP-985, supports fallback when discovery fails at one URL.
 

src/mcp/client/auth/utils.py

@@ -20,8 +20,7 @@ logger = logging.getLogger(__name__)
 
 
 def extract_field_from_www_auth(response: Response, field_name: str) -> str | None:
-    """
-    Extract field from WWW-Authenticate header.
+    """Extract field from WWW-Authenticate header.
 
     Returns:
         Field value if found in WWW-Authenticate header, None otherwise
@@ -42,8 +41,7 @@ def extract_field_from_www_auth(response: Response, field_name: str) -> str | No
 
 
 def extract_scope_from_www_auth(response: Response) -> str | None:
-    """
-    Extract scope parameter from WWW-Authenticate header as per RFC6750.
+    """Extract scope parameter from WWW-Authenticate header as per RFC6750.
 
     Returns:
         Scope string if found in WWW-Authenticate header, None otherwise
@@ -52,8 +50,7 @@ def extract_scope_from_www_auth(response: Response) -> str | None:
 
 
 def extract_resource_metadata_from_www_auth(response: Response) -> str | None:
-    """
-    Extract protected resource metadata URL from WWW-Authenticate header as per RFC9728.
+    """Extract protected resource metadata URL from WWW-Authenticate header as per RFC9728.
 
     Returns:
         Resource metadata URL if found in WWW-Authenticate header, None otherwise
@@ -65,8 +62,7 @@ def extract_resource_metadata_from_www_auth(response: Response) -> str | None:
 
 
 def build_protected_resource_metadata_discovery_urls(www_auth_url: str | None, server_url: str) -> list[str]:
-    """
-    Build ordered list of URLs to try for protected resource metadata discovery.
+    """Build ordered list of URLs to try for protected resource metadata discovery.
 
     Per SEP-985, the client MUST:
     1. Try resource_metadata from WWW-Authenticate header (if present)
@@ -127,8 +123,7 @@ def get_client_metadata_scopes(
 
 
 def build_oauth_authorization_server_metadata_discovery_urls(auth_server_url: str | None, server_url: str) -> list[str]:
-    """
-    Generate ordered list of (url, type) tuples for discovery attempts.
+    """Generate ordered list of (url, type) tuples for discovery attempts.
 
     Args:
         auth_server_url: URL for the OAuth Authorization Metadata URL if found, otherwise None
@@ -173,8 +168,7 @@ def build_oauth_authorization_server_metadata_discovery_urls(auth_server_url: st
 async def handle_protected_resource_response(
     response: Response,
 ) -> ProtectedResourceMetadata | None:
-    """
-    Handle protected resource metadata discovery response.
+    """Handle protected resource metadata discovery response.
 
     Per SEP-985, supports fallback when discovery fails at one URL.
 

src/mcp/client/client.py

@@ -67,8 +67,7 @@ class Client:
         client_info: types.Implementation | None = None,
         elicitation_callback: ElicitationFnT | None = None,
     ) -> None:
-        """
-        Initialize the client with a server.
+        """Initialize the client with a server.
 
         Args:
             server: The MCP server to connect to (Server or FastMCP instance)
@@ -139,8 +138,7 @@ class Client:
 
     @property
     def session(self) -> ClientSession:
-        """
-        Get the underlying ClientSession.
+        """Get the underlying ClientSession.
 
         This provides access to the full ClientSession API for advanced use cases.
 
@@ -194,8 +192,7 @@ class Client:
         return await self.session.list_resource_templates(params=params)
 
     async def read_resource(self, uri: str | AnyUrl) -> types.ReadResourceResult:
-        """
-        Read a resource from the server.
+        """Read a resource from the server.
 
         Args:
             uri: The URI of the resource to read
@@ -222,8 +219,7 @@ class Client:
         *,
         meta: dict[str, Any] | None = None,
     ) -> types.CallToolResult:
-        """
-        Call a tool on the server.
+        """Call a tool on the server.
 
         Args:
             name: The name of the tool to call
@@ -255,8 +251,7 @@ class Client:
         name: str,
         arguments: dict[str, str] | None = None,
     ) -> types.GetPromptResult:
-        """
-        Get a prompt from the server.
+        """Get a prompt from the server.
 
         Args:
             name: The name of the prompt
@@ -273,8 +268,7 @@ class Client:
         argument: dict[str, str],
         context_arguments: dict[str, str] | None = None,
     ) -> types.CompleteResult:
-        """
-        Get completions for a prompt or resource template argument.
+        """Get completions for a prompt or resource template argument.
 
         Args:
             ref: Reference to the prompt or resource template

src/mcp/client/experimental/__init__.py

@@ -1,5 +1,4 @@
-"""
-Experimental client features.
+"""Experimental client features.
 
 WARNING: These APIs are experimental and may change without notice.
 """

src/mcp/client/experimental/task_handlers.py

@@ -1,5 +1,4 @@
-"""
-Experimental task handler protocols for server -> client requests.
+"""Experimental task handler protocols for server -> client requests.
 
 This module provides Protocol types and default handlers for when servers
 send task-related requests to clients (the reverse of normal client -> server flow).

src/mcp/client/experimental/tasks.py

@@ -1,5 +1,4 @@
-"""
-Experimental client-side task support.
+"""Experimental client-side task support.
 
 This module provides client methods for interacting with MCP tasks.
 
@@ -37,8 +36,7 @@ ResultT = TypeVar("ResultT", bound=types.Result)
 
 
 class ExperimentalClientFeatures:
-    """
-    Experimental client features for tasks and other experimental APIs.
+    """Experimental client features for tasks and other experimental APIs.
 
     WARNING: These APIs are experimental and may change without notice.
 
@@ -108,8 +106,7 @@ class ExperimentalClientFeatures:
         )
 
     async def get_task(self, task_id: str) -> types.GetTaskResult:
-        """
-        Get the current status of a task.
+        """Get the current status of a task.
 
         Args:
             task_id: The task identifier
@@ -131,8 +128,7 @@ class ExperimentalClientFeatures:
         task_id: str,
         result_type: type[ResultT],
     ) -> ResultT:
-        """
-        Get the result of a completed task.
+        """Get the result of a completed task.
 
         The result type depends on the original request type:
         - tools/call tasks return CallToolResult
@@ -158,8 +154,7 @@ class ExperimentalClientFeatures:
         self,
         cursor: str | None = None,
     ) -> types.ListTasksResult:
-        """
-        List all tasks.
+        """List all tasks.
 
         Args:
             cursor: Optional pagination cursor
@@ -176,8 +171,7 @@ class ExperimentalClientFeatures:
         )
 
     async def cancel_task(self, task_id: str) -> types.CancelTaskResult:
-        """
-        Cancel a running task.
+        """Cancel a running task.
 
         Args:
             task_id: The task identifier
@@ -195,8 +189,7 @@ class ExperimentalClientFeatures:
         )
 
     async def poll_task(self, task_id: str) -> AsyncIterator[types.GetTaskResult]:
-        """
-        Poll a task until it reaches a terminal status.
+        """Poll a task until it reaches a terminal status.
 
         Yields GetTaskResult for each poll, allowing the caller to react to
         status changes (e.g., handle input_required). Exits when task reaches

src/mcp/client/session_group.py

@@ -1,5 +1,4 @@
-"""
-SessionGroup concurrently manages multiple MCP session connections.
+"""SessionGroup concurrently manages multiple MCP session connections.
 
 Tools, resources, and prompts are aggregated across servers. Servers may
 be connected to or disconnected from at any point after initialization.

src/mcp/client/sse.py

@@ -37,8 +37,7 @@ async def sse_client(
     auth: httpx.Auth | None = None,
     on_session_created: Callable[[str], None] | None = None,
 ):
-    """
-    Client transport for SSE.
+    """Client transport for SSE.
 
     `sse_read_timeout` determines how long (in seconds) the client will wait for a new
     event before disconnecting. All other HTTP operations are controlled by `timeout`.

src/mcp/client/stdio/__init__.py

@@ -49,8 +49,7 @@ PROCESS_TERMINATION_TIMEOUT = 2.0
 
 
 def get_default_environment() -> dict[str, str]:
-    """
-    Returns a default environment object including only environment variables deemed
+    """Returns a default environment object including only environment variables deemed
     safe to inherit.
     """
     env: dict[str, str] = {}
@@ -104,8 +103,7 @@ class StdioServerParameters(BaseModel):
 
 @asynccontextmanager
 async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
-    """
-    Client transport for stdio: this will connect to a server by spawning a
+    """Client transport for stdio: this will connect to a server by spawning a
     process and communicating with it over stdin/stdout.
     """
     read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]
@@ -217,8 +215,7 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
 
 
 def _get_executable_command(command: str) -> str:
-    """
-    Get the correct executable command normalized for the current platform.
+    """Get the correct executable command normalized for the current platform.
 
     Args:
         command: Base command (e.g., 'uvx', 'npx')
@@ -239,8 +236,7 @@ async def _create_platform_compatible_process(
     errlog: TextIO = sys.stderr,
     cwd: Path | str | None = None,
 ):
-    """
-    Creates a subprocess in a platform-compatible way.
+    """Creates a subprocess in a platform-compatible way.
 
     Unix: Creates process in a new session/process group for killpg support
     Windows: Creates process in a Job Object for reliable child termination
@@ -260,8 +256,7 @@ async def _create_platform_compatible_process(
 
 
 async def _terminate_process_tree(process: Process | FallbackProcess, timeout_seconds: float = 2.0) -> None:
-    """
-    Terminate a process and all its children using platform-specific methods.
+    """Terminate a process and all its children using platform-specific methods.
 
     Unix: Uses os.killpg() for atomic process group termination
     Windows: Uses Job Objects via pywin32 for reliable child process cleanup

src/mcp/client/streamable_http.py

@@ -509,8 +509,7 @@ async def streamable_http_client(
     ],
     None,
 ]:
-    """
-    Client transport for StreamableHTTP.
+    """Client transport for StreamableHTTP.
 
     Args:
         url: The MCP server endpoint URL.

src/mcp/client/websocket.py

@@ -19,8 +19,7 @@ async def websocket_client(
     tuple[MemoryObjectReceiveStream[SessionMessage | Exception], MemoryObjectSendStream[SessionMessage]],
     None,
 ]:
-    """
-    WebSocket client transport for MCP, symmetrical to the server version.
+    """WebSocket client transport for MCP, symmetrical to the server version.
 
     Connects to 'url' using the 'mcp' subprotocol, then yields:
         (read_stream, write_stream)
@@ -46,8 +45,7 @@ async def websocket_client(
     async with ws_connect(url, subprotocols=[Subprotocol("mcp")]) as ws:
 
         async def ws_reader():
-            """
-            Reads text messages from the WebSocket, parses them as JSON-RPC messages,
+            """Reads text messages from the WebSocket, parses them as JSON-RPC messages,
             and sends them into read_stream_writer.
             """
             async with read_stream_writer:
@@ -61,8 +59,7 @@ async def websocket_client(
                         await read_stream_writer.send(exc)
 
         async def ws_writer():
-            """
-            Reads JSON-RPC messages from write_stream_reader and
+            """Reads JSON-RPC messages from write_stream_reader and
             sends them to the server.
             """
             async with write_stream_reader:

src/mcp/os/posix/utilities.py

@@ -1,6 +1,4 @@
-"""
-POSIX-specific functionality for stdio client operations.
-"""
+"""POSIX-specific functionality for stdio client operations."""
 
 import logging
 import os
@@ -13,8 +11,7 @@ logger = logging.getLogger(__name__)
 
 
 async def terminate_posix_process_tree(process: Process, timeout_seconds: float = 2.0) -> None:
-    """
-    Terminate a process and all its children on POSIX systems.
+    """Terminate a process and all its children on POSIX systems.
 
     Uses os.killpg() for atomic process group termination.
 

src/mcp/os/win32/utilities.py

@@ -1,6 +1,4 @@
-"""
-Windows-specific functionality for stdio client operations.
-"""
+"""Windows-specific functionality for stdio client operations."""
 
 import logging
 import shutil
@@ -34,8 +32,7 @@ JobHandle = int
 
 
 def get_windows_executable_command(command: str) -> str:
-    """
-    Get the correct executable command normalized for Windows.
+    """Get the correct executable command normalized for Windows.
 
     On Windows, commands might exist with specific extensions (.exe, .cmd, etc.)
     that need to be located for proper execution.
@@ -66,8 +63,7 @@ def get_windows_executable_command(command: str) -> str:
 
 
 class FallbackProcess:
-    """
-    A fallback process wrapper for Windows to handle async I/O
+    """A fallback process wrapper for Windows to handle async I/O
     when using subprocess.Popen, which provides sync-only FileIO objects.
 
     This wraps stdin and stdout into async-compatible
@@ -140,8 +136,7 @@ async def create_windows_process(
     errlog: TextIO | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> Process | FallbackProcess:
-    """
-    Creates a subprocess in a Windows-compatible way with Job Object support.
+    """Creates a subprocess in a Windows-compatible way with Job Object support.
 
     Attempt to use anyio's open_process for async subprocess creation.
     In some cases this will throw NotImplementedError on Windows, e.g.
@@ -199,8 +194,7 @@ async def _create_windows_fallback_process(
     errlog: TextIO | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> FallbackProcess:
-    """
-    Create a subprocess using subprocess.Popen as a fallback when anyio fails.
+    """Create a subprocess using subprocess.Popen as a fallback when anyio fails.
 
     This function wraps the sync subprocess.Popen in an async-compatible interface.
     """
@@ -231,9 +225,7 @@ async def _create_windows_fallback_process(
 
 
 def _create_job_object() -> int | None:
-    """
-    Create a Windows Job Object configured to terminate all processes when closed.
-    """
+    """Create a Windows Job Object configured to terminate all processes when closed."""
     if sys.platform != "win32" or not win32job:
         return None
 
@@ -250,8 +242,7 @@ def _create_job_object() -> int | None:
 
 
 def _maybe_assign_process_to_job(process: Process | FallbackProcess, job: JobHandle | None) -> None:
-    """
-    Try to assign a process to a job object. If assignment fails
+    """Try to assign a process to a job object. If assignment fails
     for any reason, the job handle is closed.
     """
     if not job:
@@ -279,8 +270,7 @@ def _maybe_assign_process_to_job(process: Process | FallbackProcess, job: JobHan
 
 
 async def terminate_windows_process_tree(process: Process | FallbackProcess, timeout_seconds: float = 2.0) -> None:
-    """
-    Terminate a process and all its children on Windows.
+    """Terminate a process and all its children on Windows.
 
     If the process has an associated job object, it will be terminated.
     Otherwise, falls back to basic process termination.
@@ -318,8 +308,7 @@ async def terminate_windows_process_tree(process: Process | FallbackProcess, tim
     "Process termination is now handled internally by the stdio_client context manager."
 )
 async def terminate_windows_process(process: Process | FallbackProcess):
-    """
-    Terminate a Windows process.
+    """Terminate a Windows process.
 
     Note: On Windows, terminating a process with process.terminate() doesn't
     always guarantee immediate process termination.

src/mcp/server/auth/__init__.py

@@ -1,3 +1 @@
-"""
-MCP OAuth server authorization components.
-"""
+"""MCP OAuth server authorization components."""

src/mcp/server/auth/handlers/__init__.py

@@ -1,3 +1 @@
-"""
-Request handlers for MCP authorization endpoints.
-"""
+"""Request handlers for MCP authorization endpoints."""

src/mcp/server/auth/handlers/revoke.py

@@ -15,9 +15,7 @@ from mcp.server.auth.provider import AccessToken, OAuthAuthorizationServerProvid
 
 
 class RevocationRequest(BaseModel):
-    """
-    # See https://datatracker.ietf.org/doc/html/rfc7009#section-2.1
-    """
+    """# See https://datatracker.ietf.org/doc/html/rfc7009#section-2.1"""
 
     token: str
     token_type_hint: Literal["access_token", "refresh_token"] | None = None
@@ -36,9 +34,7 @@ class RevocationHandler:
     client_authenticator: ClientAuthenticator
 
     async def handle(self, request: Request) -> Response:
-        """
-        Handler for the OAuth 2.0 Token Revocation endpoint.
-        """
+        """Handler for the OAuth 2.0 Token Revocation endpoint."""
         try:
             client = await self.client_authenticator.authenticate_request(request)
         except AuthenticationError as e:  # pragma: no cover

src/mcp/server/auth/handlers/token.py

@@ -55,9 +55,7 @@ class TokenRequest(
 
 
 class TokenErrorResponse(BaseModel):
-    """
-    See https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
-    """
+    """See https://datatracker.ietf.org/doc/html/rfc6749#section-5.2"""
 
     error: TokenErrorCode
     error_description: str | None = None