Add ServerSessionT type var to Context (#271)

* Add ServerSessionT type var to Context

* Passing locally

* Try now

Author: Kludex

src/mcp/server/fastmcp/server.py

@@ -1,5 +1,7 @@
 """FastMCP - A more ergonomic interface for MCP servers."""
 
+from __future__ import annotations as _annotations
+
 import inspect
 import json
 import re
@@ -25,16 +27,10 @@
 from mcp.server.fastmcp.utilities.logging import configure_logging, get_logger
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.server.lowlevel.helper_types import ReadResourceContents
-from mcp.server.lowlevel.server import (
-    LifespanResultT,
-)
-from mcp.server.lowlevel.server import (
-    Server as MCPServer,
-)
-from mcp.server.lowlevel.server import (
-    lifespan as default_lifespan,
-)
-from mcp.server.session import ServerSession
+from mcp.server.lowlevel.server import LifespanResultT
+from mcp.server.lowlevel.server import Server as MCPServer
+from mcp.server.lowlevel.server import lifespan as default_lifespan
+from mcp.server.session import ServerSession, ServerSessionT
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
 from mcp.shared.context import LifespanContextT, RequestContext
@@ -45,21 +41,11 @@
     ImageContent,
     TextContent,
 )
-from mcp.types import (
-    Prompt as MCPPrompt,
-)
-from mcp.types import (
-    PromptArgument as MCPPromptArgument,
-)
-from mcp.types import (
-    Resource as MCPResource,
-)
-from mcp.types import (
-    ResourceTemplate as MCPResourceTemplate,
-)
-from mcp.types import (
-    Tool as MCPTool,
-)
+from mcp.types import Prompt as MCPPrompt
+from mcp.types import PromptArgument as MCPPromptArgument
+from mcp.types import Resource as MCPResource
+from mcp.types import ResourceTemplate as MCPResourceTemplate
+from mcp.types import Tool as MCPTool
 
 logger = get_logger(__name__)
 
@@ -105,11 +91,11 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
 
 
 def lifespan_wrapper(
-    app: "FastMCP",
+    app: FastMCP,
     lifespan: Callable[["FastMCP"], AbstractAsyncContextManager[LifespanResultT]],
-) -> Callable[[MCPServer], AbstractAsyncContextManager[object]]:
+) -> Callable[[MCPServer[LifespanResultT]], AbstractAsyncContextManager[object]]:
     @asynccontextmanager
-    async def wrap(s: MCPServer) -> AsyncIterator[object]:
+    async def wrap(s: MCPServer[LifespanResultT]) -> AsyncIterator[object]:
         async with lifespan(app) as context:
             yield context
 
@@ -191,7 +177,7 @@ async def list_tools(self) -> list[MCPTool]:
             for info in tools
         ]
 
-    def get_context(self) -> "Context":
+    def get_context(self) -> "Context[ServerSession, object]":
         """
         Returns a Context object. Note that the context will only be valid
         during a request; outside a request, most methods will error.
@@ -564,7 +550,7 @@ def _convert_to_content(
     return [TextContent(type="text", text=result)]
 
 
-class Context(BaseModel, Generic[LifespanContextT]):
+class Context(BaseModel, Generic[ServerSessionT, LifespanContextT]):
     """Context object providing access to MCP capabilities.
 
     This provides a cleaner interface to MCP's RequestContext functionality.
@@ -598,13 +584,13 @@ def my_tool(x: int, ctx: Context) -> str:
     The context is optional - tools that don't need it can omit the parameter.
     """
 
-    _request_context: RequestContext[ServerSession, LifespanContextT] | None
+    _request_context: RequestContext[ServerSessionT, LifespanContextT] | None
     _fastmcp: FastMCP | None
 
     def __init__(
         self,
         *,
-        request_context: RequestContext[ServerSession, LifespanContextT] | None = None,
+        request_context: RequestContext[ServerSessionT, LifespanContextT] | None = None,
         fastmcp: FastMCP | None = None,
         **kwargs: Any,
     ):
@@ -620,7 +606,7 @@ def fastmcp(self) -> FastMCP:
         return self._fastmcp
 
     @property
-    def request_context(self) -> RequestContext[ServerSession, LifespanContextT]:
+    def request_context(self) -> RequestContext[ServerSessionT, LifespanContextT]:
         """Access to the underlying request context."""
         if self._request_context is None:
             raise ValueError("Context is not available outside of a request")

src/mcp/server/fastmcp/tools/base.py

@@ -1,3 +1,5 @@
+from __future__ import annotations as _annotations
+
 import inspect
 from typing import TYPE_CHECKING, Any, Callable
 
@@ -9,6 +11,8 @@
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
+    from mcp.server.session import ServerSessionT
+    from mcp.shared.context import LifespanContextT
 
 
 class Tool(BaseModel):
@@ -68,7 +72,11 @@ def from_function(
             context_kwarg=context_kwarg,
         )
 
-    async def run(self, arguments: dict, context: "Context | None" = None) -> Any:
+    async def run(
+        self,
+        arguments: dict[str, Any],
+        context: Context[ServerSessionT, LifespanContextT] | None = None,
+    ) -> Any:
         """Run the tool with arguments."""
         try:
             return await self.fn_metadata.call_fn_with_arg_validation(

src/mcp/server/fastmcp/tools/tool_manager.py

@@ -1,12 +1,16 @@
+from __future__ import annotations as _annotations
+
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any
 
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.tools.base import Tool
 from mcp.server.fastmcp.utilities.logging import get_logger
+from mcp.shared.context import LifespanContextT
 
 if TYPE_CHECKING:
     from mcp.server.fastmcp.server import Context
+    from mcp.server.session import ServerSessionT
 
 logger = get_logger(__name__)
 
@@ -43,7 +47,10 @@ def add_tool(
         return tool
 
     async def call_tool(
-        self, name: str, arguments: dict, context: "Context | None" = None
+        self,
+        name: str,
+        arguments: dict[str, Any],
+        context: Context[ServerSessionT, LifespanContextT] | None = None,
     ) -> Any:
         """Call a tool by name with arguments."""
         tool = self.get_tool(name)

src/mcp/server/lowlevel/server.py

@@ -64,6 +64,8 @@ async def main():
 messages from the client.
 """
 
+from __future__ import annotations as _annotations
+
 import contextvars
 import logging
 import warnings
@@ -107,7 +109,7 @@ def __init__(
 
 
 @asynccontextmanager
-async def lifespan(server: "Server") -> AsyncIterator[object]:
+async def lifespan(server: Server[LifespanResultT]) -> AsyncIterator[object]:
     """Default lifespan context manager that does nothing.
 
     Args:
@@ -126,7 +128,7 @@ def __init__(
         version: str | None = None,
         instructions: str | None = None,
         lifespan: Callable[
-            ["Server"], AbstractAsyncContextManager[LifespanResultT]
+            [Server[LifespanResultT]], AbstractAsyncContextManager[LifespanResultT]
         ] = lifespan,
     ):
         self.name = name

src/mcp/server/session.py

@@ -38,7 +38,7 @@ async def handle_list_prompts(ctx: RequestContext) -> list[types.Prompt]:
 """
 
 from enum import Enum
-from typing import Any
+from typing import Any, TypeVar
 
 import anyio
 import anyio.lowlevel
@@ -59,6 +59,9 @@ class InitializationState(Enum):
     Initialized = 3
 
 
+ServerSessionT = TypeVar("ServerSessionT", bound="ServerSession")
+
+
 class ServerSession(
     BaseSession[
         types.ServerRequest,