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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`@mcp.tool()` Fails to Find Context Argument if its Type `Context` Includes Type Variables","articleBody":"**Describe the bug**\nThe code does not work because the type annotation for the `ctx` argument is not simply `Context`; it includes type variables `ServerSessionT` and `AppContext`.\n\n```python\n@mcp.tool()\ndef query_db(ctx: Context[ServerSessionT, AppContext]) -\u003e str:\n    \"\"\"Tool that uses initialized resources\"\"\"\n    db = ctx.request_context.lifespan_context.db\n    return db.query()\n```\n\nThe MCP server assumes that `ctx` will be passed by clients, as its implementation only checks whether the annotation is exactly `Context`. \n\nYou can see this limitation in the following code:\n\nhttps://github.com/modelcontextprotocol/python-sdk/blob/9ae4df85fbab97bf476ddd160b766ca4c208cd13/src/mcp/server/fastmcp/tools/base.py#L53-L58\n\nThe necessity of adding type variables to `Context` is explained in issue #xxx.\n\n\n**To Reproduce**\n1. Apply the following patch:\n\n```patch\ndiff --git a/examples/fastmcp/simple_echo.py b/examples/fastmcp/simple_echo.py                                                                                                                                                                                                                                                                                                    \nindex c261526..3069b7e 100644                                                                                                                                                                                                                                                                                                                                                     \n--- a/examples/fastmcp/simple_echo.py\n+++ b/examples/fastmcp/simple_echo.py\n@@ -1,14 +1,15 @@\n \"\"\"                                                                                                                                                                                                                                                                                                                                                                              \n FastMCP Echo Server                                                                                                                                                                                                                                                                                                                                                              \n \"\"\"                                                                                                                                                                                                                                                                                                                                                                              \n+from typing import Any                                                                                                                                                                                                                                                                                                                                                           \n                                                                                                                                                                                                                                                                                                                                                                                  \n-from mcp.server.fastmcp import FastMCP                                                                                                                                                                                                                                                                                                                                           \n+from mcp.server.fastmcp import FastMCP, Context                                                                                                                                                                                                                                                                                                                                  \n                                                                                                                                                                                                                                                                                                                                                                                  \n # Create server                                                                                                                                                                                                                                                                                                                                                                  \n mcp = FastMCP(\"Echo Server\")                                                                                                                                                                                                                                                                                                                                                     \n                                                                                                                                                                                                                                                                                                                                                                                  \n                                                                                                                                                                                                                                                                                                                                                                                  \n @mcp.tool()                                                                                                                                                                                                                                                                                                                                                                      \n-def echo(text: str) -\u003e str:                                                                                                                                                                                                                                                                                                                                                      \n+def echo(ctx:Context[Any, Any], text: str) -\u003e str:                                                                                                                                                                                                                                                                                                                               \n     \"\"\"Echo the input text\"\"\"                                                                                                                                                                                                                                                                                                                                                    \n     return text   \n```\n\n2. Run the following command:\n```\nuv run pytest tests/test_examples.py\n```\n\n \n3. The test will fail with the following error:\n\n```\nError executing tool echo: 1 validation error for echoArguments\nctx\n  Field required [type=missing, input_value={'text': 'hello'}, input_type=dict]\n    For further information visit https://errors.pydantic.dev/2.10/v/missing\n```\n\n**Expected behavior**\nType variables should be allowed in tool definitions. The MCP server should correctly handle `Context` annotations that include type variables.\n\n\n**Desktop (please complete the following information):**\n - OS: macOS 15.3.2\n - Browser N/A\n - Version N/A\n","author":{"url":"https://github.com/jkawamoto","@type":"Person","name":"jkawamoto"},"datePublished":"2025-03-24T08:49:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/357/python-sdk/issues/357"}