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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Strings parameters tool containing serialized JSON objects are automatically decoded","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\nI have a tool function in a MCP server that accepts an input parameter (\"data\") of type string (str); if a client uses a serialiazed JSON as content of this parameter it gets the error message:\n\n`Tool call result: meta=None content=[TextContent(type='text', text=\"Error executing tool print_data: 1 validation error for print_dataArguments\\ndata\\n  Input should be a valid string [type=string_type, input_value={'message': 'hello'}, input_type=dict]\\n    For further information visit https://errors.pydantic.dev/2.11/v/string_type\", annotations=None, meta=None)] structuredContent=None isError=True` \n\nI suspect that somewhere the string is decoded automatically to a dict.\n\nIt is the same behavior with both stdio and streamable-http transports.\n\n### Example Code\n\n```Python\n# server.py\n\nfrom mcp.server.fastmcp import FastMCP\nimport loguru\nimport time\nimport json\n\nmcp = FastMCP(\"test-server\", \"0.0.1\")\n\n\n@mcp.tool()\ndef print_data(data: str) -\u003e str:\n    \"\"\"\n    Print the data.\n    \"\"\"\n    loguru.logger.warning(f\"Received data: {data} (type: {type(data)})\")\n    return f\"data: {data}\"\n\n\nif __name__ == \"__main__\":\n    import sys\n    transport = sys.argv[1] if len(sys.argv) \u003e 1 else \"stdio\"\n    if transport != \"stdio\" and transport != \"streamable-http\":\n        raise ValueError(\"Transport must be 'stdio' or 'streamable-http'\")\n    mcp.run(transport=transport)\n\n# stdio_client.py\nfrom mcp import ClientSession, StdioServerParameters, types\nfrom mcp.client.stdio import stdio_client\nimport json\n\nserver_params = StdioServerParameters(\n    command=\"python\",\n    args=[\"-u\", \"server.py\"],\n    env=None,\n)\n\n\nasync def run():\n    async with stdio_client(server_params) as (read, write):\n        async with ClientSession(read, write) as session:\n            await session.initialize()\n            args = {\"data\": json.dumps({\"message\": \"hello\"})}\n            result = await session.call_tool(\"print_data\", arguments=args)\n            print(\"Tool call result:\", result)\n\n\nif __name__ == \"__main__\":\n    import asyncio\n    asyncio.run(run())\n    \n# http_client.py\nfrom mcp.client.streamable_http import streamablehttp_client\nfrom mcp import ClientSession\nimport json\n\nasync def main():\n    async with streamablehttp_client(\"http://localhost:8000/mcp\") as (\n        read_stream,\n        write_stream,\n        _,\n    ):\n        async with ClientSession(read_stream, write_stream) as session:\n            await session.initialize()\n            args = {\"data\": json.dumps({\"message\": \"hello\"})}\n            tool_result = await session.call_tool(\"print_data\", arguments=args)\n            print(\"Tool call result:\", tool_result)\n\nif __name__ == \"__main__\":\n    import asyncio\n    asyncio.run(main())\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nPython 3.13.5 and 3.12.11\nMCP SDK 1.10.1\n```","author":{"url":"https://github.com/cjg","@type":"Person","name":"cjg"},"datePublished":"2025-07-09T20:13:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/1112/python-sdk/issues/1112"}