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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"FastMCP crashes when tool return type uses Python 3.10+ `A | B | C` union syntax","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\n`FastMCP` crashes on startup when a tool function's return type annotation uses Python 3.10+ union syntax (`A | B | C`, i.e. `types.UnionType`). The error originates from `_create_wrapped_model` in `func_metadata.py`, which passes the union annotation directly to Pydantic's `create_model()` as a bare keyword value, causing `PydanticUserError`.\n\n### Example Code\n\n```Python\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP(\"test\")\n\n@mcp.tool()\nasync def my_tool(flag: bool) -\u003e dict | list | str:  # \u003c-- triggers the bug\n    if flag:\n        return {\"key\": \"value\"}\n    return [\"item\"]\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\n- mcp (Python): latest (\u003e=1.0.0,\u003c2.0.0)\n- pydantic: \u003e=2.10 (observed on 2.10.x)\n- Python: \u003e=3.10\n```","author":{"url":"https://github.com/Swcmb","@type":"Person","name":"Swcmb"},"datePublished":"2026-05-14T12:45:56.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/2591/python-sdk/issues/2591"}