11 MCP Structured Returns: BaseModel, TypedDict, Dataclass, and Plain Classes Compared
Six tools, six return types, one key question: when does structuredContent get populated and when does content give you usable JSON? This section breaks down exactly what FastMCP does with BaseModel, TypedDict, dict[str, float], plain annotated classes, @dataclass, and classes with no annotations — and where each one fails. outputSchema in list_tools tells you everything before you make a single call.
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