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MCP 2.x structured output fails despite type annotations

By PulseAugur Editorial · [1 sources] ·

The author encountered issues when migrating from MCP 1.x to MCP 2.x, specifically with the FastMCP tool, which was renamed to MCPServer. While the import path and basic tool registration were straightforward, the promised structured output feature in MCP 2.x did not function as expected. Despite annotating return types as `dict` or `TypedDict`, the SDK consistently returned a JSON string within a `TextContent` block, leaving the `structured_content` field as `None`. This forces clients to fall back to parsing the text content, negating the benefit of the new structured output contract. AI

IMPACT This issue highlights potential reliability problems in SDKs that promise structured output, impacting developers relying on these features for robust data handling.

RANK_REASON The item discusses a specific issue with a software tool's functionality and migration, not a new release or significant industry event.

Read on dev.to — MCP tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

MCP 2.x structured output fails despite type annotations

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The item discusses a specific issue with a software tool's functionality and migration, not a new release or significant industry event.
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COVERAGE [1]

  1. dev.to — MCP tag TIER_1 English(EN) · Enjoy Kumawat ·

    MCP 2.x's Structured Output Only Shows Up If Your Return Type Asks for It

    <p>"MCP Is Dead Simple. And That's Exactly the Problem" has been trending in the MCP tag this week, and it needled me into actually testing a claim I'd been resting on instead of checking: that my own MCP server's tool contracts are simple because the SDK is simple. <code>my-git-…

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