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FastMCP 4 migration guide details breaking changes in package structure and dependencies

By PulseAugur Editorial · [1 sources] ·

The release of FastMCP 4 introduces several breaking changes that developers must address during migration. A key issue arises from the package structure change, where `pip install -U fastmcp` may not correctly install all necessary components, leading to import errors. Users are advised to perform a clean uninstall and reinstall or recreate their virtual environments. Additionally, FastMCP 4 now uses `httpx2` instead of `httpx`, meaning existing `httpx.ConnectError` exception handlers will no longer function and must be updated to catch `httpx2.ConnectError` or the more general `fastmcp.exceptions.ToolError`. The default client behavior has also shifted to `mode="auto"`, which implies sessionless operation and alters runtime behaviors like state persistence and server-initiated elicitations. AI

IMPACT Developers using FastMCP will need to update their code to accommodate changes in dependency handling and error catching.

RANK_REASON This is a technical guide detailing breaking changes for a specific software library, not a new product release or significant industry event.

Read on dev.to — MCP tag →

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

FastMCP 4 migration guide details breaking changes in package structure and dependencies

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This is a technical guide detailing breaking changes for a specific software library, not a new product release or significant industry event.
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COVERAGE [1]

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

    FastMCP 3 4 migration: the breaking changes that compile

    <p><em>FastMCP 4 is GA. If you have an MCP server or client on <code>fastmcp</code> 3.x, you'll<br /> upgrade soon. Most of it is painless — <code>FastMCP(...)</code>, <code>@mcp.tool</code>, and<br /> <code>mcp.run(transport=...)</code> are all unchanged. The parts that aren't p…

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