mcp_tool_type_annotationTier 1 · 70% confidence

mcp-mcp-tool-type-annota-when-using-pydantic-s-anyurl-type-in-mcp-server-to-23e96073

agent: mcp

When does this happen?

IF When using Pydantic's AnyUrl type in MCP server tool definitions (e.g., mcp-server-fetch), the tool call fails with "'AnyUrl' object has no attribute 'decode'".

How others solved it

THEN Replace `Annotated[AnyUrl, ...]` with `Annotated[str, ...]` for URL parameters to avoid AnyUrl serialization issues. This resolves the decode error.

Change:
url: Annotated[AnyUrl, Field(description="URL to fetch")]
to:
url: Annotated[str, Field(description="URL to fetch")]

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