Skip to content

Remove deprecated tool_serializer parameter #2754

New issue
New issue
Open
Open
Remove deprecated tool_serializer parameter#2754
Labels
breaking changeBreaks backward compatibility. Requires minor version bump. Critical for maintainer attention.enhancementImprovement to existing functionality. For issues and smaller PR improvements.serverRelated to FastMCP server implementation or server-side functionality.v3Targeted for FastMCP 3
Milestone
Deprecations
@jlowin

Description

@jlowin
jlowin
opened on Dec 26, 2025

The tool_serializer parameter on FastMCP and the serializer parameter on tools were deprecated in v3.0.0.

Deprecation PR: #2753

Users should return ToolResult from their tools for explicit control over serialization. See Custom Serialization for examples.

Metadata

Metadata

Assignees

No one assigned

    Labels

    breaking changeBreaks backward compatibility. Requires minor version bump. Critical for maintainer attention.enhancementImprovement to existing functionality. For issues and smaller PR improvements.serverRelated to FastMCP server implementation or server-side functionality.v3Targeted for FastMCP 3

    Projects

    No projects

    Milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions