### Preflight Checklist - [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet - [x] This is a single bug report (please file separate reports for different bugs) - [x] I am using the latest version of Claude Code ### What's Wrong? The issue from #3084 was marked as fixed and closed, but FastMCP Pydantic models are still serialized as a JSON string in the 1.0.117 (Claude Code) releas

[BUG] FastMCP Pydantic serialization issue persists after #3084 fix

Status Closed — not planned

Maintainer reply None cached

Activity 7 comments · opened Sep 17, 2025 · closed Jan 6, 2026

Preflight Checklist

[x] I have searched existing issues and this hasn't been reported yet
[x] This is a single bug report (please file separate reports for different bugs)
[x] I am using the latest version of Claude Code

What's Wrong?

The issue from #3084 was marked as fixed and closed, but FastMCP Pydantic models are still serialized as a JSON string in the 1.0.117 (Claude Code) release, causing validation errors.

What Should Happen?

FastMCP should pass Pydantic models as dictionaries to prevent input validation errors

Steps to Reproduce

Same as #3084

Claude Model

None

Is this a regression?

Claude Code Version

1.0.117 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

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