[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?
No
Claude Code Version
1.0.117 (Claude Code)
Platform
Anthropic API
Operating System
macOS
Terminal/Shell
Terminal.app (macOS)
7 Comments
Found 3 possible duplicate issues:
This issue will be automatically closed as a duplicate in 3 days.
🤖 Generated with Claude Code
There are two issues being created at the same time for the same issue. I'm seeing the same issue in Desktop and it appears to be present in some code shared between Claude Desktop and Claude Code. The previously closed issue is _not_ resolved.
The other one logged within 24 hours of this one is also being flagged as a dupe. I believe one or both of these needs to stay open until a fix is found:
All of the noted dupes above are _closed_ and therefor not valid for deduping.
I closed #7845, I missed that this one was opened, I'll follow along here
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