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Track: FastMCP 2.14.x outputSchema incompatibility with Claude Desktop #468
Description
Summary
FastMCP 2.14.x introduces automatic outputSchema generation for all MCP tools, but Claude Desktop doesn't support this field and disables the tool toggle entirely when it encounters it. This causes MCP tools to be invisible to users despite being registered correctly server-side.
Current Status
Fixed in v0.16.3 by pinning to fastmcp==2.12.3 (PR #464)
Root Cause
FastMCP 2.14.0 Breaking Change
FastMCP 2.14.0 removed the ability to disable outputSchema generation:
output_schema=Falsewas deprecated in 2.11.4- Removed entirely in 2.14.0
- Now
outputSchemais always generated for all tools
Claude Desktop Incompatibility
When Claude Desktop receives tool definitions with outputSchema:
- The tool toggle is disabled in the UI
- Tools appear registered server-side (
tools/listreturns all tools) - But tools are not available in conversations
- Users see "tool not found" errors
Version Comparison
| Package | Working | Broken |
|---|---|---|
| fastmcp | 2.12.3 | 2.14.x |
| mcp | 1.14.0 | 1.24.0 |
User Reports
From Discord (Dec 19, 2025):
Claude Desktop connects to Basic Memory successfully – I can see in the logs that tools/list returns all 18 tools – but Claude doesn't make them available in conversations. The MCP server shows "running" in settings.
Users confirmed:
- ✅ Server shows "running" in MCP settings
- ✅ Logs show successful initialize
- ✅
tools/listreturns all 18 tools - ✅
prompts/listandresources/listwork - ❌ Tools NEVER available in conversations
Upstream Issues
- FastMCP: jlowin/fastmcp#2654 - Request for global config to disable
outputSchema - Claude Code: anthropics/claude-code#10031 - Related
outputSchemavalidation issue (fixed) - Claude Code: anthropics/claude-code#14807 - Broader MCP breakage Dec 19, 2025
Troubleshooting for Users
If you're still experiencing issues after upgrading to v0.16.3:
1. Verify FastMCP Version
pip show fastmcp
# Should show Version: 2.12.32. Clean Reinstall
# Option 1: pip
pip uninstall basic-memory fastmcp -y
pip install basic-memory
# Option 2: uvx
uvx --reinstall basic-memory
# Option 3: pipx
pipx uninstall basic-memory
pipx install basic-memory3. Restart Claude Desktop
Fully quit and restart Claude Desktop after reinstalling.
4. Windows Users
If still not working, try killing orphan Node processes:
Get-Process | Where-Object {$_.ProcessName -like "*node*"} | Stop-Process -ForceResolution Path
This issue will remain open to track when we can safely upgrade FastMCP:
- FastMCP adds global config option to disable
outputSchema(#2654) - OR Claude Desktop adds support for
outputSchema - Test upgrade to newer FastMCP version
- Remove pin and update dependency
Related
- Pin FastMCP to 2.12.3 - Version 2.14.x breaks MCP tools visibility #463 - Original issue report
- fix: Pin FastMCP to 2.12.3 to fix MCP tools visibility #464 - PR that pinned FastMCP to 2.12.3
Activity
claude commentedon Dec 23, 2025
Claude finished @phernandez's task in 1m 30s —— View job
Issue Triage Complete
Triage Analysis
Issue Classification:
Summary:
This is a well-documented tracking issue for monitoring the FastMCP
outputSchemaincompatibility. The immediate problem was already resolved in v0.16.3 by pinning tofastmcp==2.12.3(PR #464). This issue serves as a reminder to upgrade once the upstream issues are resolved.Duplicate Check:
Key Points:
outputSchemamandatory, but Claude Desktop doesn't support itoutputSchema(jlowin/fastmcp#2654)outputSchemasupportRecommendations:
Label Applied:
enhancement- This tracks when we can enhance the codebase by upgrading to newer FastMCP versions