Description

The MCP server crashes with a fatal ValueError: Circular reference detected (id repeated) when Claude Code sends a ListToolsRequest. The server starts, connects, and processes the initialize handshake successfully, but crashes during tool list serialization. This prevents any MCP tools from being used — Claude Code falls back to direct SSH.

Environment

• OS: Windows 11 Enterprise (Build 22631)
• Hardware: Dell Pro Max 16 MC16250 (Intel Core Ultra 7 265H, 16 cores, 32GB RAM)
• Python: 3.12.10
• linux-mcp-server: 1.3.2
• fastmcp: 2.14.6
• pydantic: 2.12.5
• pydantic-core: 2.41.5
• mcp: 1.27.0
• MCP Client: Claude Code 2.1.92

Steps to Reproduce

1. Install on Windows: pip install --user linux-mcp-server
2. Register with Claude Code:

claude mcp add-json "Linux-RHEL" '{"command":"C:\Users\username\AppData\Roaming\Python\Python312\Scripts\linux-mcp-server.exe","env":{"LINUX_MCP_VERIFY_HOST_KEYS":"false"}}'

3. claude mcp list shows ✓ Connected
4. Launch Claude Code and request any tool (e.g., "Use get_system_information to check host myserver")
5. Server crashes — tools never register

Expected Behavior

ListToolsRequest should return the tool schemas and tools should be available in Claude Code.

Actual Behavior

Server processes ListToolsRequest, then crashes with:

CRITICAL | Fatal error in Linux MCP Server: unhandled errors in a TaskGroup (2 sub-exceptions)
...
ValueError: Circular reference detected (id repeated)

The error originates in pydantic's model_dump() during response serialization:

Workaround

Claude Code falls back to direct SSH (bash tool), which works. The MCP tools are the only thing broken.

Notes

• Attempted downgrading pydantic to 2.11.5 — did not fix the issue
• Attempted downgrading fastmcp to 2.3.4 — incompatible with linux-mcp-server 1.3.2
• The server initializes correctly and the docket.worker starts — the crash is specifically in the ListToolsRequest response serialization
• Also noting related issue linux-mcp-server is vunerable when attacking using Man in The Middle approach #304 (MITM vulnerability with default verify_host_keys=False)