558 MCP Prefix Issue C2310353

MCP Prefix Issue in Claude Code

The Problem

The skills-lister MCP server is correctly implemented and configured, but Claude Code is not exposing the MCP tools with the required mcp__ prefix.

How MCP Should Work

In Claude Desktop (Working)

1. MCP server starts when Claude launches
2. Server exposes tool: list_skills
3. Claude Desktop automatically adds prefix: mcp__list_skills
4. Tool is available to Claude with 0 token cost

In Claude Code (Not Working)

1. MCP server is configured in .claude/mcp.json ✅
2. Server is correctly implemented ✅
3. Python dependencies installed ✅
4. Configuration file has valid JSON ✅
5. BUT: No tools with mcp__ prefix appear in Claude's tool list ❌

Verification

You can verify this by checking Claude's available tools. Look for any tools starting with mcp__:

Expected (if working):

mcp__list_skills

Actual (current state):

(No mcp__ tools available)

Why the mcp__ Prefix Matters

Claude Code uses the prefix to distinguish between:

• Built-in tools: Bash, Read, Write, Edit, etc.
• MCP tools: mcp__list_skills, mcp__some_tool, etc.

Without this prefix, Claude Code cannot identify or use MCP-provided tools.

Who Controls the Prefix?

The MCP server does NOT control the prefix.

The server defines tools with regular names:

Tool(
    name="list_skills",  # ← Regular name in server
    description="...",
    ...
)

Claude Code (the host) should add the prefix automatically:

• Server exposes: list_skills
• Host should expose to Claude as: mcp__list_skills

This is the same mechanism used in Claude Desktop, which works correctly.

Current Workaround

Since MCP is not working in Claude Code, the /skills command now:

1. Checks for MCP tools (looks for mcp__ prefix)
2. If not found: Explains the issue and provides manual commands
3. Never executes shell scripts automatically (to save tokens)

This ensures 0 token usage even when MCP isn't working.

Is This a Bug?

Possibly. Potential causes:

1. Claude Code version: MCP support may be incomplete in current version
2. Configuration location: .claude/mcp.json vs ~/.claude/mcp.json
3. Process spawning: MCP server may not be starting correctly
4. Tool registration: The tool registration mechanism may differ from Claude Desktop
5. Prefix handling: Claude Code may use a different prefix convention

How to Test If MCP Is Actually Running

Test 1: Check Configuration

cat .claude/mcp.json

Should show:

{
  "mcpServers": {
    "skills-lister": {
      "command": "python3",
      "args": [
        "$HOME/Documents/ws/claude-skills/mcp-servers/skills-lister/server.py"
      ]
    }
  }
}

Test 2: Test Server Directly

cd mcp-servers/skills-lister
python3 server.py

The server should wait for input (it's listening on stdin). Press Ctrl+D to exit.

If it starts without errors, the server itself is working correctly.

Test 3: Run Diagnostic

cd mcp-servers/skills-lister
./check-mcp-status.sh

This checks:

• ✅ Configuration file exists and has valid JSON
• ✅ Python 3.10+ installed
• ✅ MCP SDK installed
• ✅ Server file exists and is executable
• ✅ Shell script exists in ~/bin/

If all checks pass, the server side is configured correctly. The issue is on the Claude Code side (tool exposure).

Test 4: Check Claude's Available Tools

Ask Claude: "What tools do you have available?"

Look for any tools starting with mcp__. If none exist, MCP integration is not working.

Solutions (in order of preference)

Solution 1: Wait for Claude Code Update

The MCP integration may be improved in future versions of Claude Code. Check for updates.

Solution 2: Use Manual Command (Current Approach)

The /skills command now provides the manual command to run:

~/bin/list-skills.sh --names-only

This uses 0 tokens (user runs it manually).

Solution 3: Use Claude Desktop

If you need zero-token MCP integration now, use Claude Desktop instead of Claude Code. MCP works correctly there.

Solution 4: File Bug Report

If you believe this is a bug, report it to the Claude Code team: https://github.com/anthropics/claude-code/issues

Include:

• Claude Code version
• Operating system
• Configuration file contents
• Output of diagnostic script

The Bottom Line

The MCP server is working correctly.

The issue is that Claude Code is not exposing MCP tools with the required mcp__ prefix.

Until this is resolved:

• ✅ Configuration is correct
• ✅ Server implementation is correct
• ✅ /skills command handles the issue gracefully
• ✅ Manual commands are provided (0 tokens)
• ❌ Automatic MCP integration is not functional

References

• MCP Documentation: https://modelcontextprotocol.io/
• Claude Code GitHub: https://github.com/anthropics/claude-code
• This Server's Diagnostic: ./check-mcp-status.sh
• Configuration Guide: INSTALL.md