Working With Mcp

Working with MCPProxy and MCP Tools

Overview

MCPProxy can be accessed two ways:

1. MCP Tools (mcp__MCPProxy__*) - When MCPProxy is configured as an MCP server in Claude Code
2. HTTP API (curl http://localhost:8080/...) - For debugging MCPProxy itself

This skill is for #1. For debugging MCPProxy itself, use HTTP API approach sparingly.

FIRST: Detect MCPProxy Connection

When user mentions MCPProxy or MCP tools:

• "check buildkite mcp"
• "use slack mcp tools"
• Any MCPProxy tool request

YOU MUST IMMEDIATELY:

1. Look at your available tools (passive observation - don't run commands)
2. Scan for tools starting with mcp__MCPProxy__

Examples of what you're looking for:

mcp__MCPProxy__retrieve_tools
mcp__MCPProxy__call_tool
mcp__MCPProxy__upstream_servers

Decision:

• ✅ See mcp__MCPProxy__* tools? → MCPProxy is connected, use them
• ❌ Don't see mcp__MCPProxy__* tools? → MCPProxy MCP server not connected

If tools are missing, IMMEDIATELY tell user:

> "MCPProxy MCP tools aren't available in this session. The MCPProxy MCP server isn't connected. > > Try the /mcp command and select MCPProxy to reconnect. Once reconnected, you'll have access to the mcp__MCPProxy__* tools."

DO NOT:

• Try bash commands like claude mcp list or mcpproxy list-tools
• Try HTTP API with curl
• Make multiple attempts with different approaches
• Try to work around the issue

Missing mcp__MCPProxy__* tools = MCPProxy MCP server not connected = Suggest /mcp reconnect. That's it.

Core Decision Tree (Passive Observation)

This is passive observation - look at tools you already have, don't run commands.

Look at your available tools (the tool list in this session)
  ↓
Do you see ANY tools starting with mcp__MCPProxy__*?
│
├─ YES → MCPProxy is connected
│         ├─ Discover: mcp__MCPProxy__retrieve_tools
│         └─ Call: mcp__MCPProxy__call_tool
│
└─ NO → MCPProxy MCP server not connected
          └─ Tell user: "Try /mcp command to reconnect"

Key points:

• This is OBSERVATION, not execution
• Don't run claude mcp list or any bash command
• Just scan your available tool list
• Missing tools = connection issue, not "tools don't exist"
• Never fall back to HTTP API

When to Use This Skill

Use when:

• You need to discover what tools MCPProxy exposes
• You want to call a tool through MCPProxy
• MCPProxy is configured but you're not sure if MCP tools are available
• User mentions working with MCP servers or tools

Do NOT use when:

• User is asking about non-MCP topics
• The task doesn't involve MCP tools

What You're Looking For

When MCPProxy IS connected, your tool list includes:

Available tools:
- Bash
- Read
- Write
- mcp__MCPProxy__retrieve_tools    ← Look for these
- mcp__MCPProxy__call_tool          ← Look for these
- mcp__MCPProxy__upstream_servers   ← Look for these
- mcp__ide__getDiagnostics
- mcp__ide__executeCode

When MCPProxy is NOT connected:

Available tools:
- Bash
- Read
- Write
- mcp__ide__getDiagnostics
- mcp__ide__executeCode
← No mcp__MCPProxy__* tools = Connection issue

Key distinction:

• mcp__ide__* = Different MCP server (IDE integration)
• mcp__MCPProxy__* = MCPProxy tools for upstream servers
• Only mcp__MCPProxy__* tools indicate MCPProxy connection

Core Pattern: Discover Then Call

// 1. Search for relevant tools
mcp__MCPProxy__retrieve_tools({
  query: "keywords describing what you need",
  limit: 10
});

// 2. Examine tool schemas in results
// Look at inputSchema to understand parameters

// 3. Call the tool
mcp__MCPProxy__call_tool({
  name: "server:tool-name",  // From search results
  args_json: JSON.stringify({  // MUST be JSON string
    param1: "value",
    param2: "value"
  })
});

Critical: args_json must be a JSON string (use JSON.stringify()), not a plain object.

Available MCP Tools

• mcp__MCPProxy__retrieve_tools - Search for tools across all upstream servers
• mcp__MCPProxy__call_tool - Execute a discovered tool
• mcp__MCPProxy__upstream_servers - Manage server configuration
• mcp__MCPProxy__list_registries - List available MCP registries
• mcp__MCPProxy__search_servers - Find new servers in registries
• mcp__MCPProxy__read_cache - Read paginated results
• mcp__MCPProxy__quarantine_security - Manage quarantined servers

Quick Reference

| Task | Check This First | Then Use | |------|------------------|----------| | Discover tools | Are mcp__MCPProxy__* in tool list? | If yes: retrieve_toolsIf no: Suggest /mcp | | Call a tool | Are mcp__MCPProxy__* in tool list? | If yes: call_toolIf no: Suggest /mcp | | Debug MCPProxy | N/A | Use HTTP API sparingly |

If MCP Tools Are Missing

When mcp__MCPProxy__* tools are not in your tool list:

Tell the user: > "MCPProxy MCP tools aren't available in this session. Try the /mcp command and select MCPProxy to reconnect."

Do NOT:

• Fall back to HTTP API (that's for debugging MCPProxy)
• Assume the tools don't exist
• Try bash commands
• Make multiple curl attempts

Reality: The tools exist when MCP connection is established. Missing tools = connection issue.

Top 3 Common Mistakes

1. Invoking MCP Tools as Bash Commands

# ❌ WRONG - MCP tools are not bash commands
mcp__MCPProxy__retrieve_tools --query "slack tools"

// ✅ CORRECT - Call as MCP tool (function)
mcp__MCPProxy__retrieve_tools({
  query: "slack tools"
})

How to tell the difference:

• mcp__server__tool pattern → MCP tool (function call)
• mcpproxy, docker → Bash command (via Bash tool)

2. Using HTTP API Instead of MCP Tools

# ❌ WRONG - Don't do this when MCP tools should work
curl http://127.0.0.1:8080/api/v1/tools?apikey=...

// ✅ CORRECT - Use MCP tools
mcp__MCPProxy__retrieve_tools({query: "search query"})

HTTP API is only for debugging MCPProxy itself.

3. Falling Into the HTTP API Spiral

Agent tries: mcp__MCPProxy__call_tool(...)
Gets error: "No such tool available"
Agent makes:
1. curl attempt with python parsing
2. curl attempt with json.tool
3. curl attempt with grep
4. Python script wrapping curl
5. Different parsing strategy...

STOP! Multiple curl attempts = you're avoiding the real problem: MCP not connected. Suggest /mcp instead.

Red Flags - STOP Immediately

If you think any of these thoughts, STOP:

• "I'll run mcp__MCPProxy__retrieve_tools as a bash command"
• "The mcp__MCPProxy__* tools don't exist, I'll use curl"
• "HTTP API is simpler, I'll use that"
• "Let me try a different way to parse the HTTP response"
• "Maybe if I use Python/jq/grep it will work"
• "The tool error means MCPProxy isn't running"

Reality:

• mcp__MCPProxy__* tools exist when MCP connection is established
• Missing tools = connection issue, not "tools don't exist"
• HTTP API is for debugging MCPProxy, not for normal usage
• Running ≠ Connected. Process can run but MCP not connected.

Red Flags in Error Responses

When an MCP tool call returns an error, check for these patterns. If matched, STOP - retrying won't help.

Unrecoverable errors - STOP and tell the user:

| Error Pattern | What It Means | Tell User | |--------------|---------------|-----------| | authentication failed, OAuth/token authentication required, authorization required | Upstream server needs re-auth | "Server '{name}' needs re-authentication. Run mcpproxy auth login --server={name} or use the MCPProxy system tray to re-authenticate." | | is not connected, is disabled | Server offline/disabled | "Server '{name}' isn't connected. Check MCPProxy status with mcp__MCPProxy__upstream_servers(operation='list')." | | access_denied, insufficient_scope | Missing permissions | "Server '{name}' lacks permissions for this operation. May need to re-authorize with additional scopes." |

Why retrying won't help:

These errors require user action outside of Claude:

• Re-running OAuth flow through MCPProxy CLI or system tray
• Fixing server configuration
• Granting additional permissions

Don't:

• Retry the same tool with different parameters
• Try other tools on the same server (they'll fail too)
• Fall back to WebFetch/curl (won't have auth either)

Debugging MCPProxy Connection Issues

If MCPProxy itself isn't working properly (servers won't connect, Docker issues, etc.), you can use these quick checks:

Quick Health Check

# 1. Is mcpproxy running?
ps aux | grep mcpproxy | grep -v grep

# 2. Get API key
grep '"api_key"' ~/.mcpproxy/mcp_config.json

# 3. Check server status
curl -s "http://127.0.0.1:8080/api/v1/servers?apikey=YOUR_KEY" | python3 -m json.tool

# 4. Check for recent errors
tail -50 ~/Library/Logs/mcpproxy/main.log | grep -i error

Key status fields to check:

• connected: Boolean - is the server connected?
• status: String - current state (connecting, ready, error)
• last_error: String - most recent error message
• tool_count: Number - how many tools available

Common MCPProxy Issues

"the input device is not a TTY" (Docker servers)

Fix: Add "isolation": {"enabled": false} to the Docker-based server config.

"unexpected argument found" (uvx/npx servers)

Fix: Put package name as first arg: ["mcp-server-name", "--arg", "value"]

"Invalid or missing API key" after restart

Fix: Check echo $MCPPROXY_API_KEY - environment variable overrides config file.

Restart MCPProxy

pkill mcpproxy
sleep 2
open /Applications/mcpproxy.app  # macOS
# OR
mcpproxy &  # Linux/headless

Summary

The capability: Use MCPProxy's tool discovery to find and call tools across all upstream MCP servers.

The pattern:

1. Check if mcp__MCPProxy__* tools exist
2. If yes → Use them (discover then call)
3. If no → Suggest /mcp reconnect
4. Never fall back to HTTP API

Result: Fast tool usage with clear error handling.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: technicalpickles
• Source: technicalpickles/pickled-claude-plugins
• License: MIT

Install and usage instructions live in the source repository linked above.