agent-add-mcp

name	agent-add-mcp
description	Use this skill when helping users add MCP servers to their CastCodes configuration.

Adding MCP Servers to CastCodes

CastCodes supports MCP servers via native config files. Follow these steps when helping a user add an MCP server.

Step 1: Determine Scope

If the user hasn't specified, ask whether they want to configure the server globally (for all projects) or project-scoped (for a specific repository only).

Config file paths:

• Global (user-scoped): ~/.cast-codes/.mcp.json
• Project-scoped: {repo_root}/.cast-codes/.mcp.json

Step 2: Gather Server Details

If the user hasn't provided the server's connection details, use WebSearch to find the correct configuration for the named server.

If it's unclear whether the server should be run as a local CLI process (stdio transport) or connected to via URL (HTTP/SSE streaming transport), ask the user which they prefer.

Step 3: Check and Prepare the Config File

Check whether the target config file exists.

• If it does not exist, create it with mkdir -p for the directory and initialize it with an empty mcpServers wrapper key:

  {
    "mcpServers": {}
  }
  

• If it exists, read it to determine which top-level wrapper key is already in use. Recognized wrapper keys (in order of preference):

  • mcpServers (preferred)
  • mcp_servers
  • servers
  • mcp.servers (nested under a mcp key)
  • Flat map (each top-level key is a server name)

  Preserve the existing wrapper key when writing. If the existing key is unrecognized or incompatible, switch to mcpServers.

  Never remove existing server entries — only add or update the new server.

Step 4: Write the Server Configuration

Command-based server (stdio transport)

{
  "mcpServers": {
    "server-name": {
      "command": "npx",
      "args": ["-y", "@scope/package-name"],
      "env": {
        "API_KEY": "${API_KEY}"
      }
    }
  }
}

By default, CastCodes spawns stdio servers from the directory the config was discovered in:

• Project-scoped configs ({repo_root}/.cast-codes/.mcp.json) run from the repo root.
• Global configs (~/.cast-codes/.mcp.json, ~/.claude.json, etc.) run from the home directory.

If the server's command or args are relative paths (e.g. ./tooling/mcp/server.js) or the server expects a specific cwd, set working_directory to override the default:

{
  "mcpServers": {
    "server-name": {
      "command": "node",
      "args": ["./tooling/mcp/server.js"],
      "working_directory": "/absolute/path/to/repo"
    }
  }
}

URL-based server (HTTP/SSE streaming transport)

{
  "mcpServers": {
    "server-name": {
      "url": "https://example.com/mcp",
      "env": {
        "API_KEY": "${API_KEY}"
      }
    }
  }
}

For environment variables containing secrets, use ${VAR_NAME} syntax — CastCodes will substitute the value from the user's environment at runtime.

Notes

• CastCodes auto-detects changes to .mcp.json files on save — no restart required.
• Configured servers appear in CastCodes Settings under MCP, labeled "Detected from CastCodes".
• Global config applies across all sessions; project config only applies when working inside that repository.