Runtime placeholders

Agent Plugins v1 defines two client-supplied variables for portable stdio MCP servers. They are runtime values, not file-install targets.

Supported variables

• ${PLUGIN_ROOT} - absolute filesystem-resolved root of the installed plugin.
• ${PLUGIN_DATA} - writable, persistent data directory for that plugin instance.

Example:

{
  "$schema": "https://agent-plugins.org/schemas/1.0.0/mcp.schema.json",
  "mcpServers": {
    "reviewer": {
      "type": "stdio",
      "command": "./bin/reviewer",
      "args": ["--cache", "${PLUGIN_DATA}/cache"],
      "env": { "RULES": "${PLUGIN_ROOT}/config/rules.json" },
      "cwd": "${PLUGIN_ROOT}"
    }
  }
}

Safety rules

• Expansion applies to string entries in args, string values in env, and cwd; it does not apply to command, environment keys, URLs, or headers.
• command is one executable token: a bare executable or a contained ./ path. It is never a shell command string.
• Plugins must not define PLUGIN_ROOT or PLUGIN_DATA in env; the client owns both values.
• Plugin-root paths must remain inside the resolved package root. Provider emitters translate the portable configuration into native provider files.