EffectFence

Overview

EffectFence is a Model Context Protocol server that prevents agents from executing duplicate side effects. It ensures that even when thousands of identical requests race to execute simultaneously, only one actually runs, providing exactly-once execution guarantees for your agent workflows.

The server runs locally on your machine via the npm package effectfence, installed with npx -y effectfence. It communicates over stdio and requires no authentication or credentials to get started. EffectFence is released under the MIT license at version v0.1.6, with full source code available at aurumflux20/effectfence on GitHub.

EffectFence is listed in the AI & Agents category on mcp.site and works with all major MCP clients including Claude Desktop, Claude Code, Cursor, VS Code, Windsurf, and Zed. If you maintain EffectFence, you can claim this listing to verify ownership and keep information up to date.

Installation

// claude_desktop_config.json
{
  "mcpServers": {
    "effectfence": {
      "command": "npx",
      "args": [
        "-y",
        "effectfence"
      ]
    }
  }
}