once

Overview

once is a Model Context Protocol server that ensures side effects run exactly once, even under retries, redelivery, and concurrent worker conditions. Any MCP-compatible AI assistant can call once's tools directly, letting the model decide when to apply idempotent execution patterns in conversations or agent runs.

The server runs locally via the Python package once-kernel, installed through uvx once-kernel. It communicates over stdio and requires no credentials to get started. once is released as v0.2.1 under the Apache-2.0 license, with source code available at aurumflux20/once-kernel on GitHub.

once appears in the Development category on mcp.site and integrates with any MCP client including Claude Desktop, Claude Code, Cursor, VS Code, Windsurf, and Zed. Maintainers are invited to claim this listing to verify ownership, earn the Verified badge, and keep information up to date.

Installation

// claude_desktop_config.json
{
  "mcpServers": {
    "once": {
      "command": "uvx",
      "args": [
        "once-kernel"
      ]
    }
  }
}